texts: In the reaction of tri-iodination of 2-methyl propane only five isomers can be formed, from which only the branch methyl can be iodinated if both the lateral carbond are iodinated .. Is this because of the inductive effect or the steric effect is the responsible here...if it is the steric effect which hinder the iodination of the branch carbon (2-methyl) why is it possible to tri-iodinate the lateral carbons( the primary carbons other than the branch one)  ?


texts:
I am studying binding energy in the context of photoelectric spectroscopy, and I came across a graph of relative numbers of electrons vs binding energy (in units of megajoules per mole). The element in question is lithium. My chemistry resource (Khan academy, to be precise) noted that the binding energy for the furthest right peak corresponds to the binding energy for the electron in the 2s subshell, and that this is also equal to the first ionization energy. However, they go on to say that the the binding energy of the second peak is not equal to the second ionization energy, since the electrons are held more tightly by the nucleus once the outermost electron is removed. I actually have two questions about this. First, why are they more tightly held once the outermost electron is removed? Is the electric field of the protons somehow getting weakened by the presence of the outer electron? I don't think this would be the case, based on my understanding of electric fields. So what is the explanation?
My other question is why the binding energy is different from the ionization energy? Is it because the electrons are removed all at once versus them being removed one-by-one? Or is there some nuance to this situation I am not seeing?
I found a question on this website that seems to be asking a similar thing, but no one answered it. Here's the link to it anyway.
Chemistry: What's the difference between "binding energy" and "ionization energy"?
Thanks in advance for the help.


texts:
Soil based question. I have soil with to much Ammonium Nitrate in it. I am wondering if I use citric acid and water mixed will that flush out the extra Ammonium Nitrate that has built up in the soil.

Will citric acid bond to Ammonium Nitrate?

Any incite on these to compounds will be greatly appreciated.


texts:
How does Lewis acidity correlate with bactericidal activity? I read on Wikipedia that the two are positively correlated but no explanation was given.

Does it have to do with the movement of electrons - i.e. a current? I'm guessing that having a current passed through you isn't good for you.

I suppose also that the interactions (i.e. coordination bonds) of metal (ions) with bacterial components can interfere with cellular processes.


texts: How to know when dissolution is complete?

texts: In a given solubility equilibrium, I know that if $ Q >K_s$, then the salt will precipitate until $ Q =K_s$. If $ Q <K_s$, however, would the precipitate dissolve until $ Q =K_s$ too?


texts: I was wondering, how are materials like uranium, plutonium and other radioctive materials made. Do they all come from the old exploded stars and just been in the mountains for millions of years. Is there getting made more of those materials, or are we gonna run out in the future?


texts: What total energy (in kJ) is contained in 1.0 mol of photons, all with a frequency of 2.75 • 10¹⁴ Hz?

texts:
There is a question, that says:

  What total energy (in $\mathrm{kJ}$) is contained in $1.0~\mathrm{mol}$ of photons, all with a frequency of $2.75 \cdot 10^{14}~\mathrm{Hz}$?

My answer was:
The energy of photon in hydrogen atom is given by the formula $E=h\nu$, where $h$ is Planck constant and $\nu$ is the frequency. After that I got an answer different from the correct answer which is $110~\mathrm{kJ}$.

Could anyone explain why?


texts: Why aren't Boron and Aluminium assigned to group 3 of periodic table? What determines the group?

texts:
I've been curious about this 3D representation of the periodic table "Mendeleev's Flower" and was trying to study it, wondering if it reveals any regularities that are not obvious from classic periodic table.

But since it's hard to look at a 3D table on a flat computer screen, I found flat equivalents.

Theodor Benfey's periodic table (1964):

J. F. Hyde's periodic table (1975):

There I found a discrepancy: Hyde assigned Boron and Aluminium to group 3 (IIIA), whereas everywhere else B and Al are in the group 13 (IIIB).

I can totally see why Hyde split the table like this (apart from being obsessed with Silicon): such a split creates an absolutely stunning symmetry.

You can see this symmetry on the Hyde's table with colors and dashed lines that cross Silicon and connect related elements.

This can't be just a stretch/exaggeration/far-fetching/"squeezing the universe to fit into an appealing model, even though the universe resists". It's just too good!

Well, to be honest, he did do some universe-squeezing: B and Al have electron configuration of s²p¹ — just like other elements of group 13 (IIIB), whereas elements of group 3 (IIIA) have electron configuration of d¹s². Since Al and B appear in the d¹s² column, Hyde had to make a note inside B and Al cells that they actually are s²p¹.

But the symmetry is just too good. It reveals stunning periodicity that no other periodic table is capable of.

And then I remembered this short-form periodic table:

Russian Wikipedia says the short form was banned by IUPAC in 1989 for some reason, but it's still used in Russian schools because it makes figuring out valence values easier.

And this table does not have the split. Boron and Aluminium appear in the same column regardless of whether you assign them to 3 (IIIA) or 13 (IIIB). I think this is absolutely beautiful.

I believe this Al/B positioning thing has scientific significance, but I don't understand the meaning of the groups.

My questions:

What does the group actually mean? Is it simply the electron configuration and nothing else?
What determines Al and B being placed in group 3 (IIIA) and not 13 (IIIB), other than the electron configuration?
Why is the short-form periodic table considered obsolete, even though it clearly has something going on that the full-form table is not revealing?
Is the Hyde's table missing any periodical properties that the classic table has?


texts: Why do the boiling and melting points decrease as you go down group 1 and vice versa for group 7?

texts:
I used to think that because an alkali metal needs to lose one electron to complete its outer shell, when the atom increases in size (atomic radius), the electron would be easier to lose as the attraction between it and the nucleus would be lower, resulting in increased reactivity as you go down the group (the opposite being true for halogens as they need one more electron and so need to retain their electrons).

Now the trouble I am having is that why would the melting point also increase, what does the ability to lose an electron have to do with the intermolecular forces between several atoms?


texts: How to compare the reactivity of sec-butyl iodide and n-butyl bromide for SN2?

texts:
Steric crowding is greater for the secondary halide, and this is important as the neucleophile will attack the back lobe of carbon atom. But the relative ability to leave is more for iodide than bromide.

So how can I compare the $\mathrm{S_N2}$ rates for sec-butyl iodide (2-iodobutane) and butyl bromide (1-bromobutane)?


texts: Does a C=C double bond donate electron or withdraw it?

texts:
I have learned that a $\ce{C=C}$ double bond is electron-rich because of the p-p overlap and hence can easily donate an electron pair: I agree with this statement.

However, in a $\ce{C=C}$ double bond, the carbon is $\ce{sp^2}$ hybridized and I've also learned that more '$\ce{s}$-type' character increases electronegativity. Does this mean that the carbon atom - being more electronegative - tries to withdraw electrons?

For example consider the double bond connected to a positively charged Carbon, then will it donate an electron and stabilize it? or will it (being more electronegative) withdraw an electron?


texts: Steric Factors on reaction with grignard reagent

texts:
I was asked to find the products X and Y.
For the product X,Here the grignard reagent and even the carbonyl substrate was too sterically hindered to approach the carbonyl center so no reaction would take place, thus the final product will be

I thought the same for Y product .. Here the Carbonyl compound was crowded to approach so no reaction would take place but then I found the answer to be this.

So for the product Y hydrolysis occured due to presence of dilute HCl but it did not occur for the first one. But why? And was I right about the logic above?


texts: What is the temperature at which lava evaporates?

texts:
According to this article ->  http://ntrs.nasa.gov/search.jsp?R=19660041998

Lava evaporates under natural conditions, but apparently, only a little bit.

I'd like to know the temperature at which lava could fully, or mostly, evaporate at 1 atm.

I know this is extremely difficult, if not impossible, to be accomplished under standard pressure with realistic tools, but I would still like to know, if you please.


texts: Balancing redox reaction of zinc and hot diluted nitric acid

texts:
I need to balance the following reaction:
$$\ce{Zn + HNO3 -> Zn(NO3)2 + NH4NO3 + H2O}$$
I assigned all oxidation numbers:
$$\ce{\overset{0}{Zn} + \overset{+1}{H}\overset{+5}{N}\overset{-2}{O_3}\longrightarrow \overset{2+}{Zn}(\overset{+5}{N}\overset{-2}{O_3})2 +\overset{-3}{N}\overset{+1}{H_4}\overset{+5}{N}\overset{-2}{O_3} +\overset{+1}{H2}\overset{-2}{O}},$$
but I'm having troubles finding the half reactions. I know
$$\ce{Zn -> Zn^{2+} + 2 e-}$$
is one half reaction, but what about the other? Since ammonium nitrate has nitrogen atoms in two different oxidation states, what do I do with them? Do I add them to see if the nitrogen is or it is not balanced?


texts: Does glucose react with Brady's reagent?

texts:
A couple of reactions of glucose have been a source of great confusion to me. One of them is it's reaction with the Brady's reagent. It's pretty debatable whether the result is positive or negative. Could someone please shed some conclusive light onto this?

Furthermore, Glucose itself successfully produces an oxime with hydroxylamine. In contrast, it's pentaacetate fails to do so. Why does this occur? Why can't the pentaacetate, like glucose, "break out" of it's cyclic form to give a positive hydroxylamine test?

Edit:
I found out about the oxime test, I had overlooked the fact that glucose pentaacetate doesn't have any hemiacetal. The Brady's test though, is still open for debate.


texts: Differentiating esters from glycosidic linkages

texts: What exactly is the difference between an ester linkage and a glycosidic linkage? I know both release a molecule of water, but what is the difference? Is it simply which types of monomers are involved in the bond?


texts: Why is only one of the lone pairs on the oxygen in furan delocalized?

texts:
Both the lone pairs on the oxygen are in allylic positions, thus both should be able to participate in resonance. If both lone pairs can participate in resonance, why is only one of the lone pairs delocalized?

Also, for a lone pair to participate resonance, does it have to be in an unhybridized p orbital? If so that would explain why only one of the lone pairs is delocalized as one of the lone pairs would be in an unhybridized p orbital to make it sp2 for oxygen.


texts:
The $K_\mathrm{p}$ value of a reaction is $\pu{10 atm}$ at $\pu{300K}$ and $\pu{4atm}$ at $\pu{400K}$. The incorrect statement about the reaction is-

  a. The reaction is exothermic

  b. The rate of forward direction is more than that of backward
      reaction.

  c. The rate of backward reaction increases more than that of forward
      reaction with increase of temperature

  d. The difference between heat of reaction at constant pressure and that at constant volume is $RT$.

The correct answer given is option b.

But then how can we say that the difference between the heat of reaction at constant pressure and that at constant volume is $RT$ for the above reaction?

How do we go about approaching this problem? Do we need to use the van't Hoff  equation? I tried using the ideal gas equation also but didn't get this $RT$. Also I think since change in enthalpy is negative so the reaction is exothermic.


texts: Why we can't see 3-d structures of certain compounds on PubChem?

texts:
It's common to read statements like one given below on PubChem:

  Conformer generation is disallowed since MMFF94s unsupported element.

