texts: Does supersaturated caffeine polymerize?

texts:
I tried preparing a supersaturated solution of caffeine by mixing anhydrous caffeine powder with water in a thin Turkish tea glass and heating it up over a candle (desperate times...).

This worked as expected and a lot more caffeine dissolved into the clear solution after I slowly swung it back and forth over the flame. The water didn't boil, no visible vapors formed on the surface, and I was holding the glass with my fingers, so I would say that the temperature was well below 100°C.

After the solution cooled down, instead of crystals precipitating, I found the glass filled with a fibrous cotton-like white mass. I pulled it out (it mostly stuck together) and dried it up on sheets of paper. Now I have white flakes which are flexible instead of brittle (still feel very much like cotton).

This would seem an awful lot like some polymer forming - except the substance tastes a lot like crystalline caffeine, and melts and evaporates much like caffeine - with the vapors having the same distinctive acrid (but not burnt) smell.

I'm pretty sure the answer to the polymer question is no, but in case I'm right I'd be very curious as to what's actually happening.


texts: How does chromium(III) react to form chromium(VI) with hydrogen peroxide?

texts:
$$\ce{2Cr(OH)3 + 4NaOH + 3H2O2 -> 2Na2CrO4 + 8H2O}$$

How does this reaction occur? We suddenly get sodium chromate in the place of chromium hydroxide. I can't understand it. We mix two hydroxides and get a salt?

What role does the peroxide play?


texts: Why is tin(II) fluoride more effective in turning apatite into fluorapatite?

texts:
In more powerful toothpastes, like Crest Pro-Health, stannous fluoride is used in place of sodium fluoride.

According to Wikipedia:

  Stannous fluoride converts the calcium mineral apatite into fluorapatite, which makes tooth enamel more resistant to bacteria generated acid attacks. In toothpastes containing calcium minerals, sodium fluoride becomes ineffective over time, while stannous fluoride remains effective in strengthening tooth enamel

Is the stannous fluoride more effective in this reaction simply due to having two fluoride ions per mole, or is there something more complicated in the mechanism of the reaction in which the tin spectator ion is beneficial?


texts:
Is the carbon in a carbonyl functional group electropositive? I had a professor tell me so, but I immediately refuted him, because he defined electronegativity earlier as the ability to stabilize negative charge ... and taking electronegativity to be a converse property, the strong partial positive character on the carbon probably isn't able to stabilize any further positive charge.

Similarly, I know that lone pairs are defined as having an electronegativity of 0 because electrons cannot stabilize other electrons.

The guy agreed with me. So, to confirm, am I correct, and, as an extension, can we describe the electron deficient carbon as electronegative instead? I could very well see a electron-deficient carbon stabilizing negative charge.

However, if we stick with the IUPAC definition of electronegativity - which is the ability to attract electrons, the carbonyl carbon still seems to lose to oxygen. A charge-separated resonance structure can be drawn for any carbonyl group with a + on carbon and a - on oxygen.

So that leads me to another question - is electronegativity the ability to stabilize negative charge or the ability to attract electrons within a covalent framework? I guess the two go hand in hand - i.e. you wouldn't attract electrons if you weren't able to stabilize them ...

As a side question: can we calculate EN for ions?


texts: Good collection of images showing periodicity of the Table?

texts:
Mendeleev's recognition of the periodicity of atomic trends was a milestone of human understanding, the detection of a pattern in what had appeared to be chaotic data. I think the significance and sheer beauty of these patterns is lost on many people (layfolk and students alike). I am wondering whether anyone knows of a coffee-table book or some other collection of images that evoke the predicted nature of atomic properties such as atomic radius, ionization energy, melting point, etc. Sure, the information is available via interactive software, but I'm looking for something more aesthetic... if it exists yet.

Particularly, my search is provoked by this striking arrangement of oxidation state information. Were it to be cleaned up a bit, it would make a beautiful poster.


texts: The most acidic hydrogen among ethane, ethene, ethyne and allene

texts:
Among
$$\ce{CH3-CH3}\qquad\ce{CH2=CH2}\qquad\ce{HC≡CH}\qquad\ce{CH2=C=CH2}$$
I need to find out the one having highest acidic hydrogen.
As per my observations, there are two equivalent resonating structures possible in case of conjugate base of $\ce{CH2=C=CH2}.$ However, the correct answer is given as $\ce{HC≡CH}$ due to inductive effect of sp hybridization of carbon.
But the priority order in determining stability of a conjugate base is:

Aromaticity
Number of equivalent resonating structures
Resonance
Hyperconjugation
Inductive effect

As such, the correct answer should have been the allene. Where did I go wrong?


texts: Interstitial defects in crystals: "high energy configurations"?

texts:
In the Wikipedia article on crystallographic defects, a type of point defect known as an interstitial defect is described as follows:

  Interstitial defects are atoms that occupy a site in the crystal structure at which there is usually not an atom. They are generally high energy configurations. Small atoms (mostly impurities) in some crystals can occupy interstices without high energy, such as hydrogen in palladium.

What is meant here by "high energy configurations"? If these defects possess a "high energy configuration", then it sounds like they would be difficult to come about? If so, then how do these defects come about (where do they gain the energy from to occur)?

I would greatly appreciate it if people would please take the time to clarify this.


texts:
I believe that there are only two exocyclic bonds in this structure. But in some text its mentioned as three.
can someone clarify.


texts: When writing out synthetic reaction schemes, why are some reactions combined in one arrow?

texts:
An example of what I mean is here in the synthesis of compound 13. The reaction of the first arrow describes a single reaction, whereas the second arrow to form the final compound seems to have five stages combined into one. I understand that the compound is carried on as is without characterization when the parts are combined into a single step.

If one were designing a new synthesis, what factors would lead one to write (and characterize) each step of the synthesis versus combining many individual reactions in one apparent step?

P. S. This is not an organic chemistry question per se, but meta-ochem (sorry if my tags are limited in description).


texts:
I remember using both 95% ethanol and absolute ethanol quite a few times. You'd need absolute ethanol for a reaction that would fail if water was present, such as a Grignard reaction. I can't remember whether the absolute ethanol was in an ordinary bottle or the type with a rubber septum (where you keep the liquid under nitrogen to avoid contact with air). If the ethanol was to be used as a reagent, a septum bottle would be more appropriate.

http://boards.straightdope.com/sdmb/showpost.php?p=5314683&postcount=10

First, why would one use ethanol for a Grignard reaction? If a Grignard is destroyed by water (as suggested by the poster) then wouldn't a Grignard also be destroyed by ethanol?

I'm guessing that the poster just forgot all his chemistry. Ironic for someone posting on a message board that proclaims to be "fighting ignorance since 1973."

Second (assuming that one could somehow run a Grignard reaction in ethanol), do you really need anhydrous ethanol for Grignard reactions to work? Why not just use a slight excess of Grignard to account for the water present in 95% ethanol? Seems to me that anhydrous ethanol would be pretty expensive given that the usual distillation methods wouldn't get you from 95% ethanol to 100% ethanol.

Finally, is there a reaction in which using excess Grignard to account for acidic protons wouldn't chemically work in some way?


texts: Bonding in diatomic C2, a carbon-carbon quadruple bond?

texts:
Carbon is well known to form single, double, and triple $\ce{C-C}$ bonds in compounds. There is a recent report (2012) that carbon forms a quadruple bond in diatomic carbon, $\ce{C2}$. The excerpt below is taken from that report. The fourth bond seems pretty odd to me.

  $\ce{C2}$ and its isoelectronic molecules $\ce{CN+}$, BN and $\ce{CB-}$ (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one σ- and two π-bonds, but also one weak ‘inverted’ bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals.

  According to Shaik, the existence of the fourth bond in $\ce{C2}$ suggests that it is not really diradical...
  If $\ce{C2}$ were a diradical it would immediately form higher clusters. I think the fact that you can isolate $\ce{C2}$ tells you it has a barrier, small as it may be, to prevent that.

Molecular orbital theory for dicarbon, on the other hand, predicts a C-C double bond in $\ce{C2}$ with 2 pairs of electrons in $\pi$ bonding orbitals and a bond order of two. "The bond dissociation energies (BDE) of $\ce{B2, C2}$, and $\ce{N2}$ show increasing BDE consistent with single, double, and triple bonds." (Ref) So this model of the $\ce{C2}$ molecule seems quite reasonable.

My questions, since this is most definitely not my area of expertise:

Is dicarbon found naturally in any quantity and how stable is it? Is it easy to make in the lab? (The Wikipedia article reports it in stellar atmospheres, electric arcs, etc.)
Is there good evidence for the presence of a quadruple bond in $\ce{C2}$ that wouldn't be equally well explained by double bonding?


texts: In an electrocyclic reaction with two pi bonds that result in a trans product, is a mix of products seen?

texts: When I look at the highest occupied molecular orbital, the two methyl groups will end up being trans to each other. I realized based on which conrotation direction both methyl groups take that two different trans products are formed that do not overlap but the lecture only gave one of these products. Am I missing a concept here or missing something obvious?


texts: What is the qualitative definition of enthalpy in beginner terms?

texts:
So in my coursebook it says enthalpy is a property that accounts for the increase in internal energy and the accompanying work done by the system and that its equal to the sum of the internal energy $U$ and product of pressure $P$ and volume $V$. Also on the net, mostly on the internet it says:

  A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

And where it explains in other words I don't quite understand, e.g. as on Wikipedia. So please I would like anyone of you to explain (in beginner terms) that what is the meaning of enthalpy and heat content as used in the definition above.


texts:
I am conducting an Aldol formation reaction in water, and below is the experimental protocol I am following (from J. Org. Chem. 1994, 59, 3590—3596):

Formation of Aldol in Water using Yb(OTf)3 Catalyst
To commercial formaldehyde solution ($\pu{1 mL}$) and THF ($\pu{3 mL}$) were successively added $\ce{Yb(OTf)3}$ ($\pu{0.04 mmol}$, $\pu{10 mol\%}$) and silyl enol ether 1-(Trimethylsiloxy)cyclohexene ($\pu{0.4 mmol}$) in THF ($\pu{1 mL}$) at rt. The mixture was stirred for $\pu{24 h}$ at this temperature and then THF was removed under reduced pressure. Water was added and the product was extracted with dichloromethane. After the usual workup, the crude product was chromatographed on silica gel to give the pure adduct.

