texts:
Are there any water-soluble salts that don't form hydrates?

Hydration seems very thermodynamically favorable, as evidenced by its abundance. It also makes sense since the solvation and hydration thermodynamics are related. I can't find any evidence of non-hydrate-forming salts by brute force search or keyword googling. Anyone know of any?


texts: What causes the really bad smell of rotten potatoes?

texts:
Rotten potatoes have a distinctive unpleasant odour, and I can't find any definitive answer as to what the chemical involved is. I was thinking it might be Butyric acid, from the fermentation of the starch, but according to Wikipedia:

  Butyric acid is present in, and is the main distinctive smell of, human vomit.

And those puppies don't smell like vomit. They're more like rotting flesh – the reason I'm asking is that I've spent half the day looking for a dead animal that the cat brought in or something, only to discover that the horrible smell was coming from the potato box.


texts: is the adsorption of resveratrol within wine or grapes linked with the time it was kept in your mouth?

texts:
This question is about resveratrol in wine or grapes and not in some special delivery system.

If one is drinking some wine or eating grapes, does it matter if they keep it in their mouth a bit so the resveratrol within the wine or grapes get absorbed better?

I know now all nutrients can be gotten into onces body via buccal method so I am asking the experts what is the case with resveratrol from wine or grapes.


texts: How to chemically postpone a chemical reaction

texts:
This question is about delaying/postponing chemical reactions in general, but I will explain my point based on an example. Most of you will know the famous vinegar and baking soda experiment which allows you to make a volcano or a rocket. What I have noticed when doing this experiment (in particular the rocket experiment) is that if you just put soda in powder form in the bottle and pour in vinegar, a lot of the formed gas escapes before you can put the cork on thus resulting in a less spectacular launch.

In the video I linked above they wrap the soda in paper to allow some time between the addition of vinegar and the start of the reaction. This is a nice trick, but in many cases you don't want any paper inside your 'reaction' vessel. My question is: what are my options to chemically postpone the start of the reaction?


texts:
Since I have been advised by my Dentist to wear a 'Tooth Guard' at night because apparently I grind my teeth, my top front teeth under the guard are disappearing, some now only about 3mms from the gum. Are they being dissolved by acid in my saliva?
Regards,
David Bowley (Scotland)


texts:
The question had two parts.

The first part said:

Calculate the amount of water required to saturate a vessel of volume 82 L at 300 K given that the vapor pressure of water is 38 torr at 300 K.

The solution went like this:

Since we have to saturate the vessel, we have to find the minimum number of moles that will occupy the given volume (that is what I understood).

Now, applying the gas equation

$$PV=nRT$$

$$\frac{1}{20}*82 = n * 0.82 * 300$$

$$n = \frac{1}{6};  w=n * M = 3 gm$$

This part is fine.

Now the second part:

If 5 gm of water is introduced in the vessel (initially evacuated), then, calculate the amount of water remaining.

So, my teacher said that, 3 gm will evaporate and 2 gm will remain. But, 3 gm will evaporate for 82 L. So, when I pour 5 gm of water, shouldn't the volume decrease?


texts: Dependency of rate of diffusion on mean free path and velocity of molecules?

texts: What is the dependency of rate of diffusion ( Graham's law) on mean free path, molecular size , average velocity, collision frequency and pressure? I have searched everywhere on internet but just found that rate of diffusion is inversely proportional to square root of density or molecular mass.


texts: Are platinum metal whiskers very unusual?

texts:
Over 20 years ago I pulled my first set of platinum spark plugs out of a car because the engine was missing badly. Two of the six plugs each had one whisker in the gap between electrodes. One crossed the gap and one was most of the way across. They started on the platinum center electrode and grew to the Inconel side (ground) electrode. They looked like straight, fine threads.

As I had worked in metallurgical R&D most of my life, I assumed they were whiskers, odd but no big deal, threw them away and put in new regular plugs. I was retired or I could have had a much better story with XRD and electron microscope data.

The reason I ask is that today I looked up metal whiskers in Wikipedia and found mostly info on whiskers formed on various solders and nothing on platinum.


texts: Estimating for atomic number for an "unknown element"

texts:
A questions tells me there's this element with 3 isotopes (270.51 amu, 34.07% abundance; 271.23 amu, 55.12%; and 269.14 amu, 10.81%), and to solve for average atomic mass. I have done so and got 271 (270.758) for its atomic mass. Then it asks me to estimate its atomic number, and its location on the periodic table. How do I exactly "estimate"?

ps. I dont think this element actually exists...


texts: Could a safe water ocean exist on a world with predominantly chlorine atmosphere?

texts: I am researching for a Si-Fi book.  Would a water ocean on a world with a predominantly chlorine ($\ce{Cl2}$) atmosphere be safe to touch?  Would the chlorine over time saturate the water creating a "laundry bleach' solution, or would it create a ocean of hydrocloric acid ($\ce{HCl}$) and hypoclorous acid ($\ce{HOCl}$), or would the chlorine just not react with the water that much?


texts: Why is the boiling point of pentane (36.1 °C) slightly higher than that of diethyl ether (34.6 °C)?

texts: Both are chains of the same length, except diethyl ether has an oxygen atom in the middle instead of a carbon. Diethyl ether has a slight dipole from the oxygen atom, so shouldn't the intermolecular forces be stronger than for pentane which give it a higher boiling point?


texts: Energies of Virtual Molecular Orbitals (LUMO)

texts: I just had a question about molecular orbitals. I was just wondering if the energies of the LUMO (virtual orbitals) derived from CCSD(T)/cc-pTVZ calculations could be trusted. I remember reading some time ago that these orbital energies could not be trusted since they contained no electrons. If they cannot be trusted, could you suggest any calculations I could perform to obtain the LUMO molecular orbital energies (perhaps TD-DFT or some other method)?


texts: What would the chemical name be for C13H8Cl3NO

texts:
Formula

  C13H8Cl3NO

SMILES

  C1=C(C(=CC(=C1)Cl)Cl)N(C(C2=CC=CC=C2)=O)Cl

I found the diagram on the left in a book and drew the one on the right using

https://pubchem.ncbi.nlm.nih.gov/edit2/index.html

And got the SMILES description from that.

Any clues as to what might be an IUPAC name and the formal way to write it out?  Have I have even drawn it correctly? I could not do it exactly because the two rings kept connecting if I followed the same orientation.  I don't know enough about double ring compounds to even hazard a guess.

Is there a automatic naming engine out there?

A compound called British Impregnite found in The Scientific method by Louis F. Fieser p.137


texts:
Which has maximum number of equivalents per mole of the oxidant?
  $$
\begin{align}
\ce{MnO4- + H2O2 &-> Mn^2+ + O2}\tag{i}\\
\ce{Cr2O7^2- + Fe^2+ &-> 2 Cr^3+ + Fe^3+}\tag{ii}
\end{align}
$$

The answer is the second option. I wanna know why

If there's another term different from equivalent mass like equivalents, then I tried doing it using the formula = No.of moles of oxidant x n factor (no of electrons gained)


texts:
why is the log P of the esters lower than the alcohols?
Is it because of the formation of dimers?


texts: How did Mendeleev know elements from compounds or mixtures?

texts:
I read that Mendeleev initially thought didymium was an element, but it was actually a mixture.
How did he know the rest of the chemicals in the periodic table were elements and not compounds, especially since back then most elements would have been incredibly difficult to isolate and get a mono-atomic version?
It seems like an impossible task in the 19th century.


texts: Molecular orbital diagram for I₃⁺ cation

texts: Do I need to go through the whole process of reducible representations/projection operator method to come up with the bent SALCs for the $\ce{[I3]+}$ molecule? Or is it simpler than that?


texts: What do locants with prime symbols (e.g. 4’) mean?

texts:
I was reading an article about acetaminophen and it says its chemical name is 4'-hydroxyacetanilide.

What does the 4 signify and most importantly, what does the apostrophe mean? First semester chem student so if you can answer in layman terms that would be great.


texts: General Method For Calculation of Enthalpy Change for a chemical reaction

texts:
1. Enthalpy of reaction defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of substance reacts completely.
2. Enthalpy of formation defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a compound is formed from its elementary antecedents.
3. Enthalpy of combustion defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a substance burns completely with oxygen.
4. Enthalpy of hydrogenation defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of an unsaturated compound reacts completely with an excess of hydrogen to form a saturated compound.
5. Enthalpy of atomization defined as the enthalpy change required to atomize one mole of compound completely.
6. Enthalpy of neutralization defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of water is formed when an acid and a base react.
7. Standard Enthalpy of a solution defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of a solute is dissolved completely in an excess of solvent so that the solution is at infinite dilution.
8. Enthalpy of fusion defined as the enthalpy change required to completely change the state of one mole of substance between solid and liquid states.
9. Enthalpy of vaporization defined as the enthalpy change required to completely change the state of one mole of substance between liquid and gaseous states.
10. Enthalpy of sublimation defined as the enthalpy change required to completely change the state of one mole of substance between solid and gaseous states.
11. Lattice enthalpy defined as the energy required to separate one mole of an ionic compound into separated gaseous ions to an infinite distance apart (meaning no force of attraction).
12. Enthalpy of mixing defined as the enthalpy change upon mixing of two (non-reacting) chemical substances.
These definitons were taken from the wikipedia article for enthalpy.
Each enthalpy seems to come with its own formula.
Is there a general approach for solving enthalpy change-related questions for reactions instead of having to memorize a formula for each and every case?
If there is an obvious point that I'm missing kindly point it out.


texts: Why isn't cyclopropane considered unsaturated?

texts:
Looking up "cycloalkanes" on wikipedia (link), I found this:

  In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are monocyclic saturated hydrocarbons.

Of course, that implies that cyclopropane is saturated. Later I looked up "saturated and unsaturated compounds" on wikipedia (link). Here's the definition they mentioned:

  In chemistry, a saturated compound is a chemical compound (or ion) that resists the addition reactions, such as hydrogenation, oxidative addition, and binding of a Lewis base.

Now the definition mentions that a saturated compound "resists addition reactions". I am aware that cyclopropane does undergo addition reactions, as mentioned here (link):

So finally, to me it seems these two wikipedia pages are contradicting each other. My question is - why is cyclopropane considered saturated if the above definitions are correct?


texts: Can you give me a mechanism for the following reaction? Here methyl is not shifting. Other ester group is shifting.


texts: Derivation of relation between acid and base equilibrium constants

texts:
While deriving the relation between $K_a$ and $K_b$ in ionic equilibrium topic, say I take an example of

$$
\ce{NH4+ + H2O -> NH3 + H3O+} \tag{1}\label{1}
$$

The $K_a$ of this reaction is:

$$
\frac{\ce{[NH3][H3O+]}}{\ce{[NH4+][H2O]}}
$$

Now $\ce{NH3}$ is a conjugate base of $\ce{NH4+}$. Its $K_b$ can be written as:

$$
\ce{NH3 + H2O -> NH4+ + OH-} \tag{2}\label{2}
$$

$K_b$ of the reaction:

$$
\frac{\ce{[NH4+][OH-]}}{\ce{[NH3][H2O]}}
$$

In my textbooks while deriving relation between $K_a$ and $K_b$, it added the two equations. In the addition of two equations we get:

$$
\ce{NH4+ + 2H2O + NH3 -> NH3 + NH4+ + H3O+ + OH-} \tag{1 + 2}\label{1p2}
$$

My textbook cancelled out both the $\ce{[NH4+]}$ and $\ce{[NH3]}$ concentration terms from the LHS and RHS. $\ce{[NH4+]}$ on the LHS is from equation $\eqref{1}$ and $\ce{[NH4+]}$ on the RHS is from equation $\eqref{2}$.