What's the meaning of the statement, and why are certain compounds unsupported? And what is MMFF94s?
This might be something really basic, but since I have never come across I asked it.


texts: Can ethanol be both a Lewis base and a Lewis acid?

texts:
I'm asked to determine if ethanol is a Lewis acid, Lewis base, or both.

Since $\ce{O}$ has two lone pairs of electrons I would think that it could act as a base, since they could easily be donated. If it was a Lewis acid, it should have an empty orbital, and I can't find such one. All of the carbons are bonded and oxygen's orbitals are full. Still my book tells me it can act as both.

Is the textbook wrong or could one say that the 1s orbital to $\ce{H}$ (bonded to $\ce{O}$) acts as "empty", since $\ce{O}$ is more electro-negative than $\ce{H}$?


texts: Determination of metal state from protein X-ray structure

texts:
Many proteins or enzymes contain metal ions within their active sites.
These metals are often directly involved in the reaction catalysed bu the enzyme. In certain cases the function is well known and understood, including the identity of the metal and the changes in oxidation state throughout the reaction.
I wonder how the metal and oxidation state is determined a priori when a new structure is obtained.  For examples if I solved a structure of a previously unknown enzyme by crystallography I may be able to determine that it contains Zinc (iron a better example), but knowing the density map tells me nothing about the oxidation state which may be important to function.
I think it must be possible to determine this, however I do not find much reliable information (inorganic complexes typically have other techniques in addition to X-ray). The situation is made harder as many protin structures in the pdb appear to be assigned without any real study.


texts:
This is a 10th-grade question.

  The melting point of a substance decreases with increase in presence of impurities in it. The melting point of ice decreases from 0 °C to -22 °C on mixing salt in it in proper proportion. That is why salt is added to make freezing mixtures to keep ice creams frozen.

I want to ask that suppose ice melts at 0 °C and we are now adding salt to lower melting point but now when we have lowered the melting point the ice will melt more quickly, then what is the purpose of mixing salt?


texts: "Synthetic wood" from cellulose and lignin?

texts:
Is there any research into making wood from its constituents?

From what I know, wood is composed of cellulose, hemicellulose, and lignin. We can extract these from raw wood, but is there any way we can combine these to form a wood-like structure that can be made into furniture?


texts: How can I determine the quantum number of a electron lost during ion formation?

texts:
How do quantum numbers change for ions?

  A sample question: Give the set of four quantum numbers that could represent the electron lost to form the Rb ION from the Rb atom.

The answer given is $n=5; l=0; m_l=0; m_s= +\frac12$. I chose the answer: $n=4; l=1; m_l=1; m_s= -\frac12$.

I am looking for an explanation. I am confused because I thought the question asked me to describe $\ce{Rb+}$ and the answer given is the same that I would choose for $\ce{Rb}$ at ground state. If it is asking me to describe the electron, how is that technically possible if I don't know where the electron went? Sounds silly and I know I'm over thinking it. Some insight would be appreciated.


texts: Why benzene takes in more delocalized electrons from its substituents even though it has six of them

texts: Benzene has six delocalized electrons in its structure then where does it places the new electrons which it gets from other substituents like oh, etc. Why does it accepts those new ones?  Please give an elaborated answer.


texts: Sometimes I use dishwashing gloves when carving cooked duck, which is very fatty. I have noticed that over time my dishwashing gloves seem to be shrinking a little bit and getting stiff. Could this be because of the fat?


texts:
Consider for example ethanol being heated in the presence of copper at 300 °C. How does it give ethanal as a product?
Copper being in its elemental state (oxidation state 0). Since negative oxidation state is not possible for Cu, which means that it should reduce ethanol to  ethane. Certainly, high temperature should have some role here. What could it be?


texts:
Source: Carey Advanced Organic Chemistry B, problem 4.1 b

Looking for interesting problems about addition to C=C, I came across this in the back of Carey. It looks like a simple addition across the alkene, but using HOBr which is something I've never came across in the lab (and perhaps not seen used as an example since I was an undergrad...).

The most obvious way to dissociate HOBr is to lose a proton, leaving the OBr anion behind, Wikipedia confirms this:

  [...] hypobromous acid partially dissociates into the hypobromite anion OBr− and the cation H+.

So far so good. And based on this fact, you might expect the product to be as drawn below. However the original literature points towards a different product, also shown below.

Obviously, the product I've drawn as expected looks a little odd, with the unstable Br-O bond present. However, I cannot rationalise how the actual product has formed.

  Ref.: JOC, 1968, 33, 3953.
  The original literature from which the problem was taken

Three possibilities:

The HOBr doesn't dissociate as I'd imagined. There is the possibility of the Br forming the bromonium ion which is then opened by hydroxide, however to get the product as isolated, you have to attack in the anti-Markovnikov sense (i.e. the product would look like the one formed if you'd attacked a primary carbocation with hydroxide).
The product forms as I drew it, but rearranges to give the product they isolate. This is what I consider to be the most likely explanation, but I'm unable to see how this happens.
The secondary carbocation forms upon protonation, but this rearranges. Again, struggling to see how this would work as the secondary cation is the most stable one that can form in this system, as there is no possibility of a tertiary carbocation.

Any suggestions to point in the right direction? Even the name of a rearrangement would be helpful.


texts: to convert ppm gas (N2O) concentrations in microgram nitrogen per gram dry soil basis?

texts: What calculations are needed to convert  $\ce{N2O}$ gas concentrations in ppm to micrograms of nitrogen per gram dry soil basis? I measured the concentration of $\ce{N2O}$ in soil samples. I extracted a small gas sample from a container (the soil is kept in this container; the container has an head space of 80 cc). Then I injected the small gas sample in the GC and I had a measurement in ppm. I want to know how many grams of N are present in my ppm measurement, taking into account that the gas comes from 5 grams of soil.


texts: How does Mars rover Curiosity's ChemCam's infrared laser produce a 100,000 K plasma plume (kT = 8.6 eV)?

texts:
Mars rover Curiosity's ChemCam uses a pulsed diode-pumped solid-state laser (DPSS) at 1064 nm to excite material on the surfaces of rocks up to 7 meters away, and a large aperture telescope to collect emitted light for analysis.
From NASA JPL's MSL Science Corer page Chemistry & Camera (ChemCam):

LIBS Instrument
The LIBS instrument uses powerful laser pulses, focused on a small spot on target rock and soil samples within 7 m of the rover, to ablate atoms and ions in electronically excited states from which they decay, producing light-emitting plasma. The power density needed for LIBS is > 10 MW/mm^2, which is produced on a spot in the range of 0.3 to 0.6 mm diameter using focused, ~14 mJ laser pulses of 5 nanoseconds duration. The plasma light is collected by a 110 mm diameter telescope and focused onto the end of a fiber optic cable. The fiber carries the light to three dispersive spectrometers which record the spectra over a range of 240 - 850 nm at resolutions from 0.09 to 0.30 nm in 6144 channels. The spectra consist of emission lines of elements present in the samples. Typical rock and soil analyses yield detectable quantities of Na, Mg, Al, Si, Ca, K, Ti, Mn, Fe, H, C, O, Li, Sr, and Ba. Other elements often seen in soils and rocks on Earth include S, N, P, Be, Ni, Zr, Zn, Cu, Rb, and Cs. It is anticipated that 50-75 laser pulses will be required achieve the desired 10% accuracy for major elements at 7 m distance.

Wikipedia's Laser-induced breakdown spectroscopy says:

LIBS operates by focusing the laser onto a small area at the surface of the specimen; when the laser is discharged it ablates a very small amount of material, in the range of nanograms to picograms, which generates a plasma plume with temperatures in excess of 100,000 K. During data collection, typically after local thermodynamic equilibrium is established, plasma temperatures range from 5,000–20,000 K. At the high temperatures during the early plasma, the ablated material dissociates (breaks down) into excited ionic and atomic species. During this time, the plasma emits a continuum of radiation which does not contain any useful information about the species present, but within a very small timeframe the plasma expands at supersonic velocities and cools. At this point the characteristic atomic emission lines of the elements can be observed. The delay between the emission of continuum radiation and characteristic radiation is in the order of 10 μs, which is why it is necessary to temporally gate the detector.

Question: What is the process by which infrared 1064 nm (1.17 eV) photons can produce an ionized plasma that results in the emission of UV photons as short as 240 nm (5.17 eV)? how exactly does a beam of 1.17 eV photons initially produce a "plasma plume with temperatures in excess of 100,000 K" where $k_B T$ = 8.6 eV.

Images from NASA.


texts: Can an iodine clock reaction work without using a strong acid?

texts:
I'm going to do an iodine clock reaction for a project and we had to submit the materials and safety sheets for the experiment a while ago. I thought I could do one by using hydrogen peroxide, sodium thiosulfate, potassium iodide and starch, but online I'm only seeing experiments that use sulfuric acid or another strong acid of some sort which is worrying me. I talked to my teacher and it's too late to change/add materials, so I'm really hoping that it can work? I'd also like to know what the strong acid does in the reaction, and what would happen (or wouldn't happen) without sulfuric acid?

Here's a bit from wikipedia for reference about the reaction:

This reaction starts from a solution of hydrogen peroxide with sulfuric acid. To this is added a solution containing potassium iodide, sodium thiosulfate, and starch. There are two reactions occurring in the solution.

(note: I didn't try formatting anything so the numbers after the ^ are the charges)

In the first, slow reaction, iodine is produced:
H2O2  +  2I^−  +  2H^+  →  I2 + 2H2O

In the second, fast reaction, iodine is reconverted to 2 iodide ions by the thiosulfate:
2S2O3^2−  +  I2  →  S4O6^2−  +  2I^−

After some time the solution always changes color to a very dark blue, almost black.
When the solutions are mixed, the second reaction causes the triiodide ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the triiodide – starch complex appears.


texts: Deriving the volumetric thermal expansion coefficient from other properties?

texts:
I have a lot of gas properties in a model but I discovered that he volumetric thermal expansion coefficient is not among them. I've found little about how the constant relates to other properties - I only know of how it's used, and there's abundant tables of them.

Can the constant be derived from any combination of the following (some parameters vary with temperature, etc)?

Temperature
Pressure
Density
Average molecular weight
Specific heat
Entropy
Enthalpy
Internal energy
Sutherlands constant (viscosity)
Lambda constant (viscosity)
Static and kinematic viscosity
Speed of sound
Bulk modulus
For each molecular component:

Molar/volumetric percentage

Mass percentage

Molecular weight

Partial pressure

Density

I'd rather derive the thermal expansion coefficient from other existing values if possible.  If not, I guess a lookup table will be in order.


texts: Does the first step in electrophilic aromatic substitution follow equilibrium? If so, can equilibrium constant be calculated?

texts:
Using chlorine (a Lewis base) and ferric chloride (a Lewis base) as an example, the first step I saw in the textbook looks like this:

The picture above depicts the Step 1 in the EAS mechanism using chlorine as an example.