Despite multiple attempts, I was unable to get the aldol product. The procedure itself is not even that convoluted to follow. NMR shows bare starting materials. Can anyone please help me to troubleshoot my issue?
Post edit: JOC reference as follows:
S. Kobayashi, I. Hachiya, “Lanthanide Triflates as Water-Tolerant Lewis Acids. Activation of Commercial Formaldehyde Solution and Use in the Aldol Reaction of Silyl Enol Ethers with Aldehydes in Aqueous Media,” J. Org. Chem. 1994, 59(13), 3590–3596 (DOI: 10.1021/jo00092a017).


texts: How can difference in neutron number cause a difference in ionisation enthalpies?

texts:
It’s a well known fact that protium,deuterium and tritium have the same number of electrons and protons but different number of neutrons. It is expected that their ionisation enthalpies should be same as in all the cases only one proton attracts the only electron they have but their enthalpies are different.

What causes this?


texts: Is dissolving salt a chemical reaction or a physical change?

texts:
Physical changes are limited to changes that result in a difference in display without changing the composition.
  (source)

e.g. melting water or dissolving sugar in water

  Chemical reactions are the processes by which chemicals interact to form new chemicals with different compositions. (source)

e.g. burning $\ce{H2}$ to $\ce{H2O}$

But what is dissolving salt in water? Is that a physical change (like dissolving sugar) or is it a chemical reaction?


texts:
I have basically written the entire name except for the oxidation state of $\ce{Co}$.

  Mercury tetrathiocyanatocobaltate(  )

to know what is the oxidation state of cobalt, I must know the oxidation state of the mercury cation. But mercury can have $+1$ and $+2$ oxidation states. How do I know which one is it in this compound?


texts: Infrared Spectroscopy of Hexachlorocyclotriphosphazene

texts:
When assigning the IR spectrum of hexachlorocyclotriphosphazene there are two prominent peaks (one due to the nitrogen-phosphorus bond and the other due to the phosphorus-chlorine bond).

There is one peak at 1179cm⁻¹ and another at 594cm⁻¹. I think that the peak at 1179cm⁻¹ is due to the phosphorus-nitrogen bond and the peak at 594cm⁻¹ is the phosphorus-chlorine bond.

My reasoning for this is that the phosphorus-nitrogen bond is shorter and therefore stronger thus it will have the higher stretching frequency.

Is my reasoning correct?


texts: How to find the moles of a salt added to a weak base, given ph

texts:
I would really appreciate some help with how to approach this:
I'm given that:
Methylamine (CH3NH2) is a weak base used in the manufacture of pesticides. Taking into account that the degree of dissociation of said compound is 21%, calculate:
Kb=6.1*10^-4  (This is likely an arbitrary number assigned to each student, rather than being the correct Kb, as is the degree of dissociation).
a) The initial concentration of methylamine necessary to prepare 175 mL of solution.   -  This I have managed to solve, my answer is 0.01093M, the autocorrect mechanism says it's right
Edit  -  Here is what I've done:
CH3NH2(aq) |  +     |   H2O    |  <>     |   OH-   |   +   |   CH3NH3+(aq)
 1      |              -   |                 0   |       |         0     |
 1-α    |              -   |                 +α  |       |         +α    |
Co(1-α) |              -   |                 Coα |       |       Coα     |

Kb= 6x10^-4 = (Co*.21)^2/Co(1-.21)
Solving this quadratic equation yields Co = 0.01093M;  Which is correct according to the autocorrect mechanism.
b)How many moles of methylamine chloride (CH3NH3Cl) must be added to the previous solution to obtain 217 mL of final solution with a pH = 11.16?      -   This is what I cannot fathom, I've tried making ICE charts and the Henderson-Hasselbalch equation, though I'm not having any luck, it's likely that I am doing it wrong though
Edit 2
This I've tried many times to do and feel like I'm going round in circles, hence this post.
I need to create from the previous solution above 217ml of a new buffer solution with Ph 11.16.
Therefore:
POH = 2.84   so;
10^-2.84  =  0.001445439
Which should be the OH- in the new solution.
POH  =  Pkb  + log [conjugate acid / weak base]
Kb =  6.1x10^-4
PKb = -logKb  =  3.21467
-0.37  =  Log[CA] - Log [0.01093]
10^-2.34 =  0.004612644 M
Since I need 217ml, I multiply;  0.004612644  by;   .217   giving; 0.00100943 moles of conjugate acid.  This is wrong according to the autocorrect.
I have tried doing an ICE chart again , which I won't rewrite here, and I obtained a similar figure.   That said, I have also obtained other figures which are also wrong.  So I think there must be some flaw in my reasoning here, which I haven't been able to identify myself.
Edit 3   -  I won't comment on this part since, it flows from the previous part.   Any comments will be hugely appreciated!
If 2.71 mL of 0.351 M HNO3 are added to the solution in part b), what will the pH of the final solution be?      This I feel more confident about, but cannot calculate it without first getting b)
Thanks in advance!
Edit 4  -  Base at equilibrium:

****CH3NH2(aq)
+
H2O
<>
OH-
+
CH3NH3+(aq)****

0.01093

-

0

0

-.0022953

-

0.01093 x.21

0.01093 x.21

0.0086347

-

0.0022953

0.0022953

Kb=6.1x10^-4
Desired OH- =  0.001445439
Kb = [OH-][BH+]/[CH3NH2]
BH+ = 6.1x10^-4(0.0086347/0.001445439)   =  0.003643991M
0.003643991M   x   .217  = .000790746 moles needed (wrong according to autocorrect)
Likewise
2.84  =  Pkb  + Log(conjugate acid/weak base)
[CA] =  (0.0086347/10^.374670165) = 0.003643989M
0.003643989M  x.217  = 0.000790745 moles (basically the same number)
There must be something else I am getting wrong, could it be to do with the volume, changing from .175L or original solution to .217L of new solution
Edit 5
I finally got it, the problem was I was using concentrations for the calculations, and since the volume changes, I should have been using moles.


texts:
Is there a way to bring back into solution a bottle of formalin that has crystallized? The bottle has been stored for several years at room temperature.
Thanks in advance.


texts: What are the effects of Ag/AgCl electrode liquid interface contamination?

texts:
I have been using a $\ce{Ag/AgCl}$ electrode with a porous teflon tip to measure open circuit potential between two electrodes separated by a membrane. I am worried that reagents such as iron chloride and ferri/ferrocyanide have contaminated the teflon tip because we have not been using a salt bridge.

However, when I place two reference electrodes in the same solution, the difference in potential between them is very low, ~2 mV.

What effect could having reagents in the teflon tip have on my measurements?


texts: Balancing the oxidative decomposition of FeS2

texts:
Doing some practice for the GRE next year.

Please could you look at my working for this and tell me why I'm wrong?

$$\ce{? FeS2 + ? O2 + ? H2O = ? Fe(OH)3 + ? H2SO4}$$

My working so far is:

There must be the same number of of $\ce{FeS2}$ as $\ce{Fe(OH)3}$
There must be twice as many $\ce{H2SO4}$ as $\ce{FeS2}$
If there is one mole of $\ce{FeS2}$ for example, there must be 1 mole of $\ce{Fe(OH)3}$ and two moles of $\ce{H2SO4}$

Tot up the $\ce{H}$ on the right: $1\times 3 + 2\times 2 = 7$. $7$ on the right so $\ce{3.5 H2O}$ on the left. ($7/2 = 3.5$)

$\ce{O}$ on the right = $1\times 3 + 2\times 4 = 11$.

This mean $11$ on the left, too. I already have $\ce{3.5 H2O}$, so we must have $11-3.5 = 7.5$ $\ce{O}$'s from the $\ce{O2}$. So $\ce{3.75 O2}$ because $7.5/2 = 3.75$.

Following the rules I've written down I arrive at:

$$\ce{1.75 FeS2 + 3.75 O2 + 3.5 H2O -> 1.75 Fe(OH)3 + 3.5 H2SO4}$$

I've multiplied these by $4$ to give integers, because that's what the question asks of me, so $7, 15, 14, 7$ and $14$.

Why am I wrong?


texts: Normally phenols have the $\ce{OH}$ group at the 1 or 4 position. Can they be made at 2 or 3 positions, where nitro groups and $\ce{-CN}$ can attach? If not, why not?


texts: Can the 5+ and 4+ oxidation state of Vanadium ion exist independently?

texts:
From my class text book:

  The oxidation states of Vanadium ions 5+ and 4+ are shown in the compounds $\ce{VO2^{+}}$ and $\ce{VO^{2+}}$  respectively, whereas $\ce{V^{3+}}$ and $\ce{V^{2+}}$ are shown like this in the redox reaction (reduction half equation).