My question is, how can we cancel them out? How can you say the $\ce{[NH4+]}$ formed from hydrolysis of $\ce{NH3}$ in reaction $\eqref{2}$ is equal to that of $\ce{[NH4+]}$ initially taken in reaction $\eqref{1}$? Even if you initially take same concentration of $\ce{[NH3]}$ for reaction $\eqref{2}$ as formed from reaction $\eqref{1}$, that concentration of $\ce{NH3}$ on hydrolysis can never give same concentration of $\ce{[NH4+]}$ as initially taken in reaction $\eqref{1}$. Then, how can you cancel these different $\ce{[NH4+]}$ concentrations when you are adding reaction $\eqref{1}$ and $\eqref{2}$?


texts: Interpretation of the following mass spectrum

texts:
I'm given the above spectrum. $M^+ = 102$, probably $\ce{C_6H_14O}$ or $\ce{C_5H_10O_2}$.
I'm thinking of ethyl propionate. (that picture also shows the fragments I expect). I can account for the following peaks:
m/z = 73: Loss of ethyl from acyl side (expected lowish intensity)
m/z = 57: Loss of alkoxy to form $\ce{CH3CH2CO+}$ (moderate intensity)
m/z = 45: Alkoxy ion (weak)
m/z = 29: ethyl carbocation, moderate intensity.
I can't account for 74, since according to my book the alkoxy side would not proceed through normal Mclafferty, but the Mclafferty + 1 rearrangement... That should yield m/z=75.
I proposed methyl butyrate, and of course that's not right. My instructor wrote to pay attention to the m/z = 57 (M-45) and m/z = 85 (M-17). This fits well with carboxylic acids, but again cannot account for the m/z = 74. McLafferty of carboxylic acids would result in a fragment of m/z=60...
Can someone help me out, I've come to a dead end for hours now.


texts: Does Br2/H2O oxidize all aldehydes including carbohydrates?

texts:
In many books, it is mentioned that aldehydes are easily oxidized even by mild oxidizing agents. However, I have never seen bromine water explicitly mentioned. But in the carbohydrates chapter, it is mentioned that glucose etc. are oxidized by bromine water.

So is it that only carbohydrates are oxidized by bromine water or all aldehydes?

Also, do they react with bromine water under the conditions of the bromine water test for unsaturation? Or do they react only under vigorous conditions?


texts:
I want to do some spectrometry experiments with glow in the dark paints. The red paints I've read product details for all seem to require a uv light to charge. Are there any red glow in the dark paints that can be charged with visible light?

I was also wondering why most red phosphorescent pigments don't have the ability to glow as long as green pigments.


texts: Finding EMF of a galvanic cell without standard potentials

texts:
For the galvanic cell
$$\ce{Ag | AgCl(s), KCl (\pu{0.2 M}) || KBr (\pu{0.001 M}), AgBr(s) | Ag}$$
find the EMF generated given $K_\mathrm{sp}(\ce{AgCl}) = \pu{2.8e-10},$ $K_\mathrm{sp}(\ce{AgBr}) = \pu{3.3e-13}.$

This is the question from JEE exam (1992).
How to start solving the problem since the $E^°$ of individual half reactions is not given? How to write the $E^°$ for the cell without it? Or is it not needed?


texts: Acid-base reaction vs. salt dissolving reaction

texts:
I was watching a video in Khan academy about acids and bases. In the video, they asked the $\mathrm{pH}$ of a solution that is $\pu{0.15M}$ of $\ce{NH3}$ ($K_\mathrm{b}=1.8\times10^{-5}$) and $\pu{0.35M}$ of $\ce{NH4NO3}$. So, it was obvious to me that the first reaction is going to be that of a weak base because the $K_\mathrm{b}$ is given. But what wasn't clear to me that the second reaction is going to be that of a salt dissolving reaction that is dissolved completely as is claimed in the video.

Since this problem is presented in a beginners learning video I guess there is a way to determine that $\ce{NH4NO3}$ is a salt that is dissolving completely in water and not an acid or base according to the information given in the video.


texts: Grignard reagent reaction mechanism: Which bond breaks during the decomposition of the adduct?

texts:
We just finished our Organic Chem. chapter Aldehydes, Ketones and Carboxylic acids at school, but I still have one lingering doubt.

Considering the reaction between an alkyl magnesium halide and an aldehyde or ketone: (Diagram's from Wikipedia)

Now my teacher tells me that in the adduct, it's the Mg-O bond that gets cleaved during hydrolysis and not the C-O bond, resulting in an alcohol.

He isn't willing to elaborate ("Aaron, it happens... just like that... you shouldn't worry about all the complexities behind everything...") and I can't seem to find any source on the web that deals with the particulars of bond-breakage.

I have doubts regarding that point of his, since the Mg-O bond has significant ionic character (due to the electronegativity difference between the atoms. But in comparison, the C-O bond is more covalent, and therefore must be easier to hydrolyze. Because (the way my high-schooler brain sees it) ionic bonds are established by strong electrostatic forces of attraction between the ions concerned, while this isn't the case for predominantly covalent bonds. So it seems to make sense that the weaker bond is preferably hydrolyzed, hence I'm mooting for the C-O hydrolysis.

Who's right here? Could someone cite any (reliable) sources?


texts: Does bleach (without rinsing) leave residues and can vinegar, baking soda neutralize them?

texts:
It's very difficult to get accurate information concerning household bleach residues. Imagine, for instance, that you pour pure (or diluted) bleach onto fabric or synthetic foam and let it dry without rinsing (as a scientific experiment). Some sources say: it will totally "vanish" (with air and/or light), leaving water and salt, and some others say it will leave some toxic compounds behind, such as sodium hydroxide (which is added to the bleach to slow its decomposition).

Question 1: Will the bleach only degrade into water and salt ($\ce{NaCl}$), or will it leave other substances? If so, which ones? If the first case is true (i.e., the degradation products are water and salt only), why does the solution still smell like bleach for a long time?

Question 2: If bleach (a base) does leave toxic residue(s), can we use vinegar (an acid) to neutralize it? $(~\ce{OH- + H+}$ should create $\ce{H2O}$)

White vinegar: I'm confused since there is contradictory information on Wikipedia. Some says vinegar will neutralize harmful residues from bleach, while another source says it "could result in much more caustic and dangerous chemicals, such as hypochlorous acid" being produced.
Will an antioxidant such as vitamin C work as neutralizer? (as suggested by Brian)

It seems that everybody agrees about the use of peroxide, bisulfite, metabisulfite or thiosulfite, and sodium thiosulfate to neutralize bleach residues, but I wonder if more common products such as vinegar or vitamin C can be used instead: What do you think about these two? Any other suggestions?

EDIT: I removed the washing example which was confusing since it implied rinsing + I updated my question since I thought bleach was an acid.

EDIT2:

  Household bleach is, in general, a solution containing 3–8% sodium
  hypochlorite and 0.01–0.05% sodium hydroxide; the sodium hydroxide is
  used to slow the decomposition of sodium hypochlorite into sodium
  chloride and sodium chlorate. wiki

Brian and Drmoishe's answer suggest several ways to way to neutralized bleach but what about the sodium Hydroxide residues? (I check on a french bleach, it shows only on the composition: "3.7% of sodium hypochlorite", nothing else)


texts: Can I perform a stable isotope analysis at home?

texts: I'm interested in measuring the stable isotope ratios of some bird feathers. I'm aware that a mass spectrometer is required to measure stable isotope ratios. Is it possible to measure stable isotope ratios at home, using your own 'DIY' spectrometer?


texts: Why is the IUPAC definition of adsorption so different from other definitions?

texts:
NB: This question has been edited. Originally, some of the confusion was caused by me not knowing that condensed phase could refer to solid phases as well. This has been removed, but the confusion caused by the other elements still remains and even the first answerer to this post was unable to clear that up.
The Gold Book's definition of adsorption

An increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces. Adsorption can also occur at the interface of a condensed and a gaseous phase.

What I've learned is that adsorption is when a fluid, or the solute(s) within a liquid, adhere to a surface. The IUPAC definition however, only recognizes the case when the solute(s) within a substance adhere to a surface. A more limited, and to my knowledge, less useful definition. Furthermore, the formulation is very ambiguous. It does not state if it is the condensed phase or the fluid phase that contains the solute(s), or if either can contain the solute(s). The middle option would necessarily be in contradiction to the IUPAC's definition of a solution:
The Gold Book's definition of solution (shortened):

A liquid or solid phase containing more than one substance, ...

This definition does not count gaseous phases as solvents. The fluid phase can be a gas, but if the dissolved substance is always within the fluid, then in cases where the fluid phase is a gas, it would be a solvent gas. This is assuming that dissolved substance is the same as solute.
Now, for part two of this mess:
Furthermore, the IUPAC definition of absorption encroaches onto the semantic territory of adsorption, at least the territory that I've seen and been taught.
The Gold Book's definition of absorption

The process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc.

The last part there is the, or a part of the, definition of adsorption. It described surface attraction, which I thought was clearly within the domain of adsorption. The first part of the definition says that the absorbate and absorbent create a physical solution. I cannot find any definition of that, but I assume they are referring to a homogeneous mixture where the solute isn't disassociated, but still kept within a solution complex. However, an absorbate isn't (necessarily) homogeneously distributed within the absorbent. This means they're violating their own (and countless other) defintion(s) of solution.
As mentioned, a solution is a single phase containing more than one substance, and a phase, as defined by the IUPAC, is homogeneous.
The Gold Book's definition of a phase:

An entity of a material system which is uniform in chemical composition and physical state.

Are these IUPAC definitions outdated? If so, what is the most authoritative definition of adsorption and absorption. I like to use words consistently, but given how inconsistent and/or ambiguous scientific literature can be, I want to pick the best definition in order to influence the literature, if however minutely, towards more consistency and away from ambiguity.


texts: Why are the C-C bonds in benzene closer in length to double bonds than single bonds?

texts: The $\ce{C-C}$ bonds in benzene are $\mathrm{140~pm}$ long which puts them in between single bonds and double bonds (given at $\mathrm{147-154~pm}$ and $\ce{134-135~pm}$ respectively, sourced from here and here) as expected. However, since the two resonance structures for benzene contribute equally, leading to six identical bonds, one might expect that the bond lengths would be exactly intermediate between a single and a double bond, but in fact the bond lengths seem to be closer to a double bond than a single bond. Why is this the case? My teacher asked me this a while ago when I was preparing for university interviews and I have never thought of a satisfactory answer.


texts:
Mixing $\ce{Ca(OH)_2}$ with $\pu{0.1 M}$ concentration of $\ce{Ca(NO3)2}$.

What will actually happen in the process? What I will see with my eyes? I know that $\ce{Ca^2+}$ cation with concentration of $\pu{0.1 M}$ will be the first to react with $\ce{OH-}$, am I right? Would there be any changes in color? What will be the timing to the end point (approximately) of the reaction?


texts: Electrolysis rust removal - side products - identify lead

texts:
my kids want to restore an old hand tool made of mostly iron.
I've choosen electrolysis as method for rust removal. We used Natron to enhance the conductivity of the water. The process worked so far and we get as expected some chemical side products swimming on top of the solution. We have brown stuff (iron) and from the wires some blue (silver or copper), and we have back stuff. The solution was pretty black afterwards as well as some scum swimming on top of it.
I don't want my kids to come in contact with (too much) lead. Is there an easy way to determine if the hand tool has lead in it?


texts: Formose reaction - mechanism of first step

texts: The following image was taken from wikkipedia's article about the Formose reaction. What is the mechanism of the first step of the reaction sequence? How do the two formaldehyde molecules condense?


texts: I am trying to study how the pressure will affect the formation of ozone at around 5 bar. So, I am looking for a simple relation between the reaction rate coefficient and the pressure to use it in my simulation. Is there any suggestion?


texts:
I am trying to implement a small python script to detect peaks and calculate the corresponding area. The y-values are in mAU and x-values in minutes but there are some unit conversions in most HPLC softwares. For example in this image (found here):

The peak at 7.87 min has a height of 329451 but only approximately 20 V in the chromatogram. How can I calculate this height?


texts: Best consumption-safe liquid for caffeine solubility?

texts:
The question is simple:  What human-consumption safe liquid has the best caffeine solubility?