I noticed that this step has two reversible arrows. I think it means chlorine, ferric chloride, the molecular complex and the ion pair will all be present in a system and their concentration values follow chemical equilibrium.

From high school knowledge (I am a freshman), we can calculate equilibrium constant using concentration. Can we still do it in this case? If so, how?


texts: Hydrogen Bonding (with oxygen) - restricted to oxygen singly bonded to H or not?

texts:
I understand the following definition of a hydrogen bond from Merriam Webster:

  an electrostatic attraction between a hydrogen atom in one polar
  molecule (as of water) and a small electronegative atom (as of oxygen,
  nitrogen, or fluorine) in usually another molecule of the same or a
  different polar substance

However, I wasn't sure if this applied to F, N, or O atoms that were not covalently bonded to H in the first place. A question regarding hydrogen-bonding came up on an exam for which the marks scheme read that both element X (with an O-H bond) and element Y (with an O in an ether group). However my teacher doesn't not agree with the marks scheme and, "[believes that the marks-scheme is] incorrect because Y doesn't contain O-H bonds so the O will not carry a large enough δ- to create hydrogen bonds with water."

Is she correct, or is the marks scheme?

Note: I thought the same as my teacher up until I saw the MS.


texts: Sintered vs perforated plate Büchner funnels

texts:
I just bought a sintered-glass style Büchner funnel for some vacuum filtration tasks. There's two issues I'm having with it:

Filter material, especially activated charcoal, ends up stuck in the glass disc. I did a little bit of research trying to find out how to clean it out and the only thing I found (for the carbon, at least), was piranha solution, but I have neither the equipment nor the experience to use it safely so I'm SOL on cleaning. Fortunately the presence of carbon isn't a huge deal for my uses, though.
For paper filtering; the sintered disc goes all the way to the edge of the funnel, so unless I've got filter paper exactly the size of the funnel (which I don't), I'm worried that the 0.5-1.0mm gap I usually end up with around the edge of the paper gives fluid a path to bypass the paper. If I err on the side of larger paper it's still a poor fit with large channels around the edge.

It seems to me that the perforated-disc style of Büchner funnel would solve both of these problems; since the funnel itself can't trap filter material and also the holes don't go all the way to the edge of the disc:

(Incidentally, the pictured funnel on the left is exactly the one I own.)
My question is, given the two apparent disadvantages I've observed with the sintered-glass style funnel, what are the pros and cons of each type? More specifically, what could I do with a sintered-glass funnel that I couldn't do (as easily) with a perforated-disc funnel?
The context here is "kitchen chemistry"; I'm not a chemist I just do a few DIY projects here and there and all my knowledge comes from YouTube and Google searches. The only lab equipment I own is this funnel, a flask, and a vacuum pump. The only relevant PPE I own is a respirator, various gloves, and some chemical safety goggles.


texts: When using LPCVD, is it possible to pattern a silicon wafer with poly-silicon using a mask?

texts: I know it's possible to etch (i.e. pattern) poly-silicon, but I'm curious if one can just use a mask instead. Are there masks that inhibit the growth of poly-silicon?


texts: Why is a 7-endo radical cyclisation favoured over a 6-exo in this synthesis?

texts: Normally, according to Baldwin's rules, 3-exo to 7-exo cyclisations are preferred over endo cyclisations. But in the following example, a 7-endo-trig reaction is favoured over a 6-exo-trig and I am just not able to understand why. It's a real synthesis example (taken from Eur. J. Org. Chem. 2015, 2015 (15), 3240–3250). Please explain to me why the 7-endo product is favoured here.


texts: Calculation of VOCs concentration (ppm) in the headspace of a bottle

texts:
What is the right way to calculate the concentration of volatile compounds in the air in a closed container?

Suppose we have a $\pu{50 mL}$ bottle with $\pu{25 mL}$ of $100\%$ ethanol $(\ce{EtOH})$ in it.

In this case, will it be okay to use $PV = nRT?$

Doing so, it will be:

$$
\begin{align}
p(\ce{EtOH}) &= \pu{0.05872 atm}\\
V &= \pu{0.025 L}\\
R &= \pu{0.08206 L atm mol-1 K-1}\\
T &= \pu{298 K}\\
\end{align}
$$

giving us $n = \pu{6.00314E-5 mol}.$

The ppm: $\pu{1 ppm} = \pu{1 mg l-1}$

$$M(\ce{EtOH}) = \pu{46.07 g mol-1}$$

$$\pu{6.00314E-5 mol} × \pu{46.07 g mol-1} × 1000 =  \pu{2.766 mg}$$

This lead to $$[\pu{ppm}] = \frac{\pu{2.766 mg}}{\pu{0.025 l}} = \pu{110.6 ppm}$$


texts: C4v point group complexes and the group orbitals (MO Theory)

texts:
Out of curiosity, what would the group orbitals be like in a C4v complex (square planar complexes or octahedral complexes with 5 identical ligands and 1 other) look like? I get a irreducible representation of 2A1 + B1 + E, which raises the question of how the 2 different A1 group orbitals look like.

Can someone explain this to me?


texts: Relationship between rate of reaction and pressure

texts:
Given the equation

$$\ce{A(g) + B(g) → C(g) + D(g)}$$

with reaction rate given by

$$v = k[\ce{A}]^2$$

How can I prove that reaction rate goes up with pressure? I tried substituting $k$ for $Ae^{-E_A/(RT)}$ which leaves me with

$$v = Ae^{-\frac{E_A}{RT}}[A]^2$$

If I use

$$PV = nRT \implies RT = \frac{1}{n}PV \implies RT = \frac{P}{C}$$

I end up with this:

$$v = Ae^{-E_A\frac{C}{P}}[A]^2$$

This shows there is a relation, but I don't know what to do with the concentration $C$, or whether it was correct to plug in the ideal gas law in the first place.  Any help?


texts: How to Determine the Rate Law knowing time and concentrations

texts:
Consider the hypothetical reaction described by the following equation:

$$\ce{3 A + 2 B -> C}$$

Reagent B was added in excess and the following time concentration data was obtained.

\begin{array}{cr}
A\ [\mathrm{10^{-3}\ mol\cdot L^{-1}}] & \mathrm{time [s]} \\ \hline
5.0 & 10\\
2.9 & 50\\
1.9 & 80\\
1.1 & 120\\
0.7 & 150\\
\end{array}

(i) Derive the relevant integrated rate law and hence find the rate constant.

I am thinking that to solve this question you just sub the values into the integrate rate laws and see if it produces a linear relationship.

Is this correct? Also could you please suggest a faster method of solving this type of question.

Thank you.


texts: Rule of Thumb for separatory funnel volumes

texts:
Is there a general rule of thumb in terms of how one chooses the volume of separatory funnel used for an extraction? It's my understanding that it is the amount of product that determines this as opposed to the volume of the solvent it is present in. If that is correct I was wondering if someone could post what the general ranges in grams of product would be appropriate for the following sep funnel sizes:

250 mL, 500 mL, 1 L


texts: How to find the units of the rate constant?

texts:
Consider the reaction $$\ce{2A + B -> Products},$$ when the concentration of $\ce{B}$ alone was doubled, the half-life did not change. When the concentration of $\ce{A}$ alone was doubled, the rate increased by two times. The units of the rate constant is:
a) $\mathrm{s^{-1}}$
b) $\mathrm{L\ mol^{-1}\ s^{-1}}$
c) Unitless
d) $\mathrm{mol\ L^{-1}\ s^{-1}}$

I have tried it solving through this method:
According to me, when the concentration of $\ce{A}$ alone was doubled, reaction rate also increased by two time, implies that reaction is first order according to $\ce{A}$. In same way, the reaction should be 0th order according to $\ce{B}$. So the net order would be 1. So this gives me answer $\mathrm{s^{-1}}$ but the answer is $\mathrm{L\ mol^{-1}\ s^{-1}}$. Where am I going wrong?


texts: What gives saliva its adhesive properties?

texts:
The act of sealing off an envelope by licking the flap and "gluing" it down so that the envelope becomes closed rather tightly is probably rare nowadays, but remains well known to many people.

I was wondering which component of saliva gives it the ability to glue paper tightly enough so that the envelope is considered sealed off.

Here's the link for the Wikipedia entry of saliva, for composition.


texts: In an effort to prevent mold in my humidifier, I added a little white vinegar to the tap water in the tank. This creates a white bloom that floats in the water. It is odourless and, despite its appearance, is not slimy. It feels more like paper pulp. After a while it adheres to the sides of the tank as a transparent sort of gel. Is this a known phenomenon? What is the white stuff?


texts:
I need to predict the geometry of Dichlorine monoxide, using the main link theory: Lewis model, VSEPR and hybridization of molecular orbitals.

First, the Lewis structure is a graphical representation of the pairs of electrons bonding between the atoms of a molecule and the pairs of solitary electrons, which may exist.

We proceed to predict the geometry of the electron and molecular pair of the chemical species, using the VSEPR model, which is based on the number of electron pairs of the central atom (oxygen) to determine it.

As we have previously stated, the central atom is oxygen. The number of electron pairs of oxygen is four, of these four: two are shared and two are bonding. Therefore, electronic geometry is tetrahedral and molecular gometry is angular:

To conclude, we will explain geometry by hybridization of atomic orbitals. Orbital hybridization is simply the linear combination of the mathematical expressions of the wave functions of the orbitals. We know that there are two types of overlaps:

Sigma or frontal bond.
Pi or lateral bond.

Here I am asked to study the different overlaps that occur and to explain the consequences on the geometry of that overlap. I suppose there will only be sigma overlap because there are no multiple links. But I can neither say nor explain with certainty what type of overlap and between which electrons or atoms there are and, finally, explain the relevance of this in geometry.

Likewise, ideally the angle should be 120º, but experimentally it is 110.4º. This decrease is due to the presence of two pairs of solitary electrons in the oxygen atom, isn't it? Is there any way to determine such an angle other than in the laboratory, any formula or relationship?


texts:
I would like to know under what circumstances is Trouton's rule obeyed by liquids and why certain systems i.e. substances may deviate from Trouton's rule.

By the way I know what Trouton's rule is and I have a vague idea that sometimes Trouton's rule is not obeyed due to the presence of hydrogen bonding in liquids such as water but I would like to know more about other reasons why certain situations the rule is invalid.


texts: What is the mechanism by which sugars glycate proteins?

texts: I was looking at why it's ok for glucose to circulate through the blood while fructose is prevented from doing so by the liver.  Fructose glycates proteins more readily than glucose.  This is the opposite of what I expected as glucose is an aldose while fructose is a ketose.  It seems reasonable to expect that the aldose would be more reactive than the ketose in many reactions.  What is the mechanism of glycation of proteins?


texts: How does the proticity of an acid affect the enthalpy change of its reaction with a base?

texts:
For an experiment I've carried out a series of reactions between monoprotic bases (ammonia and potassium hydroxide) and several acids. Two of these acids were monoprotic (hydrochlori acid and ethanoic acid), another was  diprotic (sulfuric acid) and the other was triprotic (phosphoric acid). My results showed that the triprotic acid released more energy per mole than both the diprotic and monoprotic acids, and that the diprotic acid released more energy per mole than the monoprotic acids. This gave rise to my question: how does the proticity of an acid affect the enthalpy change of its reaction with a base?