Why is this? cannot $\ce{V^{5+}}$ and $\ce{V^{4+}}$ exist indepepently as ions like $\ce{V^{3+}}$ and $\ce{V^{2+}}$ (when measuring the reduction potential)?


texts: Could Calcium Oxide be made from CaCO3 in a bonfire?

texts: I have some chalk and I need to turn it into CaO. Would a big bonfire be hot enough to make it? If anyone has personal experience with this reaction please share, whether succesful or not


texts: How does shock trigger nucleation of gases dissolved in a liquid?

texts:
When you drop a bottle of soda, the dissolved $\ce{CO_2}$ starts nucleating. Opening it before it's 'settled' will trigger more nucleation and usually makes a mess.

Why does the shock trigger nucleation?

Secondly, is it the same mechanism in play when you shake a bottle? And what about the physical state changes as it 'settles'? i.e., why does the $\ce{CO_2}$ rapidly nucleate if you open the bottle right after you drop it, but it doesn't if you wait a bit?


texts: Calculating charge of amino acids (am I doing it right?)

texts:
Please consider this picture of a peptide chain:

I've done some examining on the different amino acids and concluded that the chain is as follows:

Lys-Leu-Gly-Ser-Citrullin (variant of arginine).

I would now like to calculate the charge of this peptide at neutral pH.

I'm using this table to orient myself: http://www.sigmaaldrich.com/content/dam/sigma-aldrich/life-science/biochemicals/migrationbiochemicals1/common-amino-acids-table.jpg

So here's my train of thought:

As we're at neutral pH the +1 and -1 of the mandatory amino and acid groups will cancel out for all amino acids.

Lysine has a pKx of barely 11 which is some higher than 7 so it is basic in our solution. It means that it will accept a proton so Lysine has a charge of +1 in our respective solution.

Leucine, Glycine and Serine have no charge on their x group so their charge is 0 and does not contribute anything to the total charge.

Citrullin, I read, is uncharged at neutral pH, opposed to Arginine which is, so it does not contribute either.

So my total answer is +1. Is this correct? I am in no way 100% sure of my method but it would be great to get a confirmation that I am doing it correctly or to be corrected on my possibly faulty method.

Thank you!


texts:
My professor is letting me use the autoclave in the future, but I am a novice. I asked him what materials were safe to autoclave for my notes, materials meaning glass or plastic-ware. He stated, "It depends on alot of factors, but some hard and opaque plastics are generally ok, also Teflon products."
I couldnt really ask him any more questions since it is still winter break and I guess he isnt responding to emails, so I decided to google the issue.
I found this source of an autoclave guide at: https://uwaterloo.ca/safety-office/sites/ca.safety-office/files/uploads/files/guideline-for-safe-use-of-autoclaves.pdf

Good Choices:
borosilicate glass (Pyrex) has very low thermo
expansion property and therefore resistant to breaking due to heating
polypropylene (PP) and polycarbonate (PC) are heat resistant plastics
stainless steel is a good heat conductor and thus facilitates
sterilization
Poor Choices:
polystyrene (PS), polylethylene (PE) and high density polyethylene (HDPE) do not resist heat well.
if there is a risk of material melting ensure they are placed in a
secondary container which is resistant to heat.

Would this be an accurate  summary of what plastics / glassware to use in an autoclave? I would ask about the validity of the website itself, but I assumed it was reliable because it was based off of a college.


texts: How to identify an unknown compound with spectroscopic data?

texts:
I need some help identifying the following unknown compound. Any suggestions or hints regarding the possible compound would be highly appreciated.

So far I am thinking the broad peak on the IR is an alcohol that somehow mixed with possible $\ce{sp^3-CH}$ bonds. And the peak around $\pu{1600 cm-1}$ is I think a $\ce{sp-C-C}$ bond. The compound should have four distinct groups of protons. Beyond this basic analysis, I couldn't really come up with a compound.

I have updated (07/09/14) the question with a newer NMR (below) and an IR.

Some other additional info I worked out in the lab:

look: white powder like solid.
melting point: $\pu{116.3-119.0 ^\circ C}$
Solubility:

Water - insoluble
NaOH - insoluble
Sulfuric Acid - soluble

Classification Tests:

Water insoluble phenol test - positive; a dark red precipitate formed.
Chromic acid test - positive; green precipitate formed.
Bromine test for unsaturation - positive; bromine discoloration happened until about $\pu{2 ml}$ of bromine was added. White precipitate formed.

This is my first time doing these tests so please don't consider their results too strictly. My Instructor said the IR is a much safer way to determine functional groups.


texts: Why is that in an external magnetic field(uniform) the degeneracy of d,f orbitals is lost but the degeneracy of p orbitals remain intact if the main cause of losing degeneracy is the difference in energy of differently oriented orbitals in the magnetic field?


texts: Hartree-Fock vs. DFT vs. Hohenberg-Kohn vs. Kohn-Sham

texts:
What is the difference between these four approaches?

My current understanding is that Hartree-Fock uses only 1 slater-determine (accounting for antisymmetry) but neglects correlation between electrons. HF orbitals are primarily determined self-consistently.

DFT is the idea that there is a functional (presently unknown) that maps energy in terms of density. Hohenberg and Kohn found a proof that it exists but did not actually find it.

$E[p] = T[p] + E(EE)[P] + E(EN)[P]$

I am rather confused about where kohn-sham theory fits into all of this. Is it's entire purpose simply to define kinetic energy? How does it do this?


texts: Boiling temperature of salt water at low pressure

texts:
With a hydro aspirator with re-circulating water (see here), the maximum vacuum you can get is set by the water vapor pressure given the temperature.

Can you lower the vapor pressure measurably by adding salt to the water (I get −700 mmHg, at 25 °C, with regular tap water already)?


texts:
Wood countertops are frequently sealed, when a non-hardening, food-safe finish is desired, with some magic cocktail of waxes and oils. Typical components might include mineral oil, beeswax, and carnauba wax, mixed in some unspecified ratio and then sold at premium prices.

Not being content paying for a bottle of magic goo, I'm seeking a scientific understanding of how these products actually work.

These products claim to (over time) build a coating of wax on the surface which is more durable than simply mineral oil would be. If the product is a mixture of oil and wax which is quite thin, how can it build a layer of wax on the surface of the wood with repeated applications? Does the wood selectively absorb the oils? When subsequent applications are made, are the thinner oils able to diffuse through whatever wax has built on the surface?

What are the relevant concepts in play here, and in what directions might I research to gain a deeper understanding?


texts:
I was doing experiment of estimation of copper and nickel ions by means of volumetric and gravimetric analysis respectively.

Suppose we analysed $\ce{Cu^2+}$ ions by precipitating with $\ce{H2S}$
But still there are some copper ions remaining in solution.

Is there possibility that they ($\ce{Cu^2+}$) can form complex with DMG
and interfere with Ni-DMG complex.


texts: Conversion factor for chemical to physical scale of atomic masses

texts:
Before the 1960s chemists and physicists used to have different atomic mass tables. The main difference was that the physicists assigned the $^{16}$O as the O=16 from mass spectrometry. Chemists on the other hand were using the ordinary oxygen which is a mixture of $^{16}$O, $^{17}$O and $^{18}$O as O=16. Of course, this confused everyone before carbon-12 was suggested as a compromise as both parties were not willing to budge. The % abundance of $^{16}$O is 99.76, $^{17}$O is 0.04, $^{18}$O is 0.2.

People were able to develop a conversion factor between a chemist's atomic mass and a physicist's atomic mass tables. A book by Guggenheim "Physicochemical Calculations" shows the example how the conversion factor was obtained. He doesn't explain anything clearly.

Could anyone shed some light, how the author is calculating this factor? I assume by $$\frac{O}{^{16}O}$$ the author mean how heavy is ordinary oxygen with respect to true $^{16}$O . I don't know where 1 is coming from and why there is a 2 besides O-18 isotope.


texts: Factors affecting mass of a gas-evolving reaction mixture over time

texts:
Excess magnesium was added to a beaker of aqueous hydrochloric acid on a balance. A graph of the mass of the beaker and contents was plotted against time (line 1).
What change in the experiment could give line 2?

The same mass of magnesium but in smaller pieces
The same volume of a more concentrated solution of hydrochloric acid
A lower temperature

Original question:

The answer is just II. However, I thought that both I and II would answer the question. Why is it just II?


texts: Stability order of chelate complexes of nickel and iron

texts:
Which stability order is correct?

$\ce{[Ni(en)2]^2+} > \ce{[Ni(dmg)2]}$
$\ce{[Fe(edta)]^2-} > \ce{[Fe(en)3]^3+}$

In the first case I think the stability depends upon the strength of the ligand so is $\ce{en}$ stronger than $\ce{dmg}$ to form a more stable compound? As the oxidation state of nickel is same for both the cases.
In the second case the oxidation state of iron is +2 and +3, respectively. But I am confused if the metal with higher oxidation state is more stable or the metal with lower oxidation state is more stable?
Does stability depends upon the number of chelating groups?


texts: Is writing D-(+)-glucose more correct than D-glucose?

texts:
In one of my books glucose is written as D-(+)-glucose everywhere.
D represents the position of
-OH group with respect to glyceraldehyde..
So D = (+)-Glyceraldehyde..
So won't writing D-(+)-Glucose make no sense as we have already mentioned the position??
Please Help..


texts: Why does the electrical conductivity increase as the temperature decreases?

texts:
I am measuring the dependence of electrical conductivity of ( NaCl and $\ce{CH3COONa}$ solutions  with concentration 9g/L) with temperature using conductivity meter.