It's 200mg/10mL of water.  That's a lot of water for one dose!

I found some ramblings on a fourm stating it's "low" solubility in propylene glycol and glycerine (glycol) but no sources or exact numbers, so that doesn't mean much.

In boiling water, it seems much higher, 66g/100mL, which is more of the range I am hoping.  So what happens if I boil a liter of water, dump 100g into it, and then let it cool?

I am hoping for something in the range of 200mg in 1mL-3mL, but as low as possible, and relatively easy to acquire.

All help very much appreciated.


texts: How does repeatedly freezing and thawing antibiotics decrease their antimicrobial effectiveness?

texts: This is something I have read on forums repeatedly, but I don't understand the mechanism behind how antibiotics lose their effectiveness. I understand how repeatedly thawing and freezing cells would damage them, but not how repeatedly thawing and freezing antibiotics make them lose effectiveness.


texts: What is "mer", the first ingredient in a type nail polish?

texts: The first ingredient in a type of nail polish is "mer". What is this? Is it some kind of chemical? How is it used? How does it work?


texts:
I am examining methods for storing silver bullion (both .999 fine and 90% silver) in such a way that it does not tarnish and want you to evaluate a suggested solution.  I know there are commercial products intended to prevent or slow tarnish, such as cloth bags that are infused with silver particles.  However, they aren't really suitable for bullion in my opinion.  A proper solution should be

Very cheap
Work for a lot of silver, in varying sizes and shapes
Work for a long time

The suggestion in question is to store the silver in a somewhat air-tight container (like a ziploc bag or rigid plastic container) in which you also store some clean copper (perhaps bare electrical wire).  In this situation, it has been suggested that atmospheric sulfur that gets in will react preferentially with the copper, leaving the silver pretty free of tarnish.  From time to time the copper could be cleaned or replaced.

Unfortunately, the source who presented this solution did not seem very authoritative to me. Can you analyze the solution and give me your opinion?  Please assume that tarnish is unacceptable, even though the silver is being kept as bullion and there are methods for removing tarnish.

Bonus question 1: Are there other atmospheric chemicals aside from sulfur that attack silver over time that one should worry about?

Bonus question 2: It as been suggested by some that the low density polyethylene in plastic bags will off-gas over time and cause additional tarnish or some other chemical reaction.  I can't imagine this is sulfur but perhaps there is a chemical released that affects copper?


texts: How does a volcano for a science fair work?

texts: In this video it shows me how to make a Volcano but what is the science behind it? http://www.youtube.com/watch?v=9b_gltKtERY


texts: The reason for bicarbonate acting as a buffer

texts:
My question is: what is the primary reason that bicarbonate ($\ce{HCO3^-}$) can act as a buffer in a solution? The two (possibly) relevant equations that I can think of are

$$ \ce {H2CO3 + H2O <=>HCO3- + H3O+ }$$

$$ \ce {HCO3- + H2O <=> CO3^{2-} + H3O+} $$

Using these two equations, I can come up with two 'reasons' as to why $\ce{HCO3-}$ can act as a buffer. The first reason is that $\ce{HCO3-}$ can create $\ce{H+}$ ions in the second equation, but can also use up $\ce{H+}$ ions in the first equation. This means that if I put an excess amount of $\ce{H+}$ ions into the solution, the $\ce{HCO3-}$ can act by Le Chatelier's principle to remove the $\ce{H+}$ ions. If I put an excess of $\ce{OH-}$ ions into solution, they will react with the $\ce{H+}$ ions, and cause a shortage. The $\ce{HCO3-}$ can then react with water to replenish the $\ce{H+}$ ions in the solution. The only requirement for this explanation to occur is that enough $\ce{HCO3-}$ exists within the solution to be able to balance out whatever changes may occur.

This was the picture I had in my head as I began to study my textbook today. However, my textbook differed, listing only the first equation as the reason $\ce{HCO3-}$ can act as a buffer. In the first equation, an excess of $\ce{H+}$ ions would drive the equation to the left, removing the excess ions from the solution. An excess of $\ce{OH-}$ ions would react with the $\ce{H+}$ ions, and drive the equation to the right, replenishing the $\ce{H+}$ ions, and restoring neutrality. This is a much simpler explanation, but it requires that there be adequate amounts of both $\ce{HCO3-}$ AND $\ce{H2CO3}$ in the solution to maintain neutrality.

Both 'reasons'seem to operate on the same fundamental rules, and yet they require different things to operate, and I am pretty sure my textbook is right and not me. One reason I can think of why is perhaps that the second equilibrium lies far on the left, so far left that barely any $\ce{CO3^{2-}}$ is ever present in solution, which would mean the second equation could never help with buffering the solution, or if it did it would be negligible help. Is this line of reasoning correct?


texts:
Shouldn't 4-isopropyl-2-cyclohexyl-3,5-dimethylheptane be written as 2-cyclohexyl-3,5-dimethyl-4-isopropylheptane, by following alphabetical order?

Or maybe you don't look at the cyclo or the di prefix when you look for alphabetical order.


texts: What is the dipole moment of borazine? I tried googling it but can't find a reliable source. I searched in JD Lee too. It might have it but I could not find.


texts:
So I came across a question on the equation of the pH curve, and I wanted to know how the user here arrived at the matter balance equation. Any help would be appreciated!

Here is the question:

The reason behind the steep rise in pH in the acid base titration curve


texts: Gravimetric analysis of barium or sulfate

texts: Which is a better way to obtain barium sulfate precipitate for gravimetric analysis: by adding $\ce{Ba^2+}$ ions to sulfate ion solution, or the other way round? I think it is better to precipitate sulfate ions by $\ce{BaCl2}$ solution, as this causes chloride ions to coprecipitate.


texts: Book recommendation for studying chemistry through scientific method

texts:
I really like physics and I think the reason is that I enjoy studying the process of discovering rather than reading known facts. I've tried to study chemistry but I feel introductory texts on chemistry doesn't focus on the process of discovery and how one could reach certain conclusions after following such and such thought processes. Instead they state facts.

For instance, while studying chemistry the author would tell you that a water molecule is $\mathrm{H_2O}$ (two atoms of hydrogen and one of oxygen) but doesn't explain how early scientists reached that conclusion.

Can someone recommend any introductory textbook/book that'll teach chemistry that way?


texts: Finding concentration and moles given final and initial pH

texts:
Questions:

How many moles of $\ce{NaOH}$ are needed to change $500~\mathrm{L}$ of solution with $\mathrm{pH}\ 2$ to $\mathrm{pH}\ 11$?
A swimming pool contains 2 million litres of water at $\mathrm{pH}\ 7.80$ and the $\mathrm{pH}$ needs to be changed to $6.80$. How many moles of $\ce{HCl}$ are needed?

Not quite sure how to tackle these types of questions.
Would I have to find the moles of hydroxide/hydrogen ions to neutralise the solution first (ie. moles of hydroxide ions needed to reach $\mathrm{pH}\ 7$) and then from there calculate the amount of hydrogen/hydroxide ions needed to get to final $\mathrm{pH}$?
Or do I just calculate the difference between the moles of ions in final $\mathrm{pH}$ with moles of ions in initial $\mathrm{pH}$?
I have tried both methods but I don't have the answer to the questions so I'm just trying to understand the concept.


texts: Canonical MOs vs. Localized MOs: Do both represent reality in the same way?

texts:
In my understanding localized molecular orbitals (LMOs) are equivalent to "standard" molecular orbitals, often called canonical orbitals (CMOs—by the way, why are they called canonical?). We can produce LMOs by linear combination of CMOs and they represent the same physical state.

I have two questions:

Is this just a mathematical gimmick or does this represent the reality? Is it possible for a molecule to be in such a state where every electron is in such a localized orbital? Or is this just a thing we do because it makes it easier to understand?
If LMOs are equivalent and have the same energy then why do calculations always end up with canonical molecular orbitals and never with LMOs? And wouldn't there be an unlimited amount of equivalent solutions?


texts: Finding new pH after adding potassium hydroxide to citrate buffer

texts:
Calculate the $\mathrm{pH}$ change of citrate buffer containing $\pu{10.26 mmol}$ of citric acid and $\pu{6.98 mmol}$ citrate caused by adding $\pu{4.72 mmol}$ of potassium hydroxide. $\mathrm{p}K_\mathrm{a} = 3.06.$

I'm trying to calculate it like initial $\mathrm{pH}$

$$\mathrm{pH_0} = 3.06 - \lg\frac{\pu{10.26 mmol}}{\pu{6.98 mmol}} = 2.89,$$

then if we add $\pu{4.72 mmol}$ $\ce{KOH},$ it's $1:3$ to citric acid. Final $\mathrm{pH}$ will be

$$\mathrm{pH_1} = 3.06 - \lg\frac{\pu{10.26 mmol} - \pu{1.57 mmol}}{\pu{6.98 mmol} + \pu{1.57 mmol}} = 3.05,$$

but this answer is incorrect. Where is the mistake?


texts:
Phenolsulfonic Acid( PSA) is produced after sulphonation of Phenol at around 100 degrees. They use this product in tinplating.
Once a stable PSA is formed after sufficient reaction , unreacted impurities are Phenol 2% max , Sulphate  3% max. PSA is to be 65%  minimum and rest is water.
After few days , Sulphate is increasing to 3.5 to 4.5% or even more, while others remain constant.
The question is:
Why sulphate concentration is increasing from the initial value of 3% on day 1 to 4% on day 5/6. I dont have a chemistry background so any help would be appreciated. Thank you


texts: How does a "sea of electrons" hold metal atom together?

texts: How do delocalized electrons hold metal atoms together? What are the forces of attraction between the atoms and the electrons called?


texts: Whats is the difference between Tris and Tris-acetate?

texts:
I would like to run the following protocol to detect plasmid presence in bacteria.
Would be possible to replace 'Tris- acetate' with just Tris?

We don't have 'Tris- acetate' in the lab.
What is the role of acetate here?

Hot alkaline method for all plasmid sizes and bacteria (Kado & Liu,1981), modified

Centrifuge 2-3 ml of culture, resuspend pellet in 1 ml of solution containing 0.04 M Tris- acetate, pH 8.0 (adjust pH with glacial acetic acid) and 2 mM EDTA
Add 2 ml of lysis buffer (0.05 M Tris, 3% SDS, pH 12.50, adjusted with 2 N NaOH) and mix
Incubate at 60-68°C for 30-45 min (strain dependent)
Add to hot samples 6 ml of phenol/chloroform (1:1) and mix gently to complete emulsification
Separate phases by centrifugation at 10.000 x g for 15-20 min at RT and transfer the upper aqueous phase carefully (avoid interphase which contains debris) to new tube containing 1 volume of chloroform. Mix and centrifuge again for separation of phases


texts: Is it really possible for a liquid to stay afloat a gas (vapor)?

texts:
I have a book at home called Chemical Thermodynamics for Process Simulation (I have the first edition) and at the very beginning of chapter 2 of the book one may read the following:

No reference is given. I have never found anything about it so far and I am curious to know whether this statement is true.


texts: Rotating dihedral and Angles And Generate File For Each Degree

texts:
I need to optimize a force field I generated. For this, I should apply Genetic Algorithm (G.A). To apply G.A I should do this for 220 molecules:

1) Find all dihedrals for each molecule ( I did this already with parmchk)

2) Rotate all dihedrals (10-20 degree step in 90 or 360 degree range) and save a mol2 file at each steps.