To be clear I compared my results between weak acids, strong acids so the strength of the acid has already been accounted for.


texts:
I've never actually been told deeper reasons for this. In organic chemistry classes (1 and 2) we're usually told that aromatic compounds are very stable whereas anti-aromatic ones are not, and also, we're taught how to identify them. Why are they, though?

(I'm pretty sure this has something to do with the de-localisation of electron pairs, which enhances stability, but I only have a vague idea. Is that the only reason?)


texts: Do molecular orbital diagrams exist for ionically bonded compounds and molecules?


texts: Is there any substance that's a 4-4-4 on the NFPA diamond?

texts:
A lot of the organometallics are rather... interesting compounds to work with. The most famous (among those who care, anyway) is tert-butyllithium or t-BuLi. It is the textbook example of a pyrophoric substance, demonstrated to pretty much every chemistry major as an air-sensitive chemical requiring special handling (Syringe and cannula transfers, gas-tight septa, argon/nitrogen blankets, that kind of thing). Despite the inherent hazards of the stuff, it's widely used in the industry (hence widespread notoriety) to add butyl groups to an organic molecule (most organometallics are useful for this kind of carbon-carbon bond formation), and you can buy it by the - airtight - gallon canister.
There are, surprisingly (except to most organic chemists), even more dangerous compounds in the average organic chemistry lab. Along the pyrophoric line, many of the multi-methyl-metallics are violently pyrophoric (even compared to BuLi), including trimethylaluminum, dimethylzinc, and dimethylmagnesium. All of these are also extremely poisonous (anything these compounds can do to your wonder-drug-in-progress, they can also do to various key structures in your own body), with dubious honors reserved for dimethylmercury. Because many of these, especially the organoalkaline compounds, react violently with water, they also automatically rate at least a 3 on the "reactivity" scale.
When leads me to wonder, because I wonder about these things from the safety of my office chair in a completely unrelated field; just how bad can it get? Specifically, is there any compound with an accepted use in laboratory or industrial application that is nasty enough to max out the entire NFPA-704 diamond? The closest I can find is trimethylaluminum, at a 3-4-3 (heath-fire-reactivity). T-BuLi is a 3-3-4. I can't find NFPA data on straight diazomethane (which may be because nobody in their right mind ever works with the stuff in its pure gaseous form; it's always used in a dilute diethyl ether solution, and even then is never sold or shipped that way) but it would probably be a finalist, as the gas is acutely toxic, autoignites at room temperature, and detonates on standing (something for everyone!). I'm thinking that these two inorganic families - light alkyl-metallics and organo-polyazides - would be the most likely candidates to produce a compound so toxic, so flammable, and so readily reactive, yet so interesting to chemistry, that the NFPA would see fit to rate it, and would give it highest honors.


texts: Is the definition of anode and cathode failing?

texts:
We know anode=oxidation=loss of electrons and cathode=reduction=gain of electrons but in the photoelectric effect the electrons are gained at the anode and lost at the cathode of the discharge tube?
References: Anode,
Cathode, Oxidation, Reduction


texts: I've read this today. From the little I know, they discover new elements by bombarding some atoms in particle accelerators (I guess) and these new elements last a few miliseconds. So what's the insight they reveal to us? Why are these discoveries useful?


texts: What significance and applications does nanotechnology have in chemistry?

texts: Specifically concerning organic chemistry, but an explanation on its significance and applications in inorganic chemistry would be great as well.


texts:
There are (obviously) a lot of chemical compounds. Wikipedia is able to translate names to molecular formulas for a lot of them.

But is there a way to enter a molecular formula and find the chemical's name?

Coming from the computer science background, I realize that there should be some canonical textual representation of the molecular formulas for such search to be possible. So is there a language that can represent a full formula, including the structure? I could only find some latex-based languages.


texts: Why is Zinc reduced instead of Hydrogen, in the electrolysis of aqueous Zinc Sulfate, using Carbon electrodes?

texts:
In the electrolysis of aqueous zinc sulfate using carbon electrodes, why are zinc ions reduced at the cathode, instead of hydrogen ions? How does this relate to over potential?

It would mean a lot to me, if someone could provide a detailed answer.


texts: Why did permanent marker come off of glass after rubbing it with my fingers?

texts:
No, my fingers don't look stained but I accidentally made a stroke of permanent marker on glass and I was looking for nail polish remover to use, couldn't find it. So I started rubbing the stroke of permanent marker with my fingers and it started coming off. I was able to get it off with 3 fingers, with the thumb getting most of it off.

I wonder, is it because permanent marker dissolves in skin oil that I was able to get it off? Or was it the small amount of sweat? Or both? Or was it just that I was rubbing my fingers on the permanent marker and the shear force got it off?

I mean yes, this was just a few minutes after the accidental stroke that I got it off with my fingers but that would have been plenty of time for it to dry, I think.


texts: Why can't the energy of an electron exceed 0 eV?

texts:
Today we were learning about atomic structure, our teacher introduced the mathematics of the 'Niels Bohr Model of Atom', and calculated the value of total energy $\frac{-13.6z^2}{n^2} \pu{eV}$. He explained that when the value of $n$ tends to infinity, the value of total energy tends to $0$. Mathematically I understood this.
I couldn't understand, or comprehend, how or why the energy of an electron can't exceed $\pu{0 eV}$? What is the reasoning behind it; why is it not possible and what is the physical significance of it.
For the sake of the argument, what would it look like if the total energy of an electron were to exceed $\pu{0 eV}$?


texts: CuI, CuSCN Solubility in Ammonia Solution

texts:
I am trying to dissolve copper(I) iodide and copper(I) thiocyanate in ammonia.  I am quite sure that they both dissolve in liquid ammonia, and are insoluble in ethanol.

If I were to purchase a 2.0M solution of ammonia in ethanol, like the one here, would the CuI and CuSCN still dissolve in the ammonia solution?

Also, are there any other solvents that you know of for these materials?


texts: White compound, insoluble in water, turns pink and then white again on adding water

texts:
Which white, ionic compound(s) that is insoluble in water turns pink when you add only a little of water but white again when you add more water?

I know that the cation is one of the following:

$$\ce{Mg^2+, Ca^2+, Sr^2+, Ba^2+, Fe^3+, Ni^2+, Co^2+, Zn^2+, Al^3+, or NH4^+}$$

I was physically examining such a compound and determined it does not change the color of litmus paper (red or blue). I thought that it could be zinc carbonate because it also precipitates hydroxide (when then compound is added to dilute nitric acid).

EDIT: I found out that the compound is lithium carbonate, and it just took a long time to dissolve. How would lithium carbonate change color like this?


texts:
I have been going through this topic in my coordination chemistry chapter. I read that in metal carbonyls, -CO group acts as a ligand and donates its lone pair of electrons into a vacant orbital of the metal to form a sigma bond. Also, there's a pi bond formation by the donation of electrons from the filled d orbital of the metal into pi* MO of the -CO molecule, and this pi bond strengthens the metal to ligand bonding.

But filling of electrons in the antibonding MO means decrease of bond order and correspondingly decrease of bond strength and bond lengths. It sounds contradictory to what's written in my book.

Where am I making a mistake?


texts: Is there a theory behind selecting elements that may be successful in potential superconductors?

texts:
Looking at something like $\ce{YBa2Cu3O7}$ which was one of the first cuprate superconductors to be discovered, I'm always curious how the selection of these substances as likely superconductors comes about?

Does it have to do with optimal crystalline structure, unpaired electrons, or is it similar to the way semiconductors are designed with doping to create "holes"?  Or are these superconductors simply determined experimentally based on small changes of what has worked in the past?

As a side question: I've always been curious as to how they stumbled upon Yttrium for this particular case.  It's not something I think of as being carried in ready supply.  Would this have been after YAG lasers were already around?


texts:
In this experiment, I used a digital temperature probe to measure the freezing point of water and some other salt solutions.

I was just wondering if there was any explanations for 1. The first plateau and 2. The dip in temperature right before the freezing point. (2.) can be observed for other salt solutions.

I think that the dip before the freezing point is caused by the formation of ice needing a 'nucleation point', which requires the temperature to be much lower such that a regular structure can be formed. I have no good explanation for 1. but it could be due to uneven cooling or some sort of issue with the probe.

Please give some suggestions that explains 1. and 2. if possible, even if you are unsure. Thank you for the help!


texts: Why is CO ligand higher than CN- in the spectrochemical series?

texts:
I've drawn MO diagrams for both molecules and they are isoelectronic. The main difference is that $\ce{C}$ is closer in electronegativity to $\ce{N}$ than it is to $\ce{O}$ so there will be less stabilisation of the bonding orbitals for $\ce{CO}$ due to the increased energy difference in the AO's.

Does this difference lead to $\ce{CO}$ having its $\pi^*$ orbital (LUMO) lower than that of $\ce{CN^{-1}}$ so it more readily accepts $\pi$ backdonation from a metal centre? Does this alone account for why $\ce{CO}$ is higher in the spectrochemical series? What about the difference between the  $\sigma$ orbitals (HOMO) in $\ce{CO}$ and $\ce{CN^{-1}}$?


texts: chemical equilibrium reaction for burned gases in engine

texts:
I do not understand how equation(3) occurs.(taken from the book Internal Combustion Engines by John B.Heywood)
$$ $$
Consider a reactive mixture of ideal gases. The reactant species $M_a,M_b$ etc.and the product species $M_l,M_m$ etc. are related by the general reaction whose stoichiometry is given by:
$$\upsilon _aM_a+\upsilon _bM_b+...=\upsilon _lM_l+\upsilon _mM_m+...$$
$$ $$

This can be otherwise written as:
$$ \sum_{i}\upsilon _iM_i=0$$
where the $\upsilon _i$ are the stoichiometric coefficients and by convention are positive for the product species and negative for the reactant species.

Let $\delta n_a$ of $M_a$ react with $\delta n_b$ of $M_b$,etc. and produce $\delta n_l$ of $M_l$,$\delta n_m$ of $M_m$,etc..

These amounts are in proportion,given by the equation (3):

(3) $$\delta n_i=\upsilon _i\delta n$$

1.Does $\delta n$ here signify 'extent of reaction'?

2.I would eventually like to use the number of moles of each species in expressing the chemical potential,with the gibbs free energy already known. But,the part I don't understant is equation 3. What does the author mean by proportional? Could you give a example?