Theoretically the conductivity should decrease as the tempreture decrease, but unfortunately that does not seem to happen.
As you can see, as I start measuring conductivity at  $\pu{85 ^\circ C}$ it decreases with decrease in temperature till $\pu{65 ^\circ C}$ ( the first three data) then again start to increase.
I repeated the experiment again and again but am getting the same result .
The conductivity start to increase around $\pu{65 ^\circ C}$! Why is this happening?


texts:
$\ce{HCN}$ is notoriously unstable: It boils at room temperature, and it tends to polymerize when concentrated in unbuffered liquid form.  Yet $\ce{HCN}$ was for decades widely used as a fumigant pesticide.  At least one commercial formulation called Zyklon became notorious after it was employed for genocide by Nazi Germany.

I was wondering what chemistry allowed large masses of $\ce{HCN}$ to be bound (presumably adsorbed?) to a solid (Zyklon was known to consist of pellets), put in cans, and then released simply by opening the cans?

The best research I have found concludes of the solid substrate:

  Zyklon-B is composed of calcium sulfate but not the soluble anhydrite of calcium sulfate.  Due to its microcrystalline structure it is possible to determine that the Zyklon-B sample is either the natural form of anhydrous calcium sulfate, also known as the mineral anhydrite or, equally likely, the insoluble anhydrite resulting from heating gypsum at temperatures above 650 degrees centigrade. Both of these forms preserve the microcrystalline structure.

Is it known, or deducible, that calcium sulfate will adsorb large masses of $\ce{HCN}$, hold it in a "shelf-stable" (nonreactive) manner when enclosed, and then release it completely at STP?

(E.g., I don't know anything about adsorption of organics, but assuming that $\ce{CaSO4}$ has adsorbed $\ce{HCN}$ to saturation then, since calcium sulfate is a powerful desiccant, I can imagine that on exposure to a humid atmosphere it would preferentially absorb water and the resulting $\ce{CaSO4·2H2O}$ must not be capable of adsorbing $\ce{HCN}$?)


texts: When writing a paper should I describe the procedure thoroughly?

texts:
I have not written any article on a chemistry subject yet, and I want to use good practices from the start.
What I have noticed reading papers, is that a lot of times it is not specified the exact procedure that is followed in a reaction, for example by saying the exact tools that are used, or the parameters used in a technique, not even in supporting information documents.
At first I thought that some information was not really relevant, but as I read more articles I found more things like “X product is centrifugated” or “Y is sonicated”, without further detailing the exact procedure that was followed, the set parameters or even the devices that were used, which can obviously be very important for a given reaction.
My opinion would be that the more information the better and I think that by omitting that information you are wasting time and resources to any researcher that tries to replicate your experiment, as well as making it difficult to check the replicability of the procedure.
Then my question is, how far should I go in describing the procedure? Should I mention even which glassware I use for a given reaction? Maybe sometimes it is pretty obvious what should be used, but it is additional information that does not hurt to know and may be useful for obtaining the exact same results. However, it is as far as I have seen not a common practice and there must be a reason behind that.
Maybe omitting this information is preferable in articles as it uses less words and it avoids making articles excessively wrong, if that is the case, what about reports or thesis with a less restricting length limits?
Thank you for your time.


texts: Solubility comparison of hexan-1-ol and cyclohexanol in water

texts:
Is hexan-1-ol or cyclohexanol more soluble in water? The answer is supposedly cyclohexanol. However, I found a statement in a book:

  Isomeric alcohols have solubilities in the order: primary alcohol > secondary alcohol > tertiary alcohol, due to decrease in polar character.

By this statement, I guessed the answer to be 1-hexanol, as it is a primary alcohol whereas cyclohexanol is a secondary alcohol. What mistake have I done?


texts: To get clear plasma out of a minute particle (such as fibrin), is it necessary to vortex sample before centrifugation?

texts: I'm going to analyse plasma samples that contain a lot of fibrin. It is necessary to centrifuge the samples to be able to pipette the plasma. After the samples are centrifuged I can't vortex them because then I mix everything again and it wont be possible to pipette. Now I wonder whether it is necessary to vortex them before I centrifuge them. I don't think so because during centrifugation particles will anyway be separated by density. Or am I wrong? And in that case why? Appreciate your thoughts!


texts: Van't Hoff factor for citric and phosphoric acids

texts:
I am trying to determine the van't Hoff Factor for citric and phosphoric acids.

When phosphoric acid is in aqueous solution, it dissociates as follows:

$$\ce{H3PO4 ->H2PO4- + H3O+}$$

Does this mean its van't Hoff factor is 2? For citric acid I cannot find what it dissociates into anywhere, so I am unable to determine the van't Hoff factor.


texts: Why should the scalar product of the components along two axes remain unchanged after transformation?

texts:
A couple of proofs from Chapter 2 of "Space Groups for Solid State Scientists" are giving me a hard time (see attached image).

So what I understand is:
"r" was the original lattice point. "r'" is the lattice point r after the mirror operation, which should also be a point of the original lattice. I also have no trouble with how the transformation r->r' works, and how dot products being zero imply a right angle between the vectors.
So why should the two dot products be equal? And how should one interpret this dot product intuitively (component of one axis onto another?) ? (I can kind of visualize how the mirror image of a triclinic lattice wouldn't coincide with the original if one of the axes is not perpendicular to the other two, but how should one prove it rigorously, and how does the aforementioned proof work)


texts: I have a chamber containing two different gases and a liquid which needs to be heated at a certain temperature to bring it to vapor state.  I have worked out that we can calculate the total pressure of a mixture of gas using Dalton's law using partial pressure of each gas. To calculate total vapor pressure we have Raoult's law. If I have both gases and vapors in the medium can we calculate the total pressure by adding them both, as in a "collecting gases over water experiment"?


texts: Can chloride attack the carbonyl bond in formaldehyde in the reaction with hydrochloric acid?

texts: I already know that a cyclic trimer and polymer of formaldehyde is formed at lower temperatures, but would chloromethanol and dichloromethane be possible at higher temperatures?


texts:
I am struggling with a calculation I should know how to do, but I feel like I may be missing crucial information. Here's the outline:

I dissolve $\pu{6.0 g}$ of $\ce{HK2PO4}$ in $\pu{20 mL}$ of water. I then add $\pu{8.0 mL}$ of $\pu{4 M}$ $\ce{HCl}$ and dilute to $\pu{30.0 mL}$. I take $\pu{25.0 mL}$ of the resulting solution and adjust the pH to 3.5 with $\pu{1 M}$ acetic acid.

What volume of acetic acid is theoretically necessary to reach pH 3.5?

Experimentally, I attempted this and the initial pH of the $\pu{25.0 mL}$ aliquot was 5.71. Technically, I should have been able to reduce the pH to 3.5 using less than $\pu{15.0 mL}$ of $\pu{1 M}$ acetic acid. However, I observed buffering effects around pH 4.0 and the total volume required was $\sim\pu{50.0 mL}$ of $\pu{1 N}$ acetic acid.


texts: In cracking, why does C form double bonds?

texts:
I've been reading about cracking and I've found various images and animations that show a process like this:

But I haven't been able to find out the sequence of events to get to this, and why this would occur.

I understand that cracking involves thermal decomposition, where a CC bond is cut. But that's as far as I can truthfully say I understand. To get this reaction below, you need:

(1) Two cuts. One cut would be between two Cs, and the other cut between a C and a H. Why does a H have to be cut off as well? Is this example I found actually one of many random and messy ones that just happens to 'give us what we want in the end'?

(2) The free H would then have to join the alkane. I get why this happens.

(3) A double bond is made. Why?


texts: Why is the solubility of calcium sulfate so low even though it is a salt?

texts: What is the reason the solubility of calcium sulfate in water is so low? Compared to other salts like potassium sulfate (110 g/L) or calcium nitrate (1470 g/L) the solubility of calcium sulfate seems to be very low (2.4 g/L).


texts: Is it possible to use diatomic carbon to utilize a new type of fuel that takes away the waste product of carbon dioxide being released into the air?

texts:
These days, we as a community what to polish and make our world a greener environment. I found a way to accomplish this task, however I don't know if it is applicable. I am wondering if the use of diatonic carbon would do the trick to produce a process that could regenerate the fuel in our vehicles and eliminate the releasing of carbon dioxide into the air. The purpose of this reaction would be to remove the Carbon Dioxide waste that is created by gasoline. Gasoline waste contains one less carbon than was originally in the reaction

(UNDER CONSTRUCTION)


texts: Conception and manufacture of the Avogadro Project's (nearly) perfect silicon sphere (and, what is “atomic feeling”?)

texts:
The Avogadro Project, an effort prior to the recent redefinition of Avogadro's constant to design a nearly perfect sphere of silicon-28, apparently relied on the fine fingers of optician Achim Leistner - at least according to the Wikipedia:

  Achim Leistner is an Australian optician of German origin.3 During
  his retirement, he was asked to join the Avogadro project to craft a
  silicon sphere with high smoothness, as automated machining does not
  match his precision.3

  In addition to precision instruments, Leistner uses his hands to feel
  for irregularities in the roundness of the sphere.3 The research
  team has called his extraordinary sense of touch "atomic feeling".3

I suppose this is largely a set of questions about machining and metrology, but materials science is important too. Also, because of the relevance to the redefinition of $N_\text{A}$ I thought it appropriate for this site. With all due respect for Achim Leistner's amazing fingers:

How was this sphere conceived and crafted? For instance: how was the size of the sphere decided? How was the material selected? What is the concentration of defects in the silicon that allows assumption that the lattice is perfect? How is this degree of quality guaranteed? Why can't "automated machining" match the precision of (assuming the alternative) "machining by hand"? What is this purported "atomic feeling"? Is this PR talk, to make the manufacture of the sphere seem more mysterious?


texts: How many electrons with l = 1 does Si in its ground state have?

texts:
I was solving practice problems for electron configuration and periodic table, and I got stuck through a question:

  An atom of silicon in its ground state has how many electrons with quantum number $l = 1$?
   a) 14
   b) 2
   c) 8
   d) 6
   c) 28

According to the solution the answer is c: 8 electrons, but that wasn't my answer.