3) Change all angles (1-2 degree step in 20 or 30 degree range)and save a mol2 file at each step.

Now, I need a program to do this because 220 molecules are too much. I tried "tleap" but it was unstable. I tried pymol but I couldn't run command directly via "cmd" or "terminal" (if there is a way I don't know). So which program should I use?


texts: Redox reaction with tin and sulfuric acid solution

texts:
I need to write the redox reaction from tin and a sulfuric acid solution. Here is the solution given:

  The first half-reaction: $\ce{Sn->Sn^2+\+2e^-}$
  The second half-reaction: $\ce{2H+\+2e^-->H2}$
  Adding both reactions will give you: $\ce{Sn\+2H+->Sn^2+\+H2}$

Why is the sulfuric acid solution replaced with $\ce{H^+}$?


texts: If a conductivity meter is calibrated with aqueous standards, can it be used to measure the conductivity of an organic-based solvent system?

texts: Trying to figure out if the calibration solutions for a conductivity probe/meter must be of a similar nature to the system which will be measured. I can only find aqueous calibration solutions online, and the solvent systems that I need to monitor are ethanol and methanol based. If I use the aqueous calibration solutions, will my measurements in the organic systems be accurate?


texts: Methylation of nitroalkanes with diazomethane

texts:
It is known that diazomethane can methylate various compounds with acidic hydrogens. Diazomethane is most well-known for the efficient synthesis of methyl esters from carboxylic acids. Clayden, Greeves, & Warren (2012) mention that it can also methylate phenols ($\ce {pK_a = 10}$), but not straight-chain alcohols ($\ce {pK_a = 16}$). The latter not being acidic enough. I was just wondering if nitroalkanes ($\ce {pK_a = 10}$) would be able to be methylated by diazomethane?

Reference

Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.


texts: Why is the entropy of solidifying solution higher than that of a pure solvent?

texts:
According to a source,

  Solutions have a greater entropy than pure liquids as the particles in a solution are more separated and solvent molecules separate the solute particles. If the number of moles of gas increases in a chemical reaction (as shown by the stoichiometry of the reaction) then the entropy also increases.

My questions are as follows:

1) How do we know the particles in a solution are more separated ?

2) How do the solvent molecules actually separate the solute particles?

3) Also, we know that the entropy of solid is less than that of a liquid, so shouldn't the addition of solute particle (which are obviously solid) actually lead to lesser entropy than pure liquid?


texts: When I place a straw in carbontated drink and blow, more gas bubbles rise to the surface?

texts:
When I place a straw in carbontated drink and blow (making bubbling noises), more gas bubbles rise to the surface. The gas bubbles which rise to the surface are from the carbon dioxide gas dissolved in the drink, I'm not talking about the air bubbles I made with the straw?

Why does this happen?


texts: How does the choice of metal (oxide) catalyst affect the range of unsaturated compounds that can be hydrogenated?

texts: In the hydrogenation of unsaturated compounds with hydrogen gas and a catalyst, the choice of palladium on carbon is able to hydrogenate alkenes and alkynes, but is unable to hydrogenate aromatic compounds. Using rhodium or Raney nickel, however, allows one to hydrogenate aromatics. How does the choice of catalyst affect what kind of unsaturated compounds can be hydrogenated?


texts: Are there any substituents other than halogens which are ortho-para directors but ring deactivators?

texts: I was reading about electrophilic aromatic substitution reactions, and in every book, I found that the only example given for ortho-para directors - which deactivate the ring - were halogens. So are there any other substituents, other than halogens, which are ortho-para directors but ring deactivators? If so, do they do it like halogens?


texts: At a recent flea market, someone was selling a surplus military device that was marked as having 900 millicuries and should be disposed of in a safe way.  I picked it up to read the words.  Whould 900 millicuries be a concerning amount of radiation?


texts:
I tried to find the value of equilibrium constant several times.
I failed..

the value of equilibrium constant at 500K
has enthalpy 250kJ/mol and entropy 48J/mol*K

the answer is $ 2.45*10^{-24} $

which formula do I have to use?


texts: Why does benzyne dimerise and not trimerise?

texts:
Why does benzyne prefer to undergo dimerisation to stabilise itself, even though the product formed (biphenylene) is anti-aromatic?

Why does it not undergo trimerisation through the following pathway, resulting in an aromatic and presumably more stable compound (triphenylene)?


texts: How exactly do sulfites prevent oxidation of wine?

texts: I know that sulfites are used in wines to prevent oxidation, but how exactly does this happen, which reactions take place and where do the products go?


texts: Synthesis question: Using reductive amination to form pyrrolidine

texts:
What molecule results from treating 4-oxopentanal with $\ce{NH4Cl}$ and $\ce{NaCNBH3}$? Why? Is this a good, high yield reaction?
I believe that pyrrolidine is formed because amination will convert one carbonyl to an amine that reacts with the other carbonyl. However, I am unsure how reasonable this is and I'm unsure what happens to the original carbonyl oxygen.


texts: Carbon dating: What to compare the new value against?

texts:
Formula for calculating age by using carbon-14 (source: HowStuffWorks)
$$
t = \frac{\ln (N_\mathrm f/N_\mathrm o)}{-0.693}t_{1/2}
$$
The page doesn't explain what the $-0.693$ represents. What is $-0.693$?
Secondly, it says that $N_\mathrm o$ is the original level of $\ce{C-14}$ and $N_\mathrm f$ is the measured value. How do you get the original value? Is it the same for all living organisms?

Wikipedia says that the age of an object (e.g. a fossil) can be calculated by comparing the amount of $\ce{C-14}$ in the fossil and the amount of $\ce{C-14}$ in a sample of living tissue. The half-life of $\ce{C-14}$ ($\pu{5,730 years}$) can be used to find out how much time has passed since the death of the organism because you should be able to calculate how much time has passed by measuring how much $\ce{C-14}$ has decayed. But how can you calculate how much has decayed if you don't know how much was there in the first place?


texts: Is temperature of crystallisation same as melting point temperature for polymers?

texts: Melting point tends to be a temperature range. Is it true that the temperature of crystallisation is the highest temperature at which the polymer melts and thus there is some distinction between temperature of crystallisation, Tc and temperature at melting point, Tm?


texts:
There are many types of plastics that can be used for food packaging, for example:

Polyethylene terephthalate (PET)
Polyethylene (PE)
Polyvinyl chloride (PVC)
Polypropylene (PP)

What are the differences between them?
Which one is most suitable for packaging pastries, for example, biscuits?


texts: Molecule or molecular ion that spontaneously breaks up in two different ways

texts:
I teach physics, and I'm looking for simple, easy examples from chemistry to use in order to illustrate basic ideas about quantum mechanics for my students. A conceptually simple example is the $\text{He}^-$ ion, which has a metastable state with a mean lifetime of 0.36 ms. This is for the totally isolated ion, i.e., we're not talking about a gas or plasma in which there can be collisions or reactions. (How would chemists refer to this? "In vacuo?") This is easy to relate to in chemical terms (a noble gas needs an electron like a fish needs a bicycle) and provides a simple example of quantum mechanical concepts like exponential decay.

What would be more fun for my purposes would be an example of a metastable molecule or molecular ion that would spontaneously break up in two different ways. I would imagine that for the ions, one could measure the half-life and branching ratios by storing them in a circular accelerator. So we could have something like the decay channel

$$\text{XYZ}^+ \rightarrow \text{XY}^+ + \text{Z}$$

competing with, say,

$$\text{XYZ}^+ \rightarrow \text{X}^+ + \text{YZ}.$$

We can then discuss things like the probabilities (branching ratios) and the fact that if the ion starts out in a well-defined state, presumably its ground state, then we can only predict these probabilities, not the actual outcome.

Part of what's making it hard for me to find examples is that I don't know the terminology to google on. Is this "autodetachment," or is that only used for the loss of an electron? "Autodissociation?"

Note: I really am looking for the process described above, not something else like breakup induced by a collision, nuclear decay, electromagnetic decay of an electronic excitation, electron emission, or chemical reactions that require two reactants to come in contact. Those other examples might be fine for some educational purposes, but they're not what this question is about.


texts: Is there a formula to know how many constitutional isomers are there for a compound? For instance in this example, most students would miss the 6th constitutional isomer and will probably only write the first 5. So I would like to kindly ask if there's a formula or pattern for it.


texts: How does pH depend on temperature, when taking into account volume expansion?

texts:
Since autodissociation of water is endothermic, increasing the temperature will drive the following equilibrium to the right, which has the effect of increasing $[\ce{H+}]$:

$$\ce{H2O <=> H+ + OH-}$$

However, increasing the temperature of liquid water will also increase its volume, which has the effect of reducing $[\ce{H+}]$ (cf. this table on Wikipedia showing how density of water varies with temperature).

So as we increase the temperature, which of the two effects dominates? Does $[\ce{H+}]$ experience a net increase or decrease?


texts: Why is BuLi a better nucleophile than base?

texts:
In Carey, Sundberg Advanced Organic Chemistry it is written:

  The relative slowness of the abstraction of protons from carbon acids by organolithium
  reagents is probably also due to the compact character of the carbon-lithium
  clusters. Since the electrons associated with the carbanion are tightly associated with
  the cluster of lithium cations, some activation energy is required to break the bond
  before the carbanion can act as a base. This kinetic sluggishness of organometallic
  compounds as bases permits important reactions in which the organometallic species
  acts as a nucleophile in preference to functioning as a strong base.

Won't the same reasons apply for nucleophilic displacement? Compactness of the aggregates should affect nucleophilicity, too. So why will BuLi add to a carbonyl instead of deprotonating the alpha carbon atom?


texts: Red Cabbage Indicator colour change with Sodium Metabisulphite

texts: I made some pH indicator by boiling and straining cabbage juice and in an effort to keep it from spoiling for a while without having to store it at fridge temperatures I added some wine making stabiliser which consists primarily of Sodium Metabisulphite. I had previously tested some of it to check if adding it would affect the pH of the indicator since I wanted a pH neutral preservative. Well strangely after leaving it for a while I came back to find that the indicator had turned a weird shade of purple I had never seen the indicator take before. At the concentration it is at the original colour was a very dark purple - almost black. Now I know that the metabisulphite ion decomposes to produce SO2 gas which is acidic under aqueous conditions but I would expect that to change the indicator to its normal purple for a not very concentrated acid. But the colour it currently is is very pale and in fact when I pipette out some of the indicator it is almost totally clear. So I guess really my question is does Potassium Metabisulphite have some kind of bleaching effect I'm not aware of?


texts: Dehalogenation of vicinal dihalo alkanes

texts:
Which mechanism will the following reaction follow?