Gibbs free energy is given as:
$$(\Delta G)_(pressure,temper_)=\sum_{i}\mu _i\delta n_i$$

which by equation(3) can be re-written as(WHY??):

$$(\Delta G)_(pressure,temper_)=\sum_{i}\mu _i\nu _i\delta n$$


texts: Platinum group metals: Why are platinum and palladium great catalysts and not the others?

texts: Platinum and palladium are great catalysts. At the same time, other metals of the so-called platinum group metals are not. What are the atomic level reasons for this?


texts: Sn + HNO3: When will the metastannic acid H2SnO3 appear?

texts:
I am dissolving tin in nitric acid.
What reactions will take place depending on the conentration of $\rm{HNO}_3$ and temperature?

Particularly, I am interested when the metastannic acid $\rm{H_2SnO_3}$ is likely to be produced, according to

$\rm{Sn + 4 HNO_3 → H_2O + H_2SnO_3 + 4 NO_2}$

P. S.
References to books or scientific articles are welcome.


texts: How do water insoluble medications get absorbed in the blood stream?

texts: If a chemical is ingested (supplement or drug) orally, which is insoluble in water, how can it be absorbed into the body?  Is there something inside a capsule that the compound is mixed with?


texts: Why do the two cycloheptadiene isomers both show four distinct shifts in carbon-13 NMR?

texts:
In the following question I have filled in the correct answer for i:

For ii I thought that should be three for Q and two for P,

For Q there are three distinct enviroments for the C atoms:

$\ce{-\mathbf{C}H2-{}}$
$\ce{-\mathbf{C}H2-C=C-{}}$
$\ce{-\mathbf{C}=C<}$

and for P there are two:

$\ce{-\mathbf{C}H2-C=C-{}}$
$\ce{-\mathbf{C}=C<}$

Howevere the correct answer is 4 for both P and Q.

So then I thought maybe they want me to include the splitting of the peaks but in both P and Q there is too much splitting for this to be the case...

Why is the correct answer 4?


texts: Practical to measure topography of a large area (mm-scale) with scanning probe microscopy (SPM)?

texts:
I'm currently very new to nanotechnology. I do understand that it is possible to achieve an excellent topography image of a sample by SPM. However, is it possible to do SPM over a large area (for instance like in SEM, which is possible to image over 1x1 mm)? Theoretically speaking, I do not see any problem except that measurement time might be very high.
So, is it possible to scan over a large area (mm-scale)? If not, what can the reason be if one excluded measurement time. If yes, which technique will give an advantage for such measurement?
I would appreciate if anyone could explain the theory/mechanism behind this or point me to a place where I can obtain a better understanding.


texts: Dimerisation of HOOC-COOH in non polar solvents

texts:
In a textbook I read, the following question is my doubt:

Why is the answer B)? Why do the other molecules not dimerise? How does it work?

(P.S- Why dosen't the B) option only form hydrogen bonds with the same type B)
?)

Edit: Turns out I was looking at the wrong answer key and the answer is actually a) :p


texts: Lithium aminoborohydride is a versatile reducing agent. In the scheme below, I don't understand how does unhindered LAB afford an alcohol in the reduction of amides while a hindered LAB affords an amine. Why are two different products obtained and by what mechanisms do these reductions take place?


texts: Are PTFE stir bars resistant to all lab solvents?

texts: It seems like glass-coated stir bars are a specialty item.  Standard stir bars are coated in PTFE.  Is PTFE resistant to all typical lab solvents?  Or is there a reference to check to ensure it is inert to the chemicals being stirred?


texts: Haloperidol synthesis - 1-chloro-4-(prop-1-en-2-yl)benzene with NH4Cl and formaldehyde

texts:
However I have no idea how the first two steps work. Especially the first step makes me wonder how the amine and the aldehyde form the 1,3-oxazinane.

Suggestions:
Ammonia and formaldehyde form via condensation the imine.
The imine and formaldehyde form the 1,3-oxazinane, however I looks kinda wrong to me. Moreover, if this is mechanism would be correct, why does the formaldehyde binding to prop-1-en and the imine don't?

Step 2 probably starts with protonating the oxygen and i think the 1,3-oxazinane cleavage follows after. However how does the methyl group bonds with the other methylene group?


texts: Graph of compressibility factor vs pressure when real gas is assigned Z=1

texts:
Problem:

According to the real gas equation, $Z = 1$ for an ideal gas and $Z$ is variable for a real gas. Suppose, in order to easy our calculations, we fixed $Z=1$ for a real gas and for ideal gas, $Z$ will become variable. $Z$ vs $P$ for an ideal gas will be similar to:

Attempt:
I imagined the answer by thinking the normal graph of $Z$ vs $P$ for a real gas and transposed each point on the locus of the real gas curve on $Z=1$ line. I then imagined where the corresponding point of the ideal gas curve would land.
This gave option (a) as the correct answer, but the book says the answer is (b). I believe then the van der Waals equation must be manipulated to get the answer, but can someone tell me how? Or is the answer given in the book wrong?


texts: Why non polar liquids wet non polar surfaces better than polar surfaces?

texts:
In semiconductor industry adhesion promoters (such as HMDS) are applied on various surfaces to improve adhesion between the surface and the photoresist:

  On SiO2 and many metals surfaces, the air humidity forms polaric
  OH-bonds. Such substrates are hydrophilic, but reveal a bad wetting
  and adhesion of the unpolaric resist. Adhesion promoters make
  hydrophilic surfaces hydrophobic thus improving the resist adhesion.
  [...] The  methyl  groups  of  the  HMDS  fragment  hereby  form  a
  hydrophobic  surface thus improving resist wetting and adhesion.
  (source)

But why should non polar (hydrophobic) substances wet non polar substrates better than polar substrates? (in the opposite case of polar substances it is clear why)


texts:
The school book and Wikipedia state that coordination isomers are:

In a coordination isomer the total ratio of ligand to metal remains the same, but the ligands attached to a specific metal ion change.
For example, a solution containing $\ce{[Co(NH3)6]^3+}$ and $\ce{[Cr(CN)6]^3−}$ is a coordination isomer with a solution containing $\ce{[Cr(NH3)6]^3+}$ and $\ce{[Co(CN)6]^3−}$.

So in this definition, consider $\ce{[Co(NH3)5(CN)][Cr(CN)5(NH3)]}$. Is it coordination isomer of $\ce{[Co(NH3)6][Cr(CN)6]}$ or not?
My teacher says yes, but I'm not quite sure according to Wikipedia.


texts: Why does a hydride shift take place in the cannizzaro reaction?

texts: Hydride is a poor leaving group so a hydride shift should not take place and instead the attacking hydroxyl should leave the carbonyl group. I don’t understand why this doesn’t happen?


texts: What does an endpoint saturated signal mean. How does it relate to equilibrium constant?

texts:
I have had a question that has been bugging me for a couple of years in my graduate school career, and I was wondering if perhaps someone could help explain it to me. Given an endpoint assay like this one I've shown below (an ELISA assay, but I assume it could work for any endpoint), how does the endpoint signal (the signal at the highest concentration) relate to binding? And how does $EC_{50}$ (the concentration between the maximum and minimum signal) relate to the equilibrium constant?

For years, biologists have used the $EC_{50}$ to give a rough approximation for the equilibrium binding constant $K_d$. However, it always bothered me that all curves features on the graph could produce nearly the same $EC_{50}$, but with different levels of maximum absorbance. For instance two of the curves are saturated at around 1.2 and the others are around 2.5.

Do all of these curves have roughly the same $K_d$, or is it worrisome that we don't get equivalence signal for the maximum concentrations?


texts:
Which is more basic: oxaziridine or diaziridine?

Oxaziridine's got one nitrogen and one oxygen as heteroatoms, while in diaziridine there are both nitrogens.


texts:
Arrange the sites in purine in order of basicity.

I feel that 9 is the least basic as nitrogen's lone pair is delocalised. However, I am unable to rank 1, 3, and 7 in a particular order. Is there a particular theory to decide basicity of sites?


texts: Who is Hræðraford, the "learned clerk" "writing in modern chemical Latin"?

texts:
This question has been re-asked on History of Science and Mathematics:
  Who is Hræðraford, the “learned clerk” “writing in modern chemical Latin”?

Truesdell, C. Rational Thermodynamics. New York, NY: Springer New York, 1984. http://dx.doi.org/10.1007/978-1-4612-5206-1, p. 41 says:

  … in the numerous papers and books whose titles and topics include "irreversible"—the learned clerk HRÆðRAFORD, writing in modern chemical Latin, has recently called their field "thermodynamice inreparabilis"—…

Who is this "learned clerk" Hræðraford?


texts: A hydrogen bond is formed between hydrogen attached to highly electronegative atoms (nitrogen, oxygen, and fluorine) which are small in size too and the non-bonding pair of electrons of another such atom that may be present in the same molecule or in another molecule but is not directly linked to it with a covalent bond. But when we talk about chlorine, it doesn't form hydrogen bond due to its large size. But what happens in the case of carbon more precisely 'carbenes'? It's small in size, has a lone pair of electrons and is somehow more electronegative too. Does it help form H-bond like N, O, and F do?


texts: What is happening at the various boundaries of a phase diagram?

texts:
Hoping to clarify a few questions I have that the textbook doesn't address.
Lets just say I have a piston (at equilibrium) with 100 moles of gaseous compound that looks like this:

And here is the phase diagram for this compound:
Now lets say I start increasing the pressure on the piston so that the gas starts to approach the gas-liquid boundary.
1.) What would start to happen as you approach the boundary? For example: Would a few moles of gas begin to condense to liquid? Is this process exponential as you approach boundary?
2.) What is happening at the boundary? For example: Are the gas and liquid in perfect equilibrium (50 moles of each)? By that logic, at the triple point, are there 33 moles of each?
3.) Now let's say we approach the critical point along the liquid-vapor boundary. I read online that its when the gas and liquid become "indistinguishable." If so, that means it has a uniform density. Does that make it another state of matter? If not, why is it special?


texts:
Electrolysis of water, means that the water is separated into hydrogen and oxygen. As beer also includes water, I initially thought electrolysis of beer would cause the water to separate, and the hydrogen and oxygen given off gas, leaving behind a higher concentrate alcohol.

However then realise that ethanol is also in beer (and is what makes you drunk it seems), and like water has an OH bond, so would the electrolysis also cause that to break?