My analysis:

Silicon is a chemical element with symbol $\ce{Si}$ and atomic number 14, this is its electron configuration:

$$\mathrm{1s^22s^22p^63s^23p^2}$$

The quantum number $l = 1$ corresponds to $\mathrm{p}$ levels only. So we have:

$$\mathrm{2p^63p^2}$$

The total number of orbitals is: $6 + 2 = 8$ orbitals.
Since each orbital has 2 electrons, the total number of electrons is $8 \times 2 = 16$ electrons.

What's wrong with my answer? Maybe the question has a wrong answer?


texts: When can a Secondary standard solution be used for standardising something instead of a Primary?

texts:
I am writing an assignment on 'Preparing and Using Standard solutions for Use in Analysis in Titrations and Spectroscopy' and one part is asking about primary and secondary standard solutions.

I know the different between them and have stated what they are and some examples, but one question I have to answer is 'Explain why Primary standard substances are not always used in standardization, and why Secondary standard substances are used instead'.

I have looked at many sources online and they all just tell me the differences between primary and secondary, was wondering if anyone could shine some light on why a secondary standard solution would be used to standardize a solution instead of a primary?


texts: Why only ketone forms enolate in Claisen condensation between ester and ketone?

texts:
Claisen condensation involves nucleophilic substitution in esters by an enolate (formed by removing an alpha-H from an ester or a ketone) giving a beta-keto ester or a diketone.

My textbook says that for the reaction between ketone and ester, the ketone always forms the enolate by losing it's alpha-H . But why can't the ester form the enolate here? What will happen if the ester has alpha-Hs and the ketone dosen't?

Attempts:

Does it the O on the alcohol side of the enolate ester help in delocalizing the negative charge via an inductive effect making it less effecive as a nucleophile?

(I can't find anything on other sites I've seen:

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch21/ch21-3-3.html

https://en.wikipedia.org/wiki/Claisen_condensation

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Organicka%20chemia%20II/Prednaska%205_Aldehydy%20a%20ketony_Reakcie%20II/Doplnkove%20studijne%20materialy/Carbonyl%20compounds_full_Blackmond.pdf

https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Reactions/Organic_Reactions/Claisen_Condensation )


texts: Can you find the mass of solvent with mass of solute, volume of solution, and solution density?

texts:
If you have, for example, $\pu{10 g}$ of a solute dissolved into $\pu{100 ml}$ of a solution, and the solution density is $\pu{1.5 g ml^-1}$, can you just multiply $100\ \mathrm{ml} \times 1.5\ \mathrm{\frac{g}{ml}}$, then subtract the $\pu{10 g}$ of solute from the result to get the mass of the solvent in grams?

Does that work or am I trying to do do this wrong?


texts: How is hair tissue mineral analysis performed?

texts: This might be a mixed chemistry / physics question, but I've always wondered what methods were used to precisely determine the levels down to trace amounts of all minerals in human tissues. I know there's a tissue preparation phase, in which I am interested though less than in the details of the actual final testing methods and processes to determine those trace amounts of all minerals.


texts: Calculating specific heat capacity from entropy

texts:
I'm currently struggling with a specific thermodynamics problem.
I'm given the entropy of a system (where A is a constant with fitting physical units): $$S(U,V,N)=A(UVN)^{1/3}$$
I'm asked to calculate the specific heat capacity at constant pressure $C_p$ and at constant volume $C_V$.
I know that the two are given by the following equation:
$$C_P=T \left(\frac{\partial S}{\partial T}\right)_{N,P}$$
$$C_V=T \left(\frac{\partial S}{\partial T}\right)_{N,V}$$
I've tried to calculate a expression for the entropy in terms of the temperature $T$ by calculating $$\left (\frac{\partial S}{\partial U}\right)=\frac{1}{T}=\frac{1}{3}(NV)^{1/3}U^{-2/3}=\frac{1}{3}\frac{S}{U}$$
but I think i'm going in a circle with this approach.
I guess I'm also having problems with the notation $\left(\frac{\partial S}{\partial T}\right)_{N,P}$ vs $\left(\frac{\partial S}{\partial T}\right)_{N,V}$. I know that this should mean "partial derivative of $S$ with respect to $T$ with constant $N$ and $P$ or $V$". But I'm not quite sure on how to evaluate that mathematically. For me personally, if I were to evaluate this two partial derivatives, they would be equal to eachother. But this can't be really true because of the relationship $$C_P-C_V=\frac{TV\alpha_P^2}{\kappa_T}$$
It would be awesome if anyone could give me starting point for this kind of problem because I'm kinda stuck with my method right now. Any help would be very much appreciated


texts: Why does SO2 have a larger bond angle than H2O, H2S, and NH3

texts:
So we have to find which of the follwing compounds has the smallest bond angle:
$\ce{H2O}$, $\ce{H2S}$, $\ce{NH3}$, $\ce{SO2}$.

So L $\propto \frac{1}{BA}$
where $L$ is the number of lone pairs and $BA$ is bond angle

So the number of lone pairs is:-

$\ce{H2O}$: 2

$\ce{H2S}$: 2

$\ce{NH3}$: 1

$\ce{SO2}$: 2

Since $\ce{NH3}$ has the fewest lone pairs, it must have the highest bond angle, so it is out.

Also $BA \propto ENC$ where $ENC$ is the electronegativity of the central atom.
The electronegativiy of the O atom in $\ce{H2O}$ is the highest, so it is out.

Also $BA \propto \frac{1}{ENS}$ where $ENS$ is the electronegativity of the surrounding atom. The electronegativiy of the H atom in $\ce{H2S}$ is the lowest of the remaining two. The only one left is $\ce{SO2}$ which must have the lowest bond angle but when I checked on google it had the highest bond angle. How is that possible?


texts: Forces of attraction of hydrated potassium ion in aqueous solution

texts:
I want to study the intermolecular forces (IMFs) in hydrated potassium ion, $\ce{K+ (aq)}$ in an aqueous solution of $\ce{KCl}$. According to my thoughts, among the 4 IMFs I know:

London dispersion forces (LDFs);
Dipole-Dipole interaction;
Hydrogen bonding;
Ion-dipole forces

LDFs are definitely included, but my question is whether dipole-dipole is included as well (because KCl is a polar substance)?


texts: Genuine confusion about the equilibrium constant

texts:
I gather that the equilibrium constant K = 1 implies that, at equilibrium, neither the forward nor the backward reactions are thermodynamically favoured. But if pressure of the system increases and this shifts the position of equilibrium, the value of K remains to be 1.

How then can K = 1 imply that, at equilibrium, neither the forward nor the backward reactions are thermodynamically favoured?

Please can I have a simple and comprehensive explanation of what the value of K actually tells us?


texts: How is knowing about the symmetry of a crystal useful

texts: I am studying the space groups and know that we use them to talk about crystal symmetry, I would like to know what material properties may be produced or predicted from knowing the symmetry or lack thereof in a crystal system of a material?


texts: The function of sulfuric acid in a reaction involving halide ions

texts:
The purpose of the following reactions is to make halogen gas:

  \begin{align}
\ce{2 NaCl (s) + MnO2 (s) + \boxed{\ce{2 H2SO4 (l)}} &-> Na2SO4 (s) + MnSO4 (s) + 2 H2O + Cl2 (g)} \\
\ce{10 KI (s) + 2 KMnO4 (s) + \boxed{\ce{8 H2SO4 (l)}} &-> 6 K2SO4 (s) + 2 MnSO4 (s) + 8 H2O + 5 I2 (g)} \\
\ce{6 KBr (s) + K2Cr2O7 (s) + \boxed{\ce{7 H2SO4 (l)}} &-> 4 K2SO4 (s) + Cr2(SO4)3 (s) + 7 H2O + 3 Br2 (g)}
\end{align}

And so we need to use a strong oxidizing agent (in these examples: $\ce{MnO2}$, $\ce{KMnO4}$, $\ce{K2Cr2O7}$) to oxidize halide ions out of their compound.

What I don't understand is the reason why we would need to use sulfuric acid in the reaction if there's another oxidizing agent available and why sulfuric acid is commonly used in this specific redox reaction. Can we use another strong acid instead?


texts: Can I express the rate of a reaction as percent per second (%/s)?

texts:
Rate is calculated as change in concentration over change in time, so usually $\pu{M/s}$ $(\pu{mol L-1 s-1}).$

Say, I'm calculating it for $20\%$ solution of ammonium hydroxide, so my final rate is in $\pu{\%/s}.$ Is this a proper format, or should I calculate for $\pu{mol L-1}?$


texts: Emergence of Lognormal distribution for the concentration of chemical compounds

texts: I'm currently reviewing the literature about lognormal distributions describing/approximating the variability of a given chemical compound across different cells/ samples etc etc. The main argument pro lognormality is the multiplicative central limit theorem, that many authors mention without providing a clear example of a reaction pathway where a targeted concentration can be approximated by the multiplication of many factors. In https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5036630/ and http://web.mit.edu/jakebeal/www/Publications/ENB17-Lognormal-Biology.pdf they make the case of autocatalytic/ catalytic processes. What would be the simplest example of a chemical reaction system where the concentration of a target chemical (not the rate) would appear Lognormal?


texts: Conductance and Conductivity of acid base titration?

texts:
I have a question with $\ce{NaOH}$ initially present with given concentration and conductivity, then an equal volume of given concentration of HCl is added and the final conductivity is given, again a same volume of HCl of same concentration is added, the conductivity of which is again given. Now I need to find molar conductivity of $\ce{HCl ,NaOH, NaCl, H2O}$.