Will it follow E2 mechanism?
If yes, then how would 2,3-dibromobutane undergo this reaction?
Does it form both cis and trans butene or is the product butyne?
Here is a link to a website that has mentioned it is E2 mechanism but has low stereospecificity.  So does Zn act as the base donating the electrons? If this happens then why isn't hydrogen leaving the substrate resulting in dehydrohalogenation and formation of alkyne? Why are two halogens leaving the substrate to give an alkene instead of alkyne


texts: Explanation for the reactions in a saltwater battery with zinc and copper electrodes

texts:
I am a physicist, not a chemist. I'm trying to get a basic understanding of the reactions taking place in a battery using a saltwater electrolyte with copper and zinc terminals. I'm writing a general science level article about it. I have found that the reactions taking place at the terminals are:

\begin{aligned}
 \text{Zinc terminal:}&&\ce{Zn(s) &-> Zn^{2+}(aq) + 2e^-} &(\Delta G = -0.76\:\mathrm{V})\\
 \text{Copper Terminal:}&&\ce{Cu^{2+}(aq) + 2e^- &-> Cu(s)} &(\Delta G = -0.34\:\mathrm{V})
\end{aligned}

I think I understand the first reaction in terms zinc leaving the terminal and going into solution kind of as if it were dissolved $\ce{ZnCl2}$ (does that make sense?). But I need help understanding the second reaction. I've inserved a copper terminal into the solution. But not copper ions. Am I to understand it like this: when I put the copper terminal in the solution, copper ions immediately drift off, perhaps for the same reason the zinc ions do, but then deposit back onto the terminal in the presence of the electrons released when the zinc goes into solution?

I'd like an explanation somewhere between that it just happens and full on redox theory, if that is possible.


texts:
Now in an electrochemical cell , we make use of the fact that whenever there is a potential drop , electrons will flow from lower to higher potential as for a potential drop , there must be an electric field .

Now suppose there are two single electrodes , what creates the electrostatic potential on each of them in the first place ?

And just to confirm , a redox reaction is used just that we can maintain a steady flow ?And so why is it that we always put in that electrolyte that has one of the ions same as the electrode element ?


texts: How to calculate the probability of a particular value of an observable being measured

texts:
A particle on a ring is prepared with a wave function equal to $\frac{1}{\sqrt{\pi}}$ between $\theta=0$ and $\theta=\pi$, and $0$ between $\theta=\pi$ and $\theta=2\pi$. If a measurement of the angular momentum is made, calculate the probability of finding a particular value $l\hbar$. [The angular momentum eigenfunctions are $|l>=(2\pi)^{-1/2}e^{il\theta}]$

I've been reading the Oxford Chemistry Primer on Quantum Mechanics and they don't provide solutions. I've been struggling with this question.

I think I need to calculate the coefficient of expansion for a particular basis eigenfunction, but I'm unsure how to proceed.


texts: Using a gas stove instead of Bunsen burner for a flame test?

texts: In all chemistry labs we use the Bunsen burner. What's the difference between it and a good old gas stove used in home? Can the stove also be used to get positive flame tests for some cations (like $\ce{Ca^2+}$)?


texts: Spins spin coupling on proton NMR for an aldehyde?

texts:
On a proton NMR, does the hydrogen in the CHO group cause coupling, or get coupled itself? Specifically, I a considering 2-methylpropanal. I cannot decide whether the H in the aldehyde group would be split into a doublet or remain as a singlet, and I am unsure as to whether the single proton on carbon number two would be a heptet (from the 6 adjacent protons), an octet ( from the 6 protons plus the proton in the aldehyde group), or if it would be split into some combination of a heptet of doublets, or a doublet of heptets...

Any advice on dealing with spin spin coupling in proton NMR would be much appreciated!


texts: Is there a chemical or solution that can penetrate silicone water repellents in fabric? Need to break down or penetrate the silicone water repellent in selected areas.


texts: Considering how electronegative oxygen is, why is the chemical shift of alcohol protons (1H-NMR) not so high? The chemical shifts of the protons on 3-propanol are about 1.20 for the carbon-1 hydrogens, about 2.2 for the alcohol proton, and about 4.01 for the carbon-2 hydrogens. Why on earth would the electrons on the carbon-2 hydrogens feel more pull from the oxygen than those on the alcohol proton?


texts: Resonance condition for absorption of energy

texts: In NMR, for example, why it is not possible to excite nuclei from one spin state to another using energy of arbitrary frequency?


texts:
E.g. for a first-order reaction, rate of reaction = k[A], I can see that a higher concentration of A leads to a higher rate due to an increased chance of effective collision.
According to this, concentration is not related to frequency factor A hence has no effect on k, the rate constant.
But A = zp, where z = collision frequency. Doesn't increased concentration of A leads to increased collision frequency z hence A hence the rate constant k? Isn't this the explanation of why the concentration of reactants can increase the reaction rate? Why it is concluded to be mutually-independent?


texts: Calculating pH when weak base is added to an strong acid

texts:
$\ce{NH3}$ solution of $\pu{0.1 mol dm-3}$ is being added to a $\pu{25.0 cm3}$ of $\pu{0.1 mol dm-3}$ $\ce{HCl}$ solution. Calculate the pH of the solution when volume of added $\ce{NH3}$ solution is $\pu{25.0 cm3}$ and $\pu{26.0 cm3}$. The $\mathrm{p}K_\mathrm{a}$ of $\ce{NH3}$ is ${9.25}$.

So I started with the equation:

$$\ce{HCl + NH3 <=> NH4Cl}\tag{1}$$

EDIT #1:

As the initial concentration of both base and acid are same, I calculated how much of base is left in the solution:

  $V_\text{miscon} = \pu{26.0 cm3 - 25.0 cm3 = 1.0 cm3}$

That's how I tried to calculate pH of the solution when $\pu{26.0 cm3}$ of $\ce{NH3}$ is added:

  $\ce{H+}$ concentration:
  $$[\ce{H+}]_\text{miscon} = [\ce{H+}]_\text{initial} \frac{V_\text{initial}}{V_\text{miscon}}\tag{2} $$ $$=  \pu{(0.1 mol dm-3} \times \pu{0.025 dm3)/0.001 dm3} = \pu{2.5 mol dm-3}$$

But, in this way, the results I am getting are completely different from marking, and even negative pH values.

But, when I tried to find $\ce{H+}$ concentration (to calculate pH) of the solution when $\pu{25.0 cm3}$ of $\ce{NH3}$ is added, I found out that initial $\ce{HCl}$ volume is completely reacted. Only ammonium chloride is left in the solution.

The marking only gives the answers, $\ce{pH = 5.28}$ and $\ce{pH = 7.85}$.

How can I calculate pH of this solution in above two conditions separately?

I can't figure it out, I am stuck in the above equation. Please give me some hints to start solving. Thanks in advance.

EDIT #2:

I finally figured it out (Adding $\pu{25.0 cm3}$ of $\ce{NH3}$).

$$\ce{[NH4+(aq)]}=n(\ce{NH4+})/V_{\mathrm{\ce{total}}}$$
$$=(\pu{0.1 mol dm-3}\cdot\pu{0.025 dm3})/\pu{0.05 dm3}$$
$$=\pu{0.0025 mol}/\pu{0.05 dm3}$$
$$\ \ \ =5.0\times 10^{-2}\  \pu{mol dm-3}$$

$\ce{NH4+}$ reacts with water to produce $\ce{H3O+}$ ions.

$$\ce{NH4+(aq) + H2O(l)  <=> H3O+(aq) + NH3(aq)}$$

Using acid dissociation constant for $\ce{NH4+}$:

$$\ce{K_\mathrm{a} = [H3O+(aq)] [NH3(aq)] / [NH4+(aq)]}$$
$$\ce{[H3O+(aq)] = \pu{( K_\mathrm{a} \cdot [\ce{NH4+(aq)}] )0.5}}$$
$$\ce{= \pu{( 5.62\times 10^{-10}\pu{mol dm-3} \cdot 5.0\times 10^{-2}\pu{mol dm-3} )0.5}}$$
$$\ce{= \pu{( 2.81\times 10^{-11}\pu{mol2 dm-6} )0.5}}$$
$$\ \ \ =5.301\times 10^{-6}\  \pu{mol dm-3}$$

  $$ \text{pH} = -(\log 5.301 - 6\log 10) = -(0.7244 - 6) = 5.28 $$


texts: If the temperature of a solution decreases, what is the sign of the enthalpy change?

texts:
Here is what seems to be a straightforward question: you dissolve some ammonium chloride in water at $25\ \mathrm{^\circ C}$, lowering the temperature of the solution. Is the change in enthalpy for the dissolving process positive or negative?

My reasoning: if the temperature of the solution decreases, it means heat was released into the surroundings, so the enthalpy change is negative.

Apparently the answer is the enthalpy change is positive. What am I doing wrong? Whatever it is, it must be stupid, but please forgive me for its late here.


texts:
In the  thermodynamics chapter of the 11th standard, I read something that I have a hard time believing to be true:

"There are two ways by which a system can interact or exchange energy with its surroundings:
(i) Heat (ii) Work."

This is what I found in various books, exactly quoted from one of them.
Why only two? Why aren't there more ways for a system to interact or exchange energy with its surroundings? If there are, what are they?


texts: Titration of barium(II) hydroxide by hydrochloric acid

texts:
$\pu{25 ml}$ of $\pu{0.024 M}$ $\ce{HCl}$ is titrated into a volumetric flask which initially contains $\pu{45 ml}$ of $\pu{0.034 M}$ $\ce{Ba(OH)2}$. Calculate the $\mathrm{pH}$.

This is how I calculated.

$$\ce{Ba(OH)2 + 2HCl -> BaCl2 + 2 H2O}$$

Moles leftover / excess moles will determine the $\mathrm{pH}$ because the others that have reacted will neutralise each other.

So calculating moles of $\ce{Ba(OH)2}$ and $\ce{HCl}$ in their respective volume
$C=\frac{n}{v}$, hence $n= c \times v $.

Moles of $\ce{Ba(OH)2} = \pu{0.034M} \times \frac{45}{1000} = 1.53 \times 10^{-3}$

Moles of $\ce{HCl} = 0.024 \times \frac{25}{1000} = 6 \times 10^{-4}$ but $2\times6\times10^{-4}$ since it’s $\ce{2HCl}$ which is $1.2 \times 10^{-3}$.

Now to find leftover moles, you’ll have to subtract as such
${1.53} \times 10^{-3} - 1.2 \times 10^{-3} = 3.3 \times 10^{-4}$, this is the excess moles of $\ce{Ba(OH)2}$.

Looking at solution of excess $\ce{Ba(OH)2}$

$\ce{Ba(OH)2  -> Ba^2+ + 2 OH-}$

Since $\ce{Ba(OH)2}$ : $\ce{2OH-}$, moles of $\ce{OH-} = 3.3 \times 10^{-4} \times 2 = 6.6 \times 10^{-4}$ moles of $\ce{OH-}$

So, concentration $= \frac{6.6 \times 10^{-4}}{\frac{25+45}{1000}} = \frac{6.6 \times 10^{-4}}{0.07} = 0.009..$

$\mathrm{pOH} = -\log \ce{[OH]} = -\log[0.009] = 2.026$

Since $\mathrm{pH} + \mathrm{pOH} = 14$, $\mathrm{pH} = 14 - \mathrm{pOH}$
So $\mathrm{pH} = 14 - 2.026 = 11.97$

HOWEVER, the book is saying $\mathrm{pH} = 12.55$. I have done this problem multiple times and cannot figure it out.


texts: How to determine the reaction of a strong electrolyte?

texts:
I'm told the $\ce{KMnO4}$ is a strong electrolyte.

How can I figure out how the reaction looks like?


texts: Do Acids/Bases undergo dilution/bio-degradation when exposed to air?

texts:
I spilled some cleaning fluid/drain opener in my trash can, a long time ago(almost 3 months ago), I didn't tried to clean it up. The cleaning fluid was diluted, and I spilled a small amount not more than 30 ml, I think.