(also does the rest of the molecule affect the strength of the OH bond, either stronger or weaker?)


texts:
For an ideal chain, Flory Huggins theory gives us a distribution which is like a Gaussian.

$$P(l, r) \sim \left(\frac{3}{2 \pi l^2}\right)^{3/2} e^{\frac{-3 r^2}{2 l^2}}$$

where, $l$ is the monomer to monomer distance(also known as Kuhn length) and $R$ is the size of the monomer

This is great but what are the approximation away from an ideal chain polymer? I want to understand how one can go beyond Flory Huggins Theory.


texts: Avogadro's number and percent error help?

texts:
Calculate the percent error in Avogadro’s number calculated by using the following experimental data.
  Pipette calibrated as 30 drops/mL
  Diameter of the watch glass: $15.5\ \mathrm{cm}$
  Number of drops to complete monolayer: 23
  Concentration of stearic acid solution: $1.2\times10^{-4}\ \mathrm{g/mL}$

Updated: My thought on this: I would take $23/30=0.77\ \mathrm{ml}$. Then take $0.77\times1.2\times10^{-4}=9.24\times10^{-5}\ \mathrm g$ of stearic acid. Then $(9.24\times10^{-5})/284.48\ \mathrm g=3.25\times10^{-7}\ \mathrm{mol}$ of stearic acid. Then how do I find the Avogadro's number (molecules per mole)?


texts: Considering ozone in liquid state, certainly there are london forces, but does the position of its molecules induce dipole intermolecular forces?


texts: Why is the Hoffmann's product the major product in the pyrolysis of an ester to form a carboxylic acid and alkene?


texts: How do we experimentally know the strength of bonds?

texts:
How do we experimentally measure the strengths of covalent versus ionic bonds? Also, for a given molecule, how can we know if two pairs of atoms is covalently or ionically bound?

Related question: Textbooks say that in solution of water, two neighboring water molecules (call them A and B) will be 'attracted' because the O in molecule A will attract the H of the 'nearby' molecule B. It is said that H will alternate states between A and B. How do we know this is true experimentally and how can it be measured?


texts: effect of overpressure on lowering viscosity of polysaccharide solution

texts:
it has been reported that agarose dissolved at 110C under slight over-pressure has a viscosity at least 10X smaller than dissolved at atmospheric pressure at 90C (ref:https://doi.org/10.1016/j.ceramint.2004.06.007).
I have not independently verified this result (but I plan to).

The context of this result is ceramic but the solution viscosity result is independent of this context.
I see this result as quite interesting with many other applications BUT
I cannot find ANY other reference of similar results with agarose gel or any other polysaccharide for that matter, which surprise me.

Do I miss something?

I am looking for references to better understand this result and the physico-chemistry behind it.
If somebody could provide some pointers it will be greatly appreciated

Thanks

JF


texts: Dissociation probability vs ionization probability

texts:
Suppose we have a diatomic molecule whose ionization energy is $I$ and dissociation energy is $D$ such that $I>D$.
Now suppose an electron with energy $E>I>D$ collides with this molecule. Which of the processes has higher probability ,the ionization or the dissociation?
Intuitively it seems that the dissociation is more probable. But I would like a justification based on quantum mechanics


texts: Why do LiOH is weak base as compared to NaOH even though it's hydration enthalpy is greater than Na?


texts: Ideal gas constant applied to water, lead, and other non-gases

texts: I've asked a number of chemists why the ideal gas constant is in the equation for Gibbs free energy, and all say the same thing: "The name is a relic of the argon gas chamber used to determine energy and temperature at different pressures and temperatures, but it applies to every thermodynamic system." This sounds great and I understand names of constants can be relics, but why can a conversion factor that only appears to pertain to ideal gases be used for other substances? R assumes elastic collisions, where energy is not lost during molecular collisions, but only ideal gases have this property. Wouldn't you use a different conversion factor for something much denser and fluid like water or even more dense like lead to convert from temperature to energy? I've heard that R is used for free electrons as well, which are pretty different from atoms much less gases. Would "R-specific" be best to use everywhere, as I've heard they do in engineering, so maybe every substance has its own temperature-energy conversion factor?


texts: What is the exact material brand and model number of Lego (ABS)?

texts:
Can anyone tell me the exact ABS material (brand and number) used for Lego?
Is it food grade material?
Kind regards


texts:
I have a molecule and I need to assign the carbon NMR to each of the carbons.

My molecule is

The peaks I have for the carbon NMR are shown below. I have also used the DEPT to work out if it's a $\ce{CH3}$, $\ce{CH2}$, $\ce{CH}$ or quaternary C.

17.7ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH3}$)
19.5ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH3}$)
25.5ppm $\ce{CH2}$
25.7ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH3}$)
32.1ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
36.7ppm $\ce{CH2}$
39.7ppm $\ce{CH2}$
66.0ppm $\ce{CH2}$
122.1ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
124.4ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
128.2ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
128.3ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
128.6ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
131.5ppm quaternary carbon
136.2ppm quaternary carbon
149.0ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
166.4ppm quaternary carbon

I think I can assign the quaternary carbon with the $\ce{C=O}$ in the ester as the 166.4ppm.

Out of the three methyl groups, would the lowest ppm of 17.7ppm be the one at the top of the molecule by itself and then would the other two at the bottom of the molecule on the alkene be 19.5ppm and 25.7ppm? I don't know which one is which, I know it is to do with cis/trans but I'm not sure which way around it is.

I also know that on the benzene ring, only three $\ce{CH}$ peaks show up as the two ortho and the two meta carbons are equivalent.


texts: Why does 2',6'-dimethylacetophenone not give iodoform test?


texts: What chemical elements does a nuclear explosion release into its mushroom cloud?

texts:
I'm interested in the mushroom cloud produced by a nuclear explosion: What is it made of?

I know that the uranium fission reaction is the following (from this web page)

$$\ce{{}^{235}U + {}^1_0n -> {}^{95}Sr + {}^{139}Xe +2~{}^1_0n + 180~MeV}$$

I know that these elements are not stable and decay, so what is left over?


texts: Explaining the colour change in the potassium permanganate titration of iron(II) ions

texts:
In the redox titration:

$$\ce{MnO4- + 8 H+ + 5 Fe^2+ -> Mn^2+ + 4 H2O + 5 Fe^3+}$$

the colour change which occurs is purple to colourless, because of the decreased concentration of permanganate ions. But doesn't this fail to account for the visible iron ions? $\ce{Fe^2+(aq)}$ is green and $\ce{Fe^3+(aq)}$ is brown.

Anyone have an explanation for why the end result is colourless and not brown solution?


texts: Given the Pair Correlation Functions for a Multi-component Fluid, is There a Simple way to Obtain the Helmholtz Free Energy?

texts:
I think it may not be possible.

Given all the pair correlation functions, it seems possible to obtain the internal energy (assuming different species interact through pair-potentials (i.e. potentials that do not involve more than two bodies)).

To obtain the free energy, the "naive" way is to obtain the partition function and take logarithm. Yet it seems not easy (except possibly through cluster expansions? which is not exact.)

Any help will be much appreciated.


texts: Effective nuclear charge on an inner electron

texts:
Is the shielding effect in a multi-electronic species caused due to mutual repulsion of electrons?

If it were so, while calculation the $Z_\text{eff}$ on an electron in say the $\mathrm{2p}$ orbital of a $\ce{Na}$ $\mathrm{(1s^2,2s^2,2p^6,3s^1)}$ atom, why don't we also consider the repulsive force of outer electrons in $\mathrm{3s}$ and add it to $Z_\text{eff}$ instead of $Z_\text{eff} = Z - \text{repulsion due to inner electrons}$?

Why doesn't the equation read

$$Z_\text{eff} = Z - \text{repulsion due to inner electrons} + \text{attraction due to outer electrons}?$$


texts:
How do we predict the products formed when we heat cyclic dicarboxylic acids? I know that in case of aliphatic dicarboxylic acids we apply Blanc's rule and predict the product formed based on the number of carbons between the acid groups.

In case of aliphatic dicarboxylic acids:

$$
\begin{array}{cc}
\hline
n & \text{Product} \\
\hline
0 & \text{Oxalic acid} \\
1 & \text{Malonic acid} \\
2 & \text{Succinic acid} \\
3 & \text{Glutaric acid} \\
\ldots & \ldots \\
\hline
\end{array}
$$

Here $n$ is a number of carbons between $\ce{-COOH}$ groups.

I tried searching on the net but could not find a convincing answer. Please correct me if I am wrong somewhere


texts: Electrolysis of brine without a membrane

texts: In the electrolysis of brine sodium hydroxide is normally formed if there is a membrane preventing the transfer of the chlorine gas. So if the membrane is removed,  will the chlorine gas react with NaOH to form sodium hypochlorite? Because I've read that the chlorine will instead just react with the hydroxide ion to produce hypochlorite. Or will it produce both? and where does the sodium ion go if it doesn't?


texts:
So far, maybe it's just that my course isn't advanced enough, but I have never seen anything called 'neutralization constant'.

Any such reaction involving an acid and a base will be complete no matter the strength of either reactant.

Buffer works the same way. If the reaction is reversible then the buffet is not effective.


texts: Forming the resonance structures for the nitrite anion

texts:
Yesterday I learned about resonance structures and came across this problem.

The resonance structures are formed when there is a delocalized π electron cloud. I understand that for benzene, there are two resonance structures. For making π bonds I know that there are three main reasons:

The two orbitals should overlap each other.
There should only be one electron for each orbital.
The two orbitals should be in one level.

In benzene there are six $\mathrm{p}_z$ unhybridized orbitals, therefore they can share their electrons with each other (delocalize the π bond electrons). So benzene has 2 resonance structures.

But I don't understand how this works for the nitrite anion, $\ce{NO2-}$.

In $\ce{NO2-}$ there is also π electron delocalization, so there are 2 resonance structures. Now I am going to say how I think the delocalization is happening.

When the first structure changes into the second structure, nitrogen has to make a π bond with the right side oxygen with each of the $\mathrm{p}_z$ orbitals before, but now that π bond's two electrons take to the right side oxygen, and then there are 2 electrons in the right side $\mathrm{p}_z$ orbital and no electrons in the nitrogen $\mathrm{p}_z$ orbital. But the left side oxygen (in the first structure, $\ce{O-}$) has 2 electrons in its $\mathrm{p}_z$ orbital. Then the nitrogen $\mathrm{p}_z$ and oxygen $\mathrm{p}_z$ overlap each other and make a π bond.

I have said before "There should only be one electron for each orbital." But, according to my understanding above, the π bond is made by $\mathrm{p}_z$ with 2 electrons (oxygen) and $\mathrm{p}_z$ with no electrons (nitrogen) overlapping each other.

So, is there any mistake in my imagination?


texts: Are alkylidene cycloalkanes planar molecules?

texts:
Above is an Alkylidene Cycloalkane. Its structure closely resembles that of allenes.

According to my knowledge, allenes are non-planar molecules due to the two $\pi$-bonds being formed on almost perpendicular planes.

So due to the similarity in structures and optical activity, are Alkylidene Cycloalkanes also non-planar?


texts: Why are porphyrins colored when porphyrinogens are colorless?

texts: In heme synthesis, porphyrinogens are colorless but upon their oxidation (from removing 6 hydrogen atoms), they become colored porphyrins. Why does the change in color occur? Is red the only color it can turn into or are there others (if so, what does that depend on)?


texts: If a sample to be carbon-dated is wrapped in a piece of paper, will you still be able to effectively measure the radioactivity of the sample?


texts: Cooling down and heating of solution unexpectedly during cation identification test

texts:
While conducting qualitative tests for identification of cations in the given salt, I encountered this unexpected temperature change. Following are the steps of tests:

Step 1: We made an aqueous solution of the salt and added dilute hydrochloric acid. There was no precipitate, which showed that Group 1 cations ($\ce{Pb^2+}$) are absent.