Molar conductivity($\lambda_m$), conductivity($\kappa$), Molarity($M$)

Using the formula $\lambda_m=\kappa/M$,firstly I found the molar conductivity of NaOH, afterwards when HCl was added, there will be only NaCl remaining, the rest being used up, so I again found molarity of NaCl and put in the formula and found molar conductivity, but in the last part I am confused, there is only HCl and NaCl remaining and I think one of these must be true:
$$\kappa_{solution}=\kappa_{\ce{NaCl}}+\kappa_{\ce{HCl}}\\\lambda_{solution}=\lambda_{\ce{NaCl}}+\lambda_{\ce{HCl}}$$
Probably the second one due to Kolrauch's Law? Also I am not aware of what is going on here because I'm just using a pretty formula. Please explain in detail the physical situation and the processes involved.

Edit:
The question is:

  The conductivity of $0.1$ M $\ce{NaOH}$ is $0.0221\;\Omega^{-1}cm^{-1}$. When an equal volume of $0.1$ M $\ce{HCl}$ solution is added, the conductivity decreases to $0.0056\;\Omega^{-1}cm^{-1}$. A further addition of $\ce{HCl}$ solution, the volume of which is equal to that of first portion added, the conductivity increases to $0.017\;\Omega^{-1}cm^{-1}$. Calculate $\Lambda_m(\ce{NaOH}),\Lambda_m(\ce{NaCl}),\Lambda_m(\ce{HCl}),\Lambda_m(\ce{H^+,OH^-})$


texts: How does salt concentration affect corrosion rate of iron?

texts:
I have conducted an experiment where I test the affect of $\ce{NaCl}$ concentration on corrosion rate of iron. I did this in an environment were all of the iron was submerged in water and the water was not aerated. The results I got are shown in the graph below.

Are there any research papers that could support or neglect my data? Can anyone explain this relationship? Is 1M just an outlier?


texts: How can I increase the current on a voltaic cell for high school lab?

texts:
I'm creating a voltaic cell project for my high school chemistry students. This is a design challenge for them to use electronegativity differences to produce enough voltage and current to power a small DIY motor and fan. So far, I have been able to generate about 1.7V but only 0.4mA using lots of spirals of Mg ribbon along with a Cu electrode using porous cups (and not a salt bridge). I've increased the concentration of Copper nitrate. What methods might increase the current? We're have enough voltage to run 1.5V DIY motors but they won't run due to lack of current.

We're doing this: http://static.nsta.org/files/tst1409_30.pdf


texts: How is the time for a test tube in a centrifuge determined?

texts:
I came across some hairy math that's used for analytical centrifugation, mostly for things like determining the sedimentation rate for liquids.

However, what I don't understand is how someone determines how long a test tube should be inside a centrifuge? Is it based off of others experimental data or is there a fixed way of doing it?


texts:
I am a physician and administer 0.9% NaCl in water ("normal saline") to patients frequently. The bag says it has a pH of 5.5 (compared to the human body of 7.4).

I am asking why the pH is so low. It is because NaCl is the salt of HCl? I read on wikipedia that the pH of NaCl aqueous solutions is 7. I teach medical students and want to give an understandable explanation why the pH on a bag of "normal saline" reads pH 5.5 and when we give lots of saline to patients, they develop acidosis in the blood.


texts: Reaction most dominant product for a low concentration nucleophile

texts:
If i got this reaction, and I determine that it will react in a Sn1 mechanism, will the dominant product will be in the results of the EtOH/H2O or CN-?

I think, the main product will be with the CN- after the hydrid shift in the Sn1 mechanism, and we will get a non chiral product, but im really not sure cause it says low concentration of the CN- nucleophile.


texts:
Many chemical bonds are known to be polarisable. Similarly, many ions are also known to be polarisable. Examples include the carbon-iodine bond and the carbonate ion. Recently, I have read that the carbon-fluorine bond is, in fact, non-polarisable. Reasons for this include the high electronegativity of fluorine and the small sizes of the carbon and fluorine atoms.

Do other non-polarisable bonds exist?
What factors would contribute to a chemical bond being non-polarisable?


texts: Which factors determine the keto:enol ratio for aldehydes, ketones, and 1,3-dicarbonyl compounds?

texts:
Arrange the following compounds in order of increasing enol content:

I think the order is D > C > B > A considering conjugation, hyperconjugation and hydrogen bonding. The enol formed in case D will have a substituted double bond (that is, more hyperconjugative structures) than the enol of C.
Both enols A and B lack hydrogen bonding. Also, enol of B is more stable because of more substituted alkene.


texts: Can citric acid protonation produce benzene and water

texts:
Is this reaction possible: $\ce{C6H8O7 + 12 H3O -> C6H6 + 19 H2O}$?
Considering that the $\ce{H+}$ can be induced by ionization of the citric acid solution the benzene can be produced in this way?


texts: How to calculate the number of geometrical isomers in allenes?

texts: Can someone explain how to calculate the number of geometrical isomers in allenes like $\ce{CH3-(CH=CH)4-CH3}$ or $\ce{CH3-(CH=CH)5-CH3}$


texts: Lennard-Jones diagram - what is on the x-axis?

texts:
I have a Lennard-Jones diagram here where I can't figure out, what is on the x-axis.

G. Ertl, Angew. Chem. Int. Ed. 2008, 47

We are looking at a diagram for a molecule $\mathrm A_2$ composed of two $\mathrm A$ atoms that comes into close contact with a surface.

If the molecule $\mathrm A_2$ forms a bond with the surface, a shallow minimum ($\mathrm A_{2,ad}$) is reached.
$E_{diss}$ is the dissociation energy needed to seperate/dissociate the two $\mathrm A$ atoms from each other (breaking the molecule). In this form (as two individual atoms), they could both seperately bind to the surface, resulting in much stronger bond formation $2 \mathrm A_{ad}$. This would be called chemisorption, if I'm not wrong.
Now combinding these two processes, we would have dissociative chemisorption. The crossing point between the two curves markes the activation energy of this.

This was just an explanation of the graph. My question is, how I should understand the horizontal dimension here - the x-axis. On a google search many diagrams pop up, but never with the dissociation included. Many of those as well as the Wikipedia article about Lennard-Jones potential show the x-axis as a distance between atoms - the interatomic distance $r$.

I cannot make sense of that on the diagram I am showing above. The distance between atoms should be smaller for the molecule than for the seperated atoms. What is on the x-axis in this case?


texts: Back titration of excess HCl used for dissolving limestone

texts:
$15.0~\mathrm{mL}$ of $1.4~\mathrm{M}\ \ce{HCl}$ was mixed with $1.00~\mathrm{g}$ of limestone (impure $\ce{CaCO3}$) until all the solid had dissolved. The solution was then transferred to a conical flask and made up to $200~\mathrm{mL}$ with water. A $20.0~\mathrm{mL}$ portion was then neutralised by $8.50~\mathrm{mL}$ of a $0.1~\mathrm{M}\ \ce{NaOH}$ solution.
Calculate:

Amount of excess $\ce{HCl}$ in the $20.0~\mathrm{mL}$ portion
Amount of excess $\ce{HCl}$ in the $200~\mathrm{mL}$ portion
Amount of $\ce{HCl}$ which reacted with $\ce{CaCO3}$

My attempt:
When the limestone is dissolved,
$$\ce{2 HCl(aq) + CaCO3(s) -> CaCl2(aq) + CO2(g) + H2O(l)}$$
$$n(\ce{HCl}) = c \times V = (15 \times 10^{-3}~\mathrm{L}) (1.4~\mathrm{M}) = 0.021~\mathrm{mol}$$
$$n(\ce{CaCO3}) = x~\mathrm{mol}$$
When the solution is diluted to $200~\mathrm{mL}$ the molarity of $\ce{HCl}$ can be found by,
\begin{align}
n_\mathrm i &= n_\mathrm f\\
c_\mathrm i V_\mathrm i &= c_\mathrm f V_\mathrm f\\
c_\mathrm f &= \frac{c_\mathrm i V_\mathrm i}{V_\mathrm f}\\
\therefore c_\mathrm f (\ce{HCl}) &= \frac{(15\times 10^{-3}~\mathrm{L})(1.4~\mathrm{M})}{(200\times 10^{-3}~\mathrm{L})} = 0.105~\mathrm{M}
\end{align}
I'm having doubts about my working up until this stage. Am I correct so far?


texts:
I'm trying to build a molecular orbital diagram for BF3 and I'm running into problems with irreducible representations on the F side.

2s for B has an irreducible representation of A1. 2p for B has an irreducible representation of E' and A''2.

2s for F considered non bonding

2p (along the bond axis) for F has an irreducible representation of E' and A'
2p (z) for F Has an irreducible representation of E'' and A''2
2p (the remaining one) has an irreducible representation of E' and A'2

From this I built A1 bonding and antibonding orbitals, A'2 bonding and antibonding orbitals, 1 A'2 and 2 E''2 non bonding orbitals.

I'm now stuck because I have 4 E' on the F side and only 2 E' on the B side.

I found the following diagram for BF3 online but it doesn't generate the E' anti bonding and also doesn't generate enough molecular orbitals.