The bottle does not say, what the cleaning fluid contains, but it probably has Sodium Hydroxide(if it is an alkali-based cleaner) or Hydrochloric acid(if it is an acid-based cleaner).

Now, what I have heard about Sodium Hydroxide, is that it reacts with carbon dioxide in the atmosphere, and lose its corrosiveness over time.

So, is it possible that NaOH got completely carboxylated in 3 months and nothing corrosive remains?

Now, about strong acid such as Hydrochloric acid, since they are hygroscopic, do they become dilute over time?


texts: What are standard controllers for automatic pH control?

texts:
My background in control theory comes from electrical engineering (robots, motors, etc.) and I want to build a control system for automatic pH control.

Can I directly apply my knowledge from classical control to chemical control? E.g. can I use a PID controller in a similar way as I am use to, or are there special pitfalls/best practices in chemistry that I should be aware of?

Short system description: My plan is to have two pumps that can add either base or acid to a solution under constant agitation (shaker plate), using a standard glass electrode sensor for measurements.


texts:
I was trying to determine whether I could synthesize methyl iodide by combining methanol, potassium iodide and sulphuric acid together... The way I see it, the sulphuric acid will protonate the OH instead of the I forming a very stable leaving group of water. Then the iodine ions would donate an electron pair to the carbon. You would be left with the salt Potassium bisulfate.... Then to remove my methyl iodide, just distil.

Will this work?


texts: Given initial moles of weak acid, find moles of conjugate base to be added to get a final pH (volume unknown)

texts:
I am preparing notes on buffers and I came across this problem on a website:

Given 10.0 mmol of a weak acid HA with $\textrm{p}K_a =5$, how many mmol of NaA needs to be added to have a final $\textrm{p}H = 5$?

The solution method shown used the Henderson-Hasselbalch equation
$$\textrm{p}H = \textrm{p}K_a + \log_{10} \frac{ [\ce{A-}] }{ [\ce{HA}] } \hspace{1in} (\textrm{Eqn 0})$$
Plugging we get
$$[\ce{A-}] = [\ce{HA}] \qquad \iff \qquad n(\ce{A-}) = n(\ce{HA})$$
Thus, at equilibrium the moles of $\ce{A-}$ needs to equal the moles of $\ce{HA}$. Next, the author assumes that this implies the initial moles will also equal
$$n(\ce{A-}) = n(\ce{HA}) \implies n_\circ (\ce{A-}) \approx n_\circ (\ce{HA})$$
Since $n_\circ (\ce{HA}) = 10$ mmol this means $n_\circ(\ce{A-}) = 10$ mmol which means the amount of NaA needed is also 10 mmol.
I wanted to solve the problem without making that assumption.
Mass Balance At equilibrium the "atoms" $\ce{A}$ appear only in the HA and $\ce{A-}$ and they only come from the initial $\ce{HA}$ of 10 mmol or from the added moles of $\ce{NaA}$ which equals the moles of $\ce{Na+}$
$$n(\ce{HA}) + n(\ce{A-}) = 10 + n(\ce{Na+}) \hspace{1in} (\textrm{Eqn 1})$$
Charge Balance
$$n(\ce{H+}) + n(\ce{Na+}) = n(\ce{A-}) + n(\ce{OH-})\hspace{1in} (\textrm{Eqn 2})$$
Equilibrium for the weak acid
$$ K_a = \frac{ [\ce{H+}] \cdot [\ce{A-}] }{ [\ce{HA}] } = [\ce{H+}] \,\frac{n(\ce{A-})}{n(\ce{HA})} \hspace{1in} (\textrm{Eqn 3})$$
Equilibrium for the water
$$
      K_w = [\ce{H+}] \cdot [\ce{OH-}] \hspace{1in} (\textrm{Eqn 4})$$
In the above we know $K_a = 10^{-5}$, $K_w= 10^{-14}$, and from the p$H$ we know $[\ce{H+}] = 10^{-5}$ mmol/mL.  The fourth equation gives $[\ce{OH-}] = 10^{-9}$ mmol/mL.
However, the final volume of the solution $V$ is not given. No volumes are given. So $n(\ce{OH-})$ can not be found from $[\ce{OH-}]$.  This means the first three equations can not be used to get $n(\ce{HA})$, $n(\ce{A-})$, and $n(\ce{Na+})$. Notice dividing the first equation by $V$ does not help since it 10 mmol would become $10/V$.
Is this correct?  Without knowing the volume I can not find a value for $n(\ce{Na+})$ unless I make the assumption given above?
Clarification Notes Added: Yes, Eqn 0 is just the negative logarithm of Eqn 3 (i.e., HH is no more than an algebraic rearrangement of the Law of Mass Action). Therefore, it is not surprising that Eqn 3 also gives $n(\ce{A-}) = n(\ce{HA})$ (after plugging in $\textrm{p}K_a=\textrm{p}H$).
The point is that I do not want to then assume that equivalence of the equilibrium mole amounts implies the same in the initial mole amounts.
In other acid-base problems this may be avoided by including an additional mass balance, charge balance, and auto-ionization of water.  Here it appears to be insufficient.  I am wondering if this is because of missing data (e.g., volume) or is there something more profound going on?


texts: Why is N-acetyl alanine methyl amide considered a dipeptide?

texts:
I'm going through an Amber Molecular Dynamics tutorial here and am a bit confused about why in step 8, we create a sequence as:

foo = seq { ACE ALA NME }

and this is called a dipeptide. I think there's only one amino acid, alanine, isn't there?


texts:
Aryl diazonium salt on treatment with copper and the corresponding hydrohalic acid, gives the aryl halide. This reaction is called the Gatterman reaction.

What is the mechanism of this reaction? Is it similar to Sandmeyer reaction?


texts: Is there a relationship between acidity of the water solution and oxidation/reduction reactions?

texts: What I want to know is if there is a relationship between the acidity of the water solution and redox reactions and if there is a relationship what favors the oxidation and what favors the reduction? Or does it depends on the reaction?


texts: What's the difference between a nucleophile and a base?

texts: Obviously it depends on the context whether you would call a particular species a nucleophile or a base but are the two terms largely synonymous or is there a difference?


texts:
It’s commonly reported that there are 9 kilocalories per gram of fat, 7 per gram of alcohol, and 4 per gram of carbohydrates or protein. But these figures (with the exception of that for alcohol, which presumably means specifically ethyl alcohol since it is discussed in the context of a human diet) are approximations given to an assumed "dietary blend" of fats, carbohydrates, or proteins respectively.

My question is this: if you could obtain a pure sample of one specific molecular type of fat, carbohydrate, or protein, is there any molecule you could choose that would provide an energy density significantly greater than 9 kcal/gram?

(I’m asking because my friends and I over at Worldbuilding SE are trying to figure out whether you could make Lembas bread without magic. My answer ("No") has been accepted and so now I’ve got raging impostor syndrome and I’m just wondering if it happens to be factually correct.)


texts: How to quickly cool a test tube without cracking it?

texts: Is there a way to safely cool a heated test tube quickly without cracking it due to sudden thermal stress in the glass? One thing that I tried was blowing on it, cooling it in slightly lukewarm water followed by cold water but this method kind of involves a "hope for the best factor". Is there a better technique?


texts: Why are chelating ligands often used to treat poisoning by heavy metals (e.g. mercury, lead)?

texts:
Why are chelating ligands such as EDTA often used to treat poisoning by heavy metals (e.g., mercury, lead)?

I have a vibe that it has something to do with the lone pairs of electrons that are able to bond to the heavy metals to form a stable coordination complex but I'm not sure about the in-depth process or explanation behind this.

Also, does it have to do with it being a polydentate ligand and not a monodentate ligand?


texts: In an emission spectrum, the limit of convergence at higher frequency corresponds to the first ionization energy

texts:
This is one of the syllabus points in the IB syllabus 12.1. We are advised to use the formula for the energy to remove one electron:

$$E = hν$$

which apparently is the same as

$$E_2 - E_1$$

where $E_2$ is the energy of the convergence limit and $E_1$ is the energy of the energy level where you remove the electron from.

However, isn't $ν$ the frequency of the emitted radiation when the electron falls from $E_2$ to $E_1$ (so with energy same as the energy difference between $E_2$ and $E_1),$ and not the frequency with the energy $E_2,$ i.e. the convergence limit?

I think I've misunderstood something and I'd appreciate if someone could explain this to me. Please use simple language because I'm only in high school.


texts:
While this question comes close, it doesn't quite answer my question.
Adding heat to oxygen & a hydrocarbon breaks the atoms apart and they reform into carbon dioxide and water. I get that.
I want to know specifically at what point the water and carbon dioxide are formed?
As I understand it the heat vibrates the atoms, breaking bonds.  Those atoms reform with oxygen forming stronger molecules ($\ce{H2O}$ and $\ce{CO2}$) which can withstand the combustion heat.  I reached this far with help from a Reddit thread.
Where that thread seemed to disagree is in my main question, on where/when the formation actually happens, it also raised another question of how long atoms are broken apart for.
In this article, it would suggest that free atoms are created which find oxygen:

$\ce{CH4 + 2O2 → CO2 + 2H2O + energy}$
[...] the arrow represents the moment the reaction happens. But there are a lot of interesting things happening that are hidden behind that arrow. A more detailed equation would look something like this:
$\ce{CH4 + 2O2 + a little energy →C + 4H + 4O →CO2 + 2H2O + lots of energy}$
The first arrow represents the breaking of the bonds, which requires energy. On the middle line are the atoms, now broken out of molecules and free to react. [...]

Which at least one of the Reddit thread contributors disagreed with.
So! (TL;DR)

When are the new molecules formed (cylindar, afterwards or both)?
How long are atoms "free", how long do they take to form new molecules, is it instantaneous or ... what?


texts: Conductometric titration curve when KCl is gradually added to AgCl solution

texts:
The situation given is that we are performing conductometric titration where KCl is gradually added to AgCl solution and we have to plot the variation of conductivity with the volume of aq. KCl added. I am new to conductometric titration and I have covered the cases of titration of acid v/s bases but I have never come across a case like this.

My thought was that since AgCl is a weak electrolyte it's initial conductivity would be low. And now we add KCl to it which is a strong electrolyte so the conductivity should increase. I also think that common ion effect would come into play and the dissociation of AgCl would be pushed back further but I'm still unsure about how an end point would be obtained here. Can someone guide me? Any help would be appreciated.


texts: Excited states using ORMAS flag in GAMESS

texts:
I am trying to calculate the excited states using single reference CISD on $\ce{CH3OH},$ using ORMAS' CI-X option in GAMESS. As a test example I am using the $\ce{CH3OH}$ that has 2 core, 7 occ. valence, 39 empty MOs (50 AOs, 48 MOs). I am unable to understand the $ormas flag in the following ormas-meoh-cisd.inp file:
!   single reference CISD on CH3OH, using ORMAS' CI-X option.
!          counting: 2 core, 7 occ. valence, 39 empty MOs (50 AOs, 48 MOs)
!          the resulting CI-SD has 53,964 Sz=0 A' determinants
!
 $contrl scftyp=rhf cityp=ormas runtyp=energy ispher=1 $end
 $system mwords=50 $end
 $basis  gbasis=ccd $end
 $guess  guess=huckel $end
 $scf    dirscf=.true. $end
 $trans  dirtrf=.true. $end
 $cidet  group=Cs stsym=Ap ncore=2 nact=46 nels=14 nstate=3 $end
 $ormas  nspace=2 mstart(1)=3,10 mine(1)=12,0
                                 maxe(1)=14,2 $end
 $data
Methanol...CISD...RHF/6-31G(d) geometry
Cs

H     1.0  -1.0616171503   0.8036449245   0.0000000000
O     8.0  -0.6870131482  -0.0653470836   0.0000000000
C     6.0   0.7093551399   0.0291827007   0.0000000000
H     1.0   1.0836641283   0.5408321444   0.8835398105
H     1.0   1.0975386849  -0.9797829903   0.0000000000
 $end

The following keywords are used in the $ORMAS section:
nspace=2
mstart(1)=3,10
mine(1)=12,0
maxe(1)=14,2

I can understand the nspace = 2 means the active space is partitioned into doubly occupied + active and valence orbitals. However, I don't understand how the numbers pertinent to the rest of the flags in $ORMAS are chosen. Any explanation would be really helpful and appreciated.


texts: What's a good way to find out how much lead is in parts being used in toys?

texts: We are using parts from China in small-scale manufacturing of toys.  We used an abotec "lead inspector" test kit on the same part from 3 sources, using abotec's leach test, in which we soaked each part in white vinegar for more than 4 hours, and then added a few ml of the vinegar used to a drop of reagent on a swab.  The swab turned black each time, indicating that the part had at least 50 ppm of lead.  The vinegar in each test tube became a little cloudy, but none turned dark.  Any thoughts about how much lead, and whether that might be unsafe?  PBGC's standard is 100 ppm.


texts: Calculating the number of energy states in a particular momentum direction

texts:
Given is a 2D scenario of a particle within a container:

The circumference shows all the possible 2D spatial directions for a given momentum value of the particle. One of those momentum directions is $p_e$.