Step 2: We then divided the solution into two parts, and through one part, passed $\ce{H2S}$ gas. There was no precipitate, which showed that Group 2 cations ($\ce{Cu^2+}$) were absent.

Here, the temperature of the solution through which we passed the gas, rose above the room temperature. It could be easily felt while holding the test tube.

Step 3: To the other part in the test tube, we added $\ce{NH4Cl}$ to make the solution completely alkaline, and then added $\ce{NH4OH}$ till the solution was ammoniacal. We did not get any precipitate, which showed that Group 3 cations (viz. $\ce{Al^3+}$, $\ce{Fe^2+}$ and $\ce{Fe^3+}$) were absent.

It is here that we encountered the temperature change. Before adding $\ce{NH4Cl}$, the solution was at room temperature. However, as we kept on adding $\ce{NH4Cl}$ to make the solution saturated and alkaline, the temperature of the solution went down considerably. After some time, it was so cold that water molecules in the atmosphere had started to condense on the outer side of the test tube. The temperature still went down when we added $\ce{NH4OH}$.

Step 4: Now we divided the solution into two parts and through one part, passed $\ce{H2S}$. We got a white precipitate, indicating the presence of Group 4 cation ($\ce{Zn^2+}$).

It must be noted here that as soon as we passed the $\ce{H2S}$ gas, the temperature of the solution again increased to room temperature.

Step 5: To the other part of the solution, we added $\ce{(NH4)2CO3}$. We got a white precipitate, which indicated that Group 5 cations (viz. $\ce{Ba^2+}$, $\ce{Sr^2+}$ or $\ce{Ca^2+}$) was present.

It must be noted that the temperature remained almost same now.

Later, by confirmatory tests, we determined the salts to be $\ce{Ca(CH3COO)2}$ and $\ce{ZnSO4}$.

Can anyone explain what chemical reactions had occurred which were exothermic and endothermic, thus resulting in a change in temperature of the solution?


texts: Grignard Reaction with Unsaturated Methyl Ester | Prevention of Acid Catalyzed Hydration

texts:
I am attempting to perform a Grignard reaction involving methyl 3-butenoate and methyl magnesium iodide in order to yield the unsaturated tertiary alcohol 2-methyl-4-penten-2-ol. The issue in doing this is that conditions are involved in the acidic workup of the bromomagnesium alkoxide the would also result in the acid-catalyzed hydration of the double bond (cold conditions). I am wondering if it is possible to replace the ice/sulfuric acid acidic workup with a solution of 3 M sulphuric acid in methanol, on an ice bath of course? Firstly, would this prevent the acid-catalyzed hydration of my double bond since no water is present? Also, would this prevent the acid-catalyzed dehydration of my newly formed tertiary alcohol since the conditions are cold (however, the fact there is no water, may result in forcing equilibrium concentrations, so would I have to keep the reaction at VERY low temperature like -78 C?). Secondly, If this would reaction would theoretically work, is there any literature available of anyone attempting it, I have searched and am not able to find any, also would methyl alcohol be the best choice in this case since it will be easy to rotovap in future steps? I will include the procedure I am looking to modify below along with a general scheme for my reaction (I have modified the procedure from Vogel's textobook of Practical Organic Chemistry page 540):

  Prepare an ether solution of methylmagnesium iodide from 12.687 g (522.0 mmol) of dry magnesium turnings, 74.093g (32.5 mL, 522.0 mmoles) of dry methyl iodide and 104.4 mL of sodium-dried ether; use the apparatus and procedure described in Exp 5.39, cognate preparation nonan-1-ol. Allow to cool (ice bath), and slowly add a solution of 26.66 g (261 mmol) of A in 26.1 mL of anhydrous ether into the vigorously stirred solution of the Grignard reagent. Reflux the mixture on a water bath for 1 hour to complete the reaction. Pour the ethereal solution into a mixture of 52.2 mL of approximately 2 M sulphuric acid and 196 g of crushed ice. Separate the upper ethereal layer and extract the aqueous solution with two 39.2 mL portions of ether. Wash the combined ethereal extracts with dilute sodium hydrogen carbonate solution, followed by a little water, then dry with anhydrous potassium carbonate or anhydrous calcium sulfate, distill off the ether on a water bath and distill the residue through a short fractionating column. Collect at 117-120 C. A further small quantity of the tertiary alcohol may be obtained by redrying the low-boiling distillate, filtering and redistilling. The yield is 90 g (88%).


texts: Is the total entropy change of all isothermal processes 0?

texts:
We define  total entropy as: $$\Delta S_\mathrm{total}=S_\mathrm{system}+S_\mathrm{surroundings}$$

I know that for an isothermal AND reversible expansion: $S_\mathrm{total}= 0$

But what about an isothermal expansion at a constant pressure? Does $S_\mathrm{total}$ also equal 0? If not, then would the equation look something like this:
$$\Delta S_\mathrm{total}=\frac{q_\mathrm{rev}}{T}+\frac{q_\mathrm{surroundings}}{T}$$

where $q_\mathrm{surroundings}$ is the heat lost or gained by the surroundings by a process and $q_\mathrm{rev}$ is the heat lost or gained by the system IF that process was to become reversible.


texts:
Don't ask me why, but I did an experiment where I tried to remove the magnetic iron oxide layer from a floppy disk by soaking it in HCl for a week. I put pieces of floppy disk in 0 - 1 M HCl, then measured their absorbance at 700 nm. HCl concentrations at 400 mM or more were able to remove about half of the absorbance, but not more than that.
We can even visibly see the change in color intensity.

This suggests that something is preventing the acid from dissolving the remaining iron oxide, and that simply increasing the concentration beyond 1 M won't help. However, I can still scratch the surface and remove some of the color, suggesting that it isn't buried throughout the plastic film, but still accessible on the surface. But I don't want to spend forever scratching the iron oxide off floppy disks, so I would still like a simple chemical method to remove the metal. Does anyone have any suggestions for removing it?


texts:
An article on Khan-Academy, states the following:

The overall reaction rate is determined by the rates of the steps up to (and including) the rate-determining step

Here, it says that rate is determined by steps before and including that of RDS. But from what I know the rate is determined by one single elementary reaction, which is the slowest, that is, the RDS. So why does it include steps previous to RDS?
(I would appreciate an example of this)


texts: Does changing the concentration of reactants alter the equilibrium constant

texts:
$$\ce{S(s) + O2(g) -> SO2(g)}$$
The reaction of sulfur with oxygen is written in equation form above. This equation can be interpreted in all of the following ways except
(A) either $\ce{S(s)}$ or $\ce{O2(g)}$ will be completely used up
(B) $Q$ must be close to $1.0$ since there is  one mole of gas on each side of the equation
(C) this reaction goes to completion
(D) adding $\ce{O2}$ will change the equilibrium constant

I do not know how to extrapolate $Q$ from just an equation with no other data. However, I also learned that the equilibrium constant is, well, constant, and adding O$_2$ should change the equilibrium concentrations, not the equilibrium constant.
The correct answer is (B). How would (D) be a valid interpretation?


texts: Extimating the repartition of water between hygroscopic compounds

texts:
Sulfuric acid is sometimes used as a drying agent.

According to literature, one possible application is, for instance, in the drying of bromine.

Some questions came to my mind: which concentration of sulfuric acid would be suitable for such a task? Under which concentration would the acid stop absorbing water from bromine, and there would then be an "equilibrium" between the water bound to bromine and that bound to sulfuric acid?

To put the problem in a general and less specific way: is there a way to estimate the capability of a compound to "catch" water from another compound, and in case of liquid compounds or of hemihydrated molecules, to estimate the maximum amount of water present before an equal repartition of water between the two substances occurs?

I was thinking about the enthalpies of hydration, but I honestly didn't come up with an idea for using them in such calculations.


texts: How do I cool my water bottle immediately for an emergency use?

texts:
What are the chemicals I can use to cool my water-bottle at once if I'm stranded in a desert and I have only chemicals to cool water?

(I'm keeping those chemicals outside my water bottle and not inside it.)


texts: Isopropyl alcohol to clean germs on countertops

texts:
My wife recently found some website recommending mixing 91% isopropyl alcohol, white vinegar, water, and corn starch and using it to wipe down countertops. This would be where we put our raw chicken and pork after washing them.

I'm concerned that this won't kill the germs as well as harsh chemicals would, although I am as eager as she is to replace those hard chemicals. I need to know, from a chemist, whether isopropyl alcohol really will kill all those E. coli and Salmonella and whatever else the raw poultry is leaving behind.

As a bonus, can anyone recommend any better solutions that we can make on our own than the one mentioned above?


texts:
For a certain first-order gas-hase reaction
$$\ce{A \rightarrow B + C}$$
the time required for half of an initial amount of A to decompose is 8.8 min. If the initial pressure of A is 400 mmHg, the time required to reduce the partial pressure of A to 50 mmHg is:
(a) 1.1 min
(b) 17.6 min
(c) 26.4 min
(d) 35.2 min
(e) 70.4 min

The correct answer is (c).
When I approached this problem, I started off thinking about it as a zeroth order reaction for simplicity.
$$\mathrm{400~mmHg \rightarrow 200~mmHg \rightarrow 100~mmHg \rightarrow 50~mmHg}$$
So, if this was a zeroth order reaction (not dependent on the amount of A remaining), then time would be $\mathrm{3 \cdot 8.8~min = 26.4~min}$. But, since this is a first order reaction, then [A] does factor into the half life and thus the time to reach $\mathrm{50~mmHg}$ should be slightly higher than $\mathrm{26.4~min}$.
However, this doesn't seem to be the case. Perhaps it's because we're using pressures instead of concentrations? But isn't pressure synonymous with concentration in the gas phase?


texts: Why is there no Pi-backbonding in BeCl2?

texts:
The $\ce{Cl}$ atoms in $\ce{BeCl2}$ donate a lone pair to the $\ce{Be}$ atom of another molecule so that Beryllium can complete its octet.

What I don't understand is why don't the $\ce{Cl}$ atoms donate the lone pair to the $\ce{Be}$ of the same molecule and form $\pi-$backbonds?


texts: Are specific heat capacity and boiling point of a substance related or proportional to each other?

texts: That is, if the substance has a high specific heat, will it also have a high boiling point? ( and vice versa)


texts: What is the reason that tertiary alcohol pKa is higher than primary alcohol pKa?

texts:
A tertiary alcohol has a $\mathrm{p}K_\mathrm{a}$ of 18 while a primary alcohol has a $\mathrm{p}K_\mathrm{a}$ of 16.
Shouldn't the tertiary alcohol be more acidic since its conjugate base is more stabilized due to induction?


texts: Why is Lithium Chloride (LiCl) soluble in alcohols?

texts:
According do J.D. Lee's Concise Inorganic Chemistry, LiCl is soluble in alcohols because it has some covalent nature, althought it is mainly polar.