Does anyone know how to mix 4 E' with 2 E'


texts:
In attempting to reveal the copper under a quarter, I used alligator clips to attach the quarter and a clump of copper wire to opposite sides of the battery. I put the electrodes in a solution of tap water and NaCl.

As the reaction occurred, the solution quickly turned into an icky (for lack of a better word) green, the water became very unclear and some cruddy particles rose to the top. Also, gas evolved from the copper electrode (which I suspect is H2). The coin blackened in this process and I took the quarter out and washed it.

Next, I reversed the polarity and the coin became the more expected copper color. As this happened, gas rose from the coin instead.

While I don't understand what occurred, my concern is the green sludge that was produced. I read solid nickel surrounds the mostly copper quarter coin. If the gas is really H2, then there must be hydroxide ions produced. If this is true, then I suspect the sludge could contain nickel(II) hydroxide. Is this possible? The "sludge" was a dark dollar green and was flaky, unlike dry nickel(II) hydroxide. It really looked like sewage more than anything else. I did not get a picture because I dumped it out before I thought of taking one (I suspected it to be dangerous). Are the conditions I described enough to get an idea of what it was?


texts: Orbitals and Pauli Exclusion Principle without Slater determinants

texts:
I'm trying to understand how to apply the pauli exclusion principle to the Born-Oppenheimer approximation of molecular structure.

Let me preface this by saying I'm not a chemist nor physicist. I'm a mathematician by trade which has the side affect of understanding electronic structure as solutions to a PDE.

So given some set of fixed nuclei and a given number of electrons I can construct the Schroedinger equation:

$$ \left(-\frac12\sum_{i=1}^N\Delta_i^2 - \sum_{i=1}^N\sum_{A=1}^M\frac{Z_A}{|r_i - r_A|} + \sum_{i=1}^N\sum_{j>i}^N \frac{1}{|r_i - r_j|} \right) \Psi = \varepsilon \Psi $$

I've read in Szabo and Ostlund that the pauli exclusion principle manifests itself by saying the wavefunction $\Psi$ must be antisymmetric in spin+location coordinates.

The next thing most people do is to introduce Slater determinants from 1-electron orbitals. This is an approximation to the solution space of wave functions, evidenced by the fact that FCI procedures must assume a sum of slater determinants for more accurate solutions. I want to keep this theoretical, so for now I don't care much for computational considerations.

The problem becomes as such, when talking about slater determinants it makes sense to append a spin variable and to talk about spin-orbitals of single electrons. Without slater determinants, single electron orbitals don't make sense (we only have the N-electron Schroedinger equation). Appending a spin coordinate also doesn't make sense because anti-symmetry of $\Psi$ and the Schroedinger equation don't see it.

So please help me out here, assuming I can solve the electronic Schroedinger equation exactly, what conditions do I need to place on the wavefunction so that the Pauli Exclusion Principle is satisfied (giving me a physical solution)?


texts: Why can't I reproduce this photolytic reaction of ferrocyanide?

texts:
I was so intrigued by the reported observation that sunlight precipitates the iron from $\ce{K4Fe(CN)6}$ (originally attributed to Matuschek, 1901) that I wanted to see it myself.  I put a saturated aqueous solution, uncovered, in direct sunlight for four hours at mid-day.  Absolutely nothing precipitated from the solution!

My understanding from the given reference is that in solution the $\ce{K4Fe(CN)6}$ gives $\ce{Fe(CN)6^{4−}}$ and then:

$\ce{Fe(CN)6^{4−} + 2H2O <-> Fe(CN)5 + (H2O)^{-3} + HCN + OH-}$

The photolysis causes $\ce{Fe(CN)6^{4−} ->[h\nu] Fe(CN)6^{3−} + e-}$,  so then we also have

$\ce{Fe(CN)6^{3−} + 2H2O <-> Fe(CN)5 + (H2O)^{-2} + HCN + OH-}$

Is it correct to assume that if a $\ce{Fe(CN)6^{4-}}$ anion absorbs a photon of adequate energy (in this case it appears to require $\lambda < 313nm$) then with 100% probability the anion will photolyze the Fe bond?

And are there models that predict (at least order of magnitude) the absorption probability of a photon by an aqueous anion (being irradiated with a given spectral flux)?

Furthermore, what is the reaction that would precipitate iron in this case?  When we bring the K back into the equations it looks like the HCN would react to produce KCN.  I don't know what should happen to $\ce{Fe(CN)5}$.

In fact, after several days in open sunlight the solution is subjectively getting darker and I do see a small amount of red sediment at the bottom of the dish that appears to redissolve on swirling.  Could be an iron oxide, but would also be consistent with $\ce{K3Fe(CN)6}$.  In either case I don't have any indication from the source what should happen to the $\ce{(CN)5}$ groups if iron is precipitating, or what else might take one of the K atoms from the initial compound.

(Ultimately I had hoped to quantify the rate of photolysis by weighing the precipitate.)


texts: Can anyone please briefly explain to me the concept of backbonding along with necessary conditions for backbonding with some example? As I understand, backbonding is a donation of electron from electron-rich to vacant orbital, but I don't know anything else.


texts: Photoelectric effect — Expected observations for waves

texts:
I understand why treating light as particle-like photon packets leads to the observed results.  I seek to understand why one would expect different results if light were treated as a wave (and thus why these results imply particle-like behaviour).

Experiment 1:  Increasing the intensity.
I imagine a light wave incident on the surface as Ψ = Asin(kx-ωt).  I imagine increasing the intensity of light as adding multiple parallel waves incident on the surface.  If each wave deposited enough energy to kick out 1 electron per second, wouldn’t adding multiple waves kick out more electrons per second without changing the energy per electron?

Is treating increased intensity as more parallel rays too much like particles?  If I instead treat increased intensity as increased amplitude squared, then I think of the wave as pouring energy into the metal surface at a faster rate, like a thicker stream of water from a spout. In this case, wouldn’t the light have deposited enough energy to free an electron more quickly, and thus still eject more electrons per second?

As a corollary here, is it right to equate A² ∝ number of photons?

Experiment 2:  Increased frequency.
I do understand why a continuous stream of energy from a wave is inconsistent with a threshold frequency to elicit a current.

Why would increasing the frequency (and therefore energy) of a wave not increase the kinetic energy of the most energetic ejected electrons?

Wouldn’t a single, particle-like wave-packet, also have energy proportional to its amplitude² as well as its frequency, and thus kick off more energetic electrons?


texts: Why is it necessary to heat an esterification reaction?

texts:
Why do we heat the mixture acid and alcohol?

The reaction of esterification is exothermic so it is not affected by increasing temperature, then what is the use of heat ?


texts: How to create a Potential Energy Surface in Chemcraft?


texts: Fragmentation involving formation of enamine

texts:
I have a problem on fragmentation which requires mechanisms involving formation of an enamine from a cyclic ketone and cyclic secondary amine. This then reacts with a cyclic aldehyde via conjugate addition, but the intermediate cannot be reached.

I've been told that I have to exchange the enamine of the ketone with the enamine of the aldehyde. What does this mean?

  PROBLEM 17

  Give mechanisms for these reactions, commenting on the fragmentation.

  Purpose of the problem
  Revision of conjugate addition of enols, another ring expansion with an enolate as leaving group and an interesting piece of stereochemistry.

  Suggested solution
  The first step is enamine formation and the second is conjugate addition. This appears to lead to a dead end as we cannot find a way to make the intermediate from the product.


texts: how is the relation between Gibbs free energy and cell potential?

texts:
First of all what is exactly Gibbs free energy in regards to an electrochemical process? How can we visualise it? How does Gibbs free energy change in an electrochemical reaction? How is it related to the cell potential?

Isn't potential the amount of work done per unit charge to bring it from infinite to that point?

Now the ionisation in cell can be thought of as occurring in multiple steps, for example:

$$\begin{aligned}
\ce{Zn(s)} & \ce{->Zn(g)} && (1) \\
\ce{Zn(g)} & \ce{->Zn+(g) + e-} &&(2) \end{aligned}$$

Then the work done to remove the charge should be the change in Gibbs free energy of second reaction and that energy should be equal to potential energy, but we take the combined change in Gibbs free energy as change in electric potential energy of the electron. Why is that so?


texts: Production of hexavalent chromium during electrolysis using a stainless steel as the cathode

texts:
I want to do rust removal by electrolysis (sodium carbonate, water solution) Can I use a stainless steel basket as a cathode or will it produce hexavalent chromium ?

I know if I use stainless steel as the anode it does produce hexavalent chromium, but what if it is a stainless cathode ?

I have a solution of sodium carbonate, water and hexavalent chromium. What can I do to eliminate the chromium or make it safe, preferably safe enough to dump in the drain if that is possible.


texts: How can I extract cellulose in plants using acids?

texts: Me and my classmates are doing a research project about the extraction of nano cellulose from plants to form a biocomposite, which can be a material to form an effective biodegradable material. We've done a quick research about it and most of the articles that we came across is to conduct an acid hydrolysis on the plant that we opt to use. We are kinda lost and confused as to what step or procedure we need to follow. Some articles also suggested that we can just boil the sample to extract the nano cellulose. Thank you for answering my question.


texts: What is the role of POCl₃ in the reaction to form (NPCl2)3 (hexachlorophosphazene)?

texts:
I did a reaction to form $\ce{(NPCl2)3}$. The reagents I used were:

phosphorus pentachloride $\ce{PCl5}$
ammonium chloride $\ce{NH4Cl}$
chlorobenzene - the solvent
phosphorus oxychloride $\ce{POCl3}$

The reaction is $\ce{3 PCl5 + 3 NH4Cl -> (NPCl2)3 + 12HCl}$

Where the chlorobenzene is the solvent.
But what role does the phosphorus oxychloride $\ce{POCl3}$ have in this reaction?