From what I understand, the number of states within that $p_e$ is given by:
$$n_e = \sqrt{\frac{L_x \cdot p_{e,x}}{h}^2 + \frac{L_y \cdot p_{e,y,}}{h}^2}$$
Where $p_{e,x}$ and $p_{e,y}$ are the projections of $p_e$ in the $x$ and $y$ coordinates.

Since $p_e$ has a spatial direction, this means that the container has a length in that same spatial direction. Let's call that container length in that same spatial direction $L_e$, illustrated as follows:

Does this $L_e$ have any relationship with the number of states in momentum $p_e$? For example, is the following equation valid?
$$n_e = \sqrt{\frac{L_x \cdot p_{e,x}}{h}^2 + \frac{L_y \cdot p_{e,y,}}{h}^2} = \frac{L_e p_e}{h}$$

EDIT: I forgot to state to use periodic boundary condition in such a case, since rigid boundaries gives zero momentum to particles. So my question is: is it possible to calculate the number of states in a single momentum that has just one direction in p-space? If so, how is this derived from the formula:
$$n=\frac{V \cdot p^3}{h^3}$$


texts: Making 1,3,5-Tribromobenzene from Phenol

texts:
The problem was to make 1,3,5-Tribromobenzene from Phenol.

I thought to do it the following way:

React phenol with $\ce{Br2/H2O}$.
Then add $\ce{Zn}$ dust to get the product.

Now I am in doubt whether the second step would even work.

Are there any better method of doing this ?


texts: Could the difference between absorption time of Diclofenac Sodium and Diclofenac Potassium be related to the cation?

texts:
Diclofenac is a common non-steroidal anti-inflammatory drugs (NSAID) that comes in a variety of formulations. Two of the most common forms are diclofenac sodium and diclofenac potassium (the links refer to the FDA prescribing information of both forms). I have heard several times that the Potassium salt starts to act faster than the sodium salt.

In the second page of both leaflets above there is a table with pharmacokinetic parameters:

For diclofenac potassium, Tmax (the time it takes to reach maximum concentration after one dose) is 1.0 hour, and the half-life (the time it takes the drug concentration to go down by 50%) is 1.9 hours.
For diclofenac sodium, Tmax is 2.3 hours, and the half-life (the time it takes the drug concentration to go down by 50%) is 2.3 hours too. This means that for the sodium salt is being "processed" (absorbed, metabolized, eliminated) by the body more slowly than the potassium salt.

One more important point of difference to note is that the sodium salt comes in enteric-coated tablets, while the potassium salt comes in immediate-release tablets.

My question is: Can the difference in "processing time" of both salts by the body stem from the different salts, or is it due to the pharmaceutical formulation (EC tablets compared to IR tablets)?

It might seem at first glance that the difference in formulation should be the answer, but if it is so, why is the cation different between formulations?

I hope anyone can shed light on the mechanism, I was unable to find any useful information that actually addressed this issue.


texts:
How do you separate metal alloys into their constituent elements, especially industrially? I don't think you can always use melting points, because alloys, depending on the composition (looking at phase curves) can have one common melting point.

Here's a phase diagram for a copper-nickel alloy, which I think is a system where both components are completely soluble in one another in all ratios.

This composition cannot be separated by cooling it down, as I understand it (even though, as mentioned here, the composition would not be uniform if the molten alloy is not cooled down slowly enough). As described here, the solid that would start to separate (the alpha phase) is itself an alloy.

EDIT: What about electrolysis? Could that work here?


texts: What causes hydrogen abstraction in the radical chain mechanism of alkane halogenation?

texts: In the first propagation step of the radical chain mechanism of alkane halogenation, why does the halogen actually abstract hydrogen from the alkane? My organic chemistry textbook so far has went into details on MOs, hybridized orbitals, and general thermodynamics of reactions but has been lacking in the by and large why of reactions. Is the $\ce{C-H}$ bond broken by sufficient collisions from halogen free atoms and the hydrogen goes to the halogen as a result of $\ce{F}$/$\ce{Cl}$/$\ce{Br}$ being more electronegative than carbon, or what should cause this to happen?


texts: Which one of the compounds listed would we use in order to get the highest concentration of sulphate ion in a saturated solution?

texts:
Solutions are prepared by dissolving a sulphate salt in water. Which one of the compounds listed would we use in order to get the highest concentration of sulphate ion, SO42-, in a saturated solution? What would be the concentration of sulphate ion in this case?
A. $\ce{CaSO4}$    $K_{sp}$ = 7.1E-5
B. $\ce{SrSO4}$    $K_{sp}$ = 3.4E-7
C. $\ce{BaSO4}$    $K_{sp}$ = 1.1E-10
D. $\ce{RaSO4}$    $K_{sp}$ = 4.0E-11

My first instinct was to simply look at the $K_{sp}$ values.. Since $\ce{CaSO4}$ has the largest $K_{sp}$ value, it should be the most soluble, right?
But then I looked at a data page referring to the solubility of common compounds in water (Please see image. According to the table, $\ce{SO4^2-}$ has low solubility with $Ca$, $Sr$, and $Ba$.
My question is- does the solubility depend on the $K_{sp}$ value (was my first instinct right), or is the table correct?
Any ideas would  be appreciated.


texts: Catalytic energy profiles (is this Wikipedia image misleading?)

texts:
I am looking at the image below from the Wikipedia page for energy profiles in chemistry. I do not understand why the "$E_a$ (with catalyst)" is labeled as it is. I would argue that is not the activation barrier with the catalyst. Shouldn't all of the individual steps need to be considered to get the rate of production of product (or, if one step has an especially high barrier, then that step can be assumed to be rate-limiting and that step's $E_a$ is the barrier with the catalyst)? I suppose in other words, if I do $r = A\exp(-E_a/k_{B}T)$ where $E_{a}$ is the activation energy with catalyst, that will not get me the rate, right?

For the record, I have taken grad-level courses in chemistry, but I got myself into a position of questioning everything I once learned and here I am.


texts: Why is conjugation not possible if there is not present alternate $\pi$ molecules according to MO theory?

texts:
I was reading this page at Chemwiki; there they were discussing conjugated-$\pi$ bond.

In order to show that conjugation requires  alternate double bonding, they cited the example of 1,3,6-heptatriene:

  To be considered conjugated, two or more π bonds must be separated by only one single bond – in other words, there cannot be an intervening sp$^3$-hybridized carbon, because this would break up the overlapping system of parallel 2p$_z$ orbitals. In the compound below, for example,  the C1-C2 and C3-C4 double bonds are conjugated, while the C6-C7 double bond is considered to be isolated, due to the effect of the sp$^3$-hybridized C5.

I've a problem with their explanation; firstly are they following MO theory or VB theory? While describing molecules with MO, they, suddenly from nowhere, used the concept of hybridisation. Why?

Can anyone explain only with he help of MO theory why, there needs to be alternate $\pi$ bonding in the molecule for conjugation to happen without use of hybeidisation?


texts: What is Y-aromaticity? Is the trinitromethanide anion aromatic?

texts:
The wikipedia article on trinitromethane claims that:

  There is some evidence that the anion (which obeys the $4n+2$ Hückel rule) is aromatic.

Unfortunately the citation is behind a paywall so I can't read it but my first obvious question is how can the trinitromethane anion obey Hückel's rule? Two of the requirements for the rule are that the molecule must be planar (or nearly planar) and must be cyclic and I fail to see how either of these conditions are satisfied since the anion is roughly trigonal pyramidal around the carbon and definitely not cyclic.

This appears to about Y-aromaticity. All the papers linked are behind paywalls for me so can someone please explain what Y-aromaticity is and how it applies to the example given. I found this previous answer which gives some discussion of Y-aromaticity in nitrate ions. If someone could provide a similar MO analysis of the trinitromethane anion this might be very interesting.


texts: Why is o-nitroaniline not more basic than aniline?

texts:
We know from SIR effect (Steric Inhibition of Resonance) that in o-nitroaniline, the nitrogen's p-orbitals will change their plane, causing the conjugation/resonance of its lone pairs to stop. So, the only effect acting on it now is −I effect from the $\ce{NO2}$ group.

Comparing with just aniline, the $\ce{NH2}$ lone pairs are in constant resonance with the benzene ring making them unavailable for donation. Thus, aniline should be less basic than o-nitroaniline. But it isn't as per experimental values. So what's going on? What's the logic behind this?


texts: Order of reaction greater than molecularity?

texts:
We were taught that the order of a reaction is either less than or equal to it's molecularity. But then I came across a mechanism called E1CB, where the fact was totally reversed. Here molecularity is Unimolecular while the order of reaction is 2.
So, is it any kind of exception or am I missing something?


texts:
The catalytic efficiency of an enzyme is given by $k_{cat}/k_M$ where $k_{cat}$ is the turnover number, or the number of molecules that can be produced per second per active site of an enzyme.
$K_{M}$ is a measure of the affinity of the enzyme with the substrate, or the likelihood of binding.
Why bother dividing the $k_{cat}$ by $K_{M}$? Isn't the affinity of the enzyme already encoded into the quantity of $k_{cat}$? How could you be an enzyme that has low affinity, but still have a huge turnover? To me this doesn't seem possible, and thus it is redundant to divide by $K_{M}$. Likewise, could there be a situation where $k_{cat}$ is low, but $K_{M}$ is high?


texts:
My understanding was that cross-linking would generally make a material harder and less flexible, but if latex does have cross-linking, how would this affect its shape and hence its properties, especially as a material with elasticity?

In addition, is there a method of testing if a substance has crosslinks?

I have googled to no avail.


texts: Memorize Transition Metal Electronic Ground State

texts:
In the course of my undergraduate studies it has become clearer and clearer that it is quite helpful to know the ground electronic state of transition metals by heart, if only to speed up the process of analysing a reaction or to evaluate a compound in respect to its number of valence electrons.

Edit: Note that when I say ground electronic state, I don't mean the state in vacuum, but when the metal is bonding to ligands. As a consequence all the electrons are found in the $d$ orbitals.

My Method:
I have now put the $d$-Block of the periodic table as a wallpaper on my screen background so I get constantly exposed to the arrangement of the elements.