Why would LiCl have covalent character? Why doesn't this happen to NaCl?

Also, is this explanation true? A friend of mine said this solubility was because of the polar nature of small alcohols. Which of them is right?


texts: How do reaction mechanisms go from proposed to approved?

texts: Pretty much what it says in the title, I've just been wondering lately, what makes a mechanism be recognized by the scientific community as the most likely thing that happens on the molecular/atomic level when the corresponding chemical reaction takes place in real life?


texts: Perchlorate salts: (explosive) reactivity with methane?

texts:
I read everywhere about the explosive reactivity of perchlorate salts (e.g., $\ce{Na}$, $\ce{Ca}$, $\ce{NH4}$ [especially]) when mixed with organic materials or solvents, yet I find no information about its reactivity with methane.

Can anyone comment please?


texts: Why are the oxides usually insulating?  It would be nice to get an explanation with chemistry terms (e.g., electronegativity, electron affinity, etc...) and a different explanation involving the language of band structure (i.e. energy gaps).


texts: Dipole moment of molecule (isolated vs in solution)

texts: Would you expect a dipole moment determined in solution to be different from that in an isolated molecule? If so, why?


texts:
I have been looking into certain approximations for band gap energies in quantum dots and I found a paper [1]. However, I noticed that the equations which they obtain are not useful to the situation which I would like to use them in.

What I would like to do is gather information from other articles that show the absorption and emission spectra and understand the size of the quantum dots if the sizes are not given.

I have looked at several papers where $\ce{ZnSe}$ QDs are absorbing and emitting around $\pu{\sim 300 .. 450 nm}$ with sizes from $\pu{2 .. 6 nm}$ in diameter.  However, if you use the LDA+C equation for $\ce{ZnSe}$ quantum dots (QDs) ($E_g = 2.54/d^{1.54}$) with diameters of $\pu{\sim 2 nm}$, you get band gaps around $\pu{0.9 eV}$ or $\pu{\sim 1370 nm}$, not $\pu{\sim 400 nm}$ as you would expect.

Is there something I am missing here?  Is this a $E_{11}$ transition band gap energy and I am seeing like a $E_{66}$ absorption to $E_{55}$ fluorescence energy emission?

Also, I noticed that there are exciton binding energies which may take up a large portion of the total exciton energy. Are these binding energies important to this problem?

If so, is there a better way to search for and obtain some literature on a simple correlation for these type of energy transitions?

Li, J.; Wang, L.-W. Phys. Rev. B 2005, 72 (12), 125325. DOI 10.1103/PhysRevB.72.125325.


texts: How does the melting point of alkynes change with the position of the triple bond?

texts:
$$
\begin{array}{cc}
\text{1-hexyne}&\pu{-132 ^\circ C}\\
\text{2-hexyne}&\pu{-89.5 ^\circ C}\\
\text{3-hexyne}&\pu{-103 ^\circ C}\\
\end{array}
$$

These are the melting points of six carbon chain alkynes. The boiling point of a molecule is mainly affected by its shape. So, if all this molecules have the same shape, why is the range of temperatures so high ($\pu{42.5 ^\circ C}$)?


texts: How can I calculate the percentage of urea in animal feed?

texts:
I'am having some doubts in the calculation  that concerns the urea percentage in animal feed using the method AOAC $967.07$

  How can I calculate the urea percentage using the following data:

The formula is: $\text{Urea %} = \displaystyle\frac{1.0 \times A (\text{test solution}) \times 100}{A \text{ (standard solution})\cdot \text{mg (test portion aliquot)}} $

If ($A$) absorbance of test solution = $0.187$ and its correspondence concentration from calibration curve = $\pu{0.41 mg/5ml}$.

If ($A$) absorbance of standard solution = $0.502$ and its correspondence concentration from calibration curve = $\pu{1.09 mg/5ml}$

Sample is prepared by weighing $\pu{1g}$ in $\pu{500 ml}$.

What can be the urea percentage value?


texts: Comparision of the Ionisation Energy Change of the alkali metals and noble gases?

texts:
Currently I compare the ionisation energys of the alkali metals:

Li: $\pu{0.52 eV}$
Na: $\pu{0.50 eV}$
K: $\pu{0.42 eV}$

Where only a slight drop (appr. remain constant) occurs.

This ionisation energys occur in comparision at the noble gases:

Ne: $\pu{2.08 eV}$
Ar: $\pu{1.52 eV}$
Kr: $\pu{1.35 eV}$

Here we can observe a faster drop in the ionisation energy.

This diagram visualizes the numbers:

Source: https://www.tf.uni-kiel.de/matwis/amat/mw1_ge/kap_2/illustr/b2_1_1.html

How can we explain that? My first attempt was for the alkali metals: Due to the rising effective nuclear charge (values of Clementi: Li 1.2, Na 2.5, K: 3.5) the orbital energys of the "highest" s-Orbital raughly remain constant (despite of rising quantum number n) and so the ionisation energy does (Koopmanns-Rule).

But the same explanation can be taken for the noble gases ($Z_\mathrm{eff}$ for highest s orbital: Ne 5.7, Ar 7,7, Kr 11.3) Here we have even a faster rise of the effective nuclear charge … but an immense drop of the ionisation energy!

So something is missing in my explanation. Does the energy of the s orbitals influenced by another important factor? Does somebody have a better explaination?

Data from Huheey: "Inorganic chemistry"


texts: Why does ice cream not go back to its original form upon refreezing?

texts: I came from a birthday party and someone gave me a sundae. I'm already full so I just decided to eat it at home. After arriving at home the sundae melted. So I left it in the freezer overnight. When I looked at my sundae it just formed ice crystals. It didn't went back to its original form. Why is that?. Why is it when I do this to ordinary ice cream it goes back to it's original form?


texts:
Well, according to the proton-electron ratio $\ce{O-}$ should be bigger than $\ce{F-}$

What about the charge/electron density in $\ce{F}$? Will it not affect the size of the atom of $\ce{F-}$?


texts: Why are limes alkaline, but lemons are acidic? They taste about the same

texts: So, this is kinda a dumb question, but I'm curious. Both are citrus fruits and very related to each other and taste sour which corresponds to both being acidic. Why then are limes said to be alkaline, but lemons are acidic?


texts: Why is $\ce{CrCl3}$ acidic? I know that it has a high charge density given that it is a transition metal ion, and can hence polarize neighbouring water molecules, resulting in hydrolysis and formation of $\ce{H+}$. However, I am a little confused over how this happens? Is the water molecule that it polarizes the ones that have formed a coordinate bond with it, or simply any water molecule? Also, with the high charge density, it should be attracting electrons from the water molecule. But does it mean that it attracts the 2 electrons forming the bond between $\ce{O}$ and $\ce{H}$? And if it attracts them to itself, why doesn't it form an extra coordinate bond? Why is there still only 1 coordinate bond? Another thing that I don't understand is that $\ce{OH-}$ has a lower d-orbital splitting ability than $\ce{H2O}$, doesn't that mean coordinate bonds with $\ce{H2O}$ are preferred? Why do they still polarize $\ce{H2O}$ to get $\ce{OH-}$ instead?


texts: Is there an uncertainty associated with the value 0 K for absolute zero?

texts:
When I say absolute zero, I’m not talking about the hypothetical temperature 0 K; I’m talking about the temperature at which a thermodynamic system has the lowest energy.

Everywhere I look, sites maintain that this should be –273.15 ºC, but I can’t find any uncertainty quoted with this prediction. Could someone shed some light on this?

To add a comment that I left on theorist’s answer:
I know that the Celsius scale is defined to be an easy conversion from the kelvin scale. What I mean to ask is the following: The Boltzmann constant $k_{\rm B}$ has a fixed value; the reference temperature $273.15\;\rm K$ is also a fixed value used in the relation $t/\mathrm{°C}=T/\mathrm{K}-273.15$. Therefore, how can we know that a thermodynamic system has the lowest energy at exactly $0\;\rm K$? If the triple point of water $T_{\rm TPW}=273.16\;\rm K$ has a standard relative uncertainty of $3.7\times10^{-7}$, then why shouldn’t $T_{\rm A0}$, especially since it has never been achieved? Something has to give and not be by definition, so to speak.


texts: Any popular experiment in chemistry that digital signal processing played a crucial role in?

texts: Is there any famous experiment in chemistry where digital signal processing played an important part? I don't mean using a machine that relies on such techniques (they all do) but an experiment where signals were first recorded (e.g. temperature/pressure/voltage/etc. as a function of time) and then analyzed with some sophisticated methods (e.g. time-frequency analysis with wavelets).


texts: Why does beryllium lose two electrons when it’s orbitals are already full?

texts:
I have a rudimentary understanding of orbitals, as in what they are, the shapes (l, I think) and of the principles.

So I was doing the electronic configuration for beryllium, it has 4 electrons. So 1s2, 2s2. Why does berylium lose two electrons to complete its “inner duplet”? I thought that atoms want to gain, lose or share electrons so their orbitals are complete and thus don’t react.

It’s orbitals are complete, is it something to do with the promotion of an electron to a higher energy level, but won’t that be temporary change? Or perhaps there is some fundamental misunderstanding on my part.

(I’m in my IGCSE, this isn’t in our syllabus, just trying to understand, so I hope my ignorance is forgiven)

Edit: I am not familiar with hybridisation but I don’t think it applies to this, unless you are giving a bonding example)


texts: Scission of carbon dioxide bonds using electric arc

texts:
I've done my research on this so I've seen the questions asking about the same premise without using plants.

Basically, this is a thought experiment I came up with. Whether or not I perform it depends on the feasibility. This is the reaction I'm thinking of:

$$
\ce{CO_2 \rightarrow C + O_2}
$$

From NIST the formation energy of carbon dioxide back into carbon and oxygen is given as -393.52 kJ/mol.

The basic premise I was thinking of was that $\ce{CO_2}$ could be pressurized in a chamber with electrodes striking an arc, most likely from an HF driver source. The energy imparted to the carbon dioxide could be used to provide the dissociation energy to break the carbon-oxygen bonds.

If the electrodes were designed with sufficient voltage in mind to break the dielectric constant of the gas, a few rudimentary numbers suggest that even at 5% efficiency, a sustained power input of 1kW would break the bond of 1 mol of $\ce{CO_2}$ in about 2 hours.

My concerns with the feasibility of such a process are:

Would it work? (i.e. would using an electric arc be a viable way to perform the bond decomposition? I'm not looking for an industrially viable process, just something that could produce a small end result.)
Would I have to worry about the carbon and oxygen re-forming? Obviously, this would render moot the entire experiment. I know that $\ce{CO_2}$ decomposition doesn't appear to be an equilibrium process, but it's still a point of concern.
If indeed it would work, how would the carbon likely present itself? I was thinking that if it sputtered itself against the vessel, the vessel could be weighed to determine the amount of carbon formed (and therefore the reaction efficiency, once the energy input was quantified).