Is it some sort of catalyst? This reaction also needs to be performed under more acidic conditions, so it is the phosphorus oxychloride that creates the acidic conditions?


texts: Why do species not absorb an electron in mass spectrometry?

texts:
The following is a direct quote from my high-school chemistry text-book regarding mass spectrometry:

  The ionization process in the mass spectrometer involves an electron from an electron gun hitting the incident species and removing an electron. The collision can be so energetic that it causes the molecule to break up into different fragments.

This fails to make sense to me for two reasons:

Why wouldn't the majority of these electrons simply pass through a given substance, like the alpha particles in Rutherford's Gold Foil Experiment?
Why do positive ions/functional groups, such as ammonia, not absorb these electrons, instead of allowing them to break the species apart (and make them more positive in the process)?

Any justifications for these discrepancies would be greatly appreciated.


texts:
According to most sources, $\ce{CrO3}$ has the following Lewis structure (courtesy ChemSpider):

However, it is also not planar (courtesy Wikipedia):

Implying that there is a lone pair somewhere.

What is the electron configuration of the $\ce{Cr}$ atom?
Which orbital is responsible for the three double bonds?
Where does the lone pair come from?
How would its MO diagram look like?


texts: Solubility of oxygen in liquid hydrogen; is anything near 1:2 (molar ratio) possible?

texts:
In a question on another Stack Exchange site a proposal was forwarded to premix oxygen and hydrogen in the stoichiometric molar ratio of 1:2 to be used as rocket fuel, as opposed to the standard method of storing each liquified gas (LOX, LH2) separately.

According to Wikipedia (here and here) the freezing and boiling points of oxygen (presumably at standard pressure) are around 54 and 90 Kelvin, while those of hydrogen are around 14 and 20 Kelvin, respectively, suggesting that oxygen would freeze well before gaseous hydrogen would condense into a liquid.

However, one might (naively) wonder if oxygen might still exist as a solution in liquid hydrogen at some useful fraction, and at a pressure of roughly one atmosphere or less for practicality of storage on a weight-conscious rocket.

Even the wording of this abstract makes this sound fairly unlikely, but without some confirmation from someone more knowledgable it's hard to address the other linked question.


texts: What happens to electrons in metal complexes after excitation by visible light?

texts:
My book writes:

  When white light shines through a solution of a complex ion of a transition metal, photons of a particular frequency are absorbed and their energy promotes an electron from lower energy level to upper energy level. We perceive the complementary color to the light absorbed.

However, I have a question about this. Once the electron is promoted to a higher energy level, won't it fall back down and give off a photon?

Surely the electron cannot remain at the higher energy level forever, as that would mean that no further electronic transitions could taking place, and the solution would turn colourless.


texts: Which has more ionic character, LiF or K2O?

texts:
After drawing the structures of both and considering dipole moment, doesn't it seems that all K2O should have more ionic character than LiF?
My textbook says its LiF.


texts: Lindemann-HInshelwood mechanism difference between high and low concentration

texts:
I have a gas reaction which follows the Lindemann-Hinshelwood mechanism and want to calculate the rate constant at low and at high concentrations of the gas but I find myself stuck at what my next step should be and hope someone here could push me in the right direction.
I have that the effective rate constants are $1.01×10^{−5}$ s$^{−1}$  at a concentration of $4.6×10^{−6}$ M and $2.21×10^{−5}$ s$^{−1}$ at a concentration of $8.71×10^{−5}$ M (which are relative low concentrations). Hence, I get that the effective rate constant can be written as:
$$k_\mathrm{eff} = \frac{k_\mathrm{a}k_\mathrm{b}[\ce{A}]}{k_\mathrm{b} + k^{'}_\mathrm{a}[\ce{A}]}\tag{1}$$
and since $k_b$ >> $k^{'}_a [A]$, which then gives:
$$k_\mathrm{eff} = \frac{k_\mathrm{a}k_\mathrm{b}[\ce{A}]}{k_\mathrm{b}} ={k_\mathrm{a}[\ce{A}]}\tag{2}$$
So
$$k_a = \frac{k_{eff}}{[A]}=\frac{k_{eff,2}-k_{eff,1}}{[A]_2 - [A]_1} = \frac{2.21×10^{−5}-1.01×10^{−5}}{8.71×10^{−5} - 4.6×10^{−6}} = 0.145 M^{-1} s^{-1}$$
HOWEVER from what I have seen online it should be that
$$\frac{1}{k_{eff}} =\frac{1}{k_a [A]}$$
So
$$k_a =\frac{1/k_{eff,2} - 1/k_{eff,1}}{1/[A]_2 - 1/[A]_1} = 3.83 M^{-1}s^{-1}$$
Which I don't think this is correct, but that is the only way that I have seen it been calculated. But why does it go from being $k_{eff}$ to being $\frac{1}{k_{eff}}$? Or is my way of doing it correct?
Further, if I then want to calculate the rate constant at high concentration of the gas, $k_a [A]>>k_b$, then  eq. (1) can be rewritten as:
$$k_\mathrm{eff} = \frac{k_\mathrm{a}k_\mathrm{b}}{k^{'}_\mathrm{a}}\tag{3}$$
But since I don't have $k_b$ or $k_{a}^{'}$ how can I calculate $k_a$ at $298$ K?
I truly hope someone can help me with this, I have tried to find similar problems online but I can only find one and I don't understand why the expression for $k_{eff}$ is as it is. I also can't find anything regarding calculation for high concentrations without being given the other rate constants. Thank you!


texts: Are polyhydroquinones considered to be polyphenols?

texts:
Looking at the Wikipedia article for polyphenol, I was struck by how the monomer units shown are all substituted meta or ortho for the hydroxyl groups.

https://en.wikipedia.org/wiki/Polyphenol#Chemical_structure_and_synthesis

I assumed, naively, that polyhydroquinones would be considered polyphenols, but I am uncertain now. From what I can see, there is no IUPAC definition of polyphenols, so I am interested to know what commonly accepted criteria (and what contentious aspects, if any) are there for polyphenols.


texts:
I am trying to understand in a qualitative way photosynthesis. I am not a chemist so please have some patience.

I read on the Wikipedia page for photosynthesis that the way plants use $\ce{CO2}$ is through the reaction:

$\ce{energy + 6 CO2 + 6 H2O -> C6H12O6 + 6O2}$

but then for cellular aerobic respiration, I find exactly the opposite reaction:

$\ce{ C6H12O6 + 6O2 -> 6 CO2 + 6 H2O + energy}$

So, from this simplified view where only net reactions are written it looks that this mechanism can be used to transfer and store energy. But from a $\ce{CO2}$ point of view it looks that plants do not "consume" carbon dioxide since what they use via photosynthesis they give back through respiration. Clearly this cannot be true. So where is the catch ?


texts:
Recently I was reading an article about DNA nanostructures and I was wondering--

I know that DNA in living systems is monodisperse, but when we make DNA synthetically, is it truly monodisperse or does the dispersity vary depending on how it is synthesized?

If it has the same molecular structure, why does the fact that it's made synthetically change the dispersity?

Thanks


texts: How are reactivity and dipole moment related?

texts:
Recently, I came across a question:  which of the two compounds has a greater reactivity; $\ce{NH_{3}}$ or $\ce{NF_{3}}$?

It is known that despite the same structure the dipole moment of $\ce{NH_{3}}$  is greater because of the electronegativity difference. So, I though that the reactivity of $\ce{NF_{3}}$ should be more as it is easy to break the bond due to the high electronegativity of fluorine but the answer turned out to be $\ce{NH_{3}}$ and I can't figure out why.


texts: What do we know about the structure of the amylopectin iodine complex?

texts:
I am studying a method of finding the ratio between amylose and amylopectin using the difference in color of the amylose-iodine complex(blue) and the amylopectin-iodine complex (red/brown).
Through the use of techniques like x-ray crystallography, the structure of the amylose-iodine complex seems to be fairly well understood, as iodine inside a helical chain of amylose.
However, I cannot find many papers that explicitly claim the structure of the amylopectin-iodine complex. Perhaps the branched nature makes it more complex, or the variety in structure unlike amylose makes it impossible to find a general structure.
Are we aware of any certain structure of the amylopectin iodine complex? Any directions to relevant papers would be appreciated.


texts: Acidic character and anion stability across periods and groups of the periodic table

texts:
I understand that to compare relative acidity one must consider the stability of the conjugate bases.
Across a period the electronegativity of an element increases. And that is for example why $\ce{HF}$ is more acidic than $\ce{H2O}$, as the negative charge of the fluoride anion is drawn closer to the positively charged nucleus and thus more stabilized than that of an oxide anion.
Down the group the atom radius increases. And that is for example why $\ce{HI}$ is more acidic than $\ce{HF}$, as the negative charge of the iodide anion is distributed over a greater volume and thus more stabilized than that of a fluoride anion.
To me, these two trends are contradictory.
How can the stability of an anion increase as the negative charge gets closer to the nucleus(observed trend across periods) but at the same time decreases as the negative charge gets further away from the nucleus(observed trend down groups)?
Is there a concept I have misunderstood?


texts: Why are bridge bonds weaker than terminal bonds and, H-B-H terminal bond angles larger than H-B-H bridging bond angles in diborane?

texts:
In Concise Inorganic Chemistry by J.D.Lee (Adapted by Sudarsan Guha, Fourth Edition), on page