What is your way to memorize what metals have which $d^x$ ground state?


texts: New relationship between exponential factors and entropy of a reaction?

texts:
Consider the reaction $\ce{A <=> B}$. Let $k_f$, $E_A$, and $A$ denote the forwards rate constant, activation energy, and pre exponential factor, respectively. Let $k_r$, $E_A'$, and $A'$ denote the backwards rate constant, activation energy, and pre exponential factor, respectively. I seem to have determined the following relationship:

$\ln{\frac{A}{A'}} = \frac{\Delta S}{R}$

First, is this a correct equation? Second, is this a known equation? I have not seen it in any of the textbooks that I have read. Third, is this a useful equation? I imagine it could be used to determine the ratio between the pre exponential factors from the entropy.

My derivation is below:
The equilibrium constant $K$ is equal the ratio of the forward and backwards rate constants $\frac{k_f}{k_r}$, so $\ln{(K)} = \ln{\left(\frac{k_f}{k_r}\right)}$. Furthermore, the Arrhenius equation expresses forward and backwards rate constants as $k_f = Ae^{-E_a/RT}$ and $k_r = A'e^{-E_a'/RT}$, where $A$ is the pre-exponential factor of the forward reaction and $A'$ is the pre-exponential factor of the reverse reaction. Substituting these expressions yields $\ln{(K)} = \ln{\left(\frac{Ae^{-E_a/RT}}{A'e^{-E_a'/RT}}\right)} = \ln{\frac{A}{A'}}-\frac{(E_a-E_a')}{RT}$.
In Chapter 5 of Atkins' Chemical Principles 7th edition, the difference in activation energies of the forwards and backwards reactions $E_a-E_a'$ is equated to the reaction enthalpy $\Delta H$. Applying this relationship, we obtain: $\ln{(K)} = \ln{\frac{A}{A'}}-\frac{\Delta H}{RT}$.
It is also known, that since $\Delta G = -RT\ln{K} = \Delta H - T\Delta S$, $\ln{(K)} = -\frac{\Delta H}{RT} + \frac{\Delta S}{R}$.
Combining these two equations yields: $\frac{\Delta S}{R} = \ln{\frac{A}{A'}}$.


texts: Why is "Spontaneous" a part of 2nd law wording?

texts:
The way I understand the law, any process is implausible if the total entropy of the universe does not increase. Why is "Spontaneous" even a part of the wording, what does it mean?

Is it perhaps a part of the probabilistic theory of thermodynamics, in which it is not "impossible" for heat to flow from cold to hot, simply very very unlikely?


texts: Chemical equilibrium — why multiplication, rather than addition?

texts:
Why do we multiply (and not take the sum) the concentration of products and reactants when more than one entity is present in either side and rise the power to the coefficient term?

Suppose either

$$\ce{aA + bB <=> cC +dD}$$

described by

$$K_{\mathrm{eq}} = \frac{[\ce{C}]^c \cdot [\ce{D}]^d} {[\ce{A}]^a \cdot [\ce{B}]^b}$$

where we multiply the concentrations and rise the power to the coefficient. Or the dissociation of

$$\ce{A_aB_b <=> aA+ + bB-}$$

described by
$$K_{\mathrm{sp}} = [\ce{A}]^a \cdot [\ce{B}]^b$$

again based on a product, and not the sum.

This seeks a clarification of math based on chemistry. How to explain it to an 8th grader?


texts: Double bonds in polyunsaturated fatty acids

texts:
$\omega - 3$ fatty acids are very frequently discussed.  $\omega - 6$ is also mentioned.  However, I don't recall mention of $\omega - 1$, $\omega - 2$, or $\omega - 4$.  Also, in polyunsaturated fatty acids, the double bonds are frequently all multiples of $3$ from the $\omega$ end.

Is it just some quirk of evolution that has led to this pattern or is there a chemical explanation?  For example, are double bonds at these positions more favourable than other positions?  If an $\omega - 2$ or $\omega - 4$ fatty acid was synthesised might it isomerise to $\omega - 3$?

In the cases with multiple double bonds 3 apart, is it really: pure single, pure single, pure double repeating?  Or is there some degree of delocalisation and the "single" bonds are actually a bit more than single and the "double" bond a bit less than double?  I guess that it does not go to the extreme of benzene in which all the bonds are equal or we would have developed more accurate names and notations to represent this.


texts: Why is acetylsalicylic acid still more acid than benzoic acid?

texts:
In this question, it is explained why salicylic acid is a stronger acid than benzoic acid. So, why acetylsalicylic acid (aspirin), whose conjugate base cannot hydrogen bond since there is no hydrogen, is still more acid than benzoic acid?

aspirin: $\mathrm{p}K_\text{a}$ = 3.5
benzoic acid: $\mathrm{p}K_\text{a}$ = 4.2

Acetoxyl is a moderate activating group for the aromatic electrophilic substitution, so it should mean that the resonance-driven electron-donating effect prevails over the inductive electron-withdrawing effect, therefore it should raise the $\mathrm{p}K_\text{a}$ of the acid.

Is it different between electrophilic substitution reactivity and acidity? Does the inductive effect prevail in this case?


texts: How to calculate the concentration based on dilution of a stock solution?

texts:
One of my exam questions is about lipstick and calculating the amount of lithium in the lipstick by AAS. The determination took place by making standard solutions with known concentrations of lithium. For the stock solution $653.3~\mathrm{mg}$ lithium chloride, $\ce{LiCl}$, was dissolved in $100~\mathrm{ml}$.

$M(\ce{Li}) = 6.49~\mathrm{g/mol}$
$M(\ce{LiCl}) = 42.4~\mathrm{g/mol}$

From the stock solution the following volumes were pipetted into $100~\mathrm{ml}$ flasks: $3$, $5$, $8$ and $10~\mathrm{ml}$. Also $1~\mathrm{ml}$ $\ce{KCl}$ solution, $1.0~\mathrm{M}$, was added to prevent ionisation.

I calculated the concentration Lithium in the stock as follows:

The mass of Lithium in the weighed $\ce{LiCl}$ is equal to the molar mass of lithium divided by the molar mass of lithium chloride multiplied by the mass that was weighed. So in the form of an equation:

$$m(Li)=\frac{M(\ce{Li})}{M(\ce{LiCl})}\cdot m(\text{total})=99.9~\mathrm{mg}$$

Therefore the concentration of the stock solution is equal to the mass of lithium divided by the volume of the flask:

$$C(\text{Li, stock}) = \frac{m(\ce{Li})}{V(\ce{Li})}=\frac{99.9}{0.1}=999~\mathrm{mg/l}$$

The concentration of the standard solutions therefore would be:

$$C(\text{standard}) = C(\text{stock}) \times \frac{V(\text{stock})}{V(\text{flask volume})}$$

With the concentration of the standard in $\mathrm{mg/L}$ and the volumes of both stock and flash volume in $\mathrm{mL}$.

This would give concentrations of $29.9$; $49.9$; $79.9$ and $99.9~\mathrm{mg/L}$ to the standard solutions where $3$, $5$, $8$ and $10~\mathrm{ml}$ were pipetted into. The answer sheet however provides us with $300$, $500$, $800$ and $1000~\mathrm{\mu g/L}$. I suspect I've made a mistake in unit conversion somewhere, due to the factor 100 mistake in the answer, although the units don't correspond either.


texts: Does acid anhydride give a positive iodoform test?

texts:
I thought that acetic anhydride $\ce{CH3COOCOCH3}$  would give a positive Iodoform test because it has a $\ce{CH3-(CO)}-$ group.But it is given that this does not happen.

Is this true? If so, can anybody explain the reason? Thanks


texts: Is this a counterexample to ΔU = 0 J for isothermal processes?

texts:
In class, I have learned that for isothermal processes the following is true at all times.

$$\Delta U=q+w=0\text{ J}$$

But suppose I lift an object against the force of gravity. Then I (the surroundings) have done work on the object (the system) without exchanging heat (neglecting friction).

Now, I claim that because some nonzero work has been done on the system, and no heat has been exchanged, the change in internal energy is nonzero:

$$\Delta U=w\neq0\text{ J}$$

Are my assumptions about internal energy incorrect or is there something totally wrong about my approach?


texts:
If we value a 20 mL solution of 0.25 M acetic acid with a 0.1M solution of NaOH

The end point of the titration is reached when 50 mL of NaOH is added from the burette. At this point, the pH of the solution is 8.81

Since the pH at the end point is 8.81, phenolphthalein, whose pKa is 9.7, has been used as an indicator.

I need to calculate the error associated with using this indicator, because as the turn does not exactly coincide with the pH at the end point, there must be some error.


texts: In the DNA backbone, which bonds exactly are considered ester bonds?

texts:
Following are the three different answers to this question that I reached by googling. Which of them is right?

The covalent bond between the 3' C and the O, and the covalent bond between the 5' C and the O:

2. The covalent bond between the P and the O that is bonded to the 5' C, and the covalent bond between the P and the O that is bonded to the 3' C (deduced from the following image):

3. The -O- (two covalent bonds with an O) that link between the P and the 5' C, and the -O- that link between the P and the 3' C (deduced from the following image):

Maybe my confusion stems from a more general misunderstanding:
When two alcohols are linked through an ester linkage (i.e., both OH groups are replaced with an -O-), we get R1-O-R2. In this case, do we say that each of the covalent bonds with the O is an ester bond? Or is the whole -O- considered a single ester bond?

(Note that I am not asking what exactly a phosphodiester bond is, which is (if I understand correctly) everything that connects the 5' C and the 3' C of adjacent nucleotides.)


texts: Is this a spontaneous or a non-spontaneous process?

texts:
An insulated tank initially contains a liquid-vapor mixture of water. Then, an electric heater is turned on and kept inside the tank until all the liquid becomes saturated vapor.

According to the 2nd Law of Thermodynamics, a spontaneous process occurs when the change in entropy of the universe is greater than zero.

In this case, since the tank is insulated, $Q=0$ and the change in entropy of the surroundings is also equal to $0$; hence, $\Delta S_\mathrm{universe}$ = $\Delta S_\mathrm{system}$. Since adding work to the system (via the electric heater) results in an increase of the thermal energy of water and its evaporation, $\Delta S_\mathrm{system}>0$. Therefore, $\Delta S_\mathrm{universe} > 0$, and the process is spontaneous.

But wouldn't the necessity of adding work to the system (in order for the water to evaporate) mean the process is non-spontaneous?


texts: Is a SR2+ substituent really more electron-withdrawing than NH3+?

texts:
As the electronegativity of nitrogen and sulfur are around 3.04 and 2.58, respectively (according to the Pauling scale). So according to that, $\ce{NH3+}$ should pull the electrons more towards it than what $\ce{SR2+}$ should. This argument has been put forth by Jan in his answer to a similar question here.

However, my textbook (Organic Chemistry by Subrata Sengupta) states that the exact opposite is true. Who is correct here?


texts: Does the slippery feeling of bases always appear at certain pH?

texts:
My guess is that the saponification reaction is only dependent on the concentration of $OH^{-1}$ ions and of course the coposition and concentration of fats. I also guess that the fat part of the situation is rather constant. My guess means that the cation would have no or negligible impact.

This would suggest that the degree of slipperyness would be only pH dependant (and not dependent on the cation). Does this mean that there is a certain pH level where the solution starts to feel slippery? If that's the case about which pH level is it?


texts:
I have a problem with making the reaktion mechanism Hydrolysis of cis-1-bromo-4-methylcyclohexane.
I know that a general hydrolysis looks like but i am not sure how he endproducts of this reaction are supposed to look like, where does the H and OH bind and what are the products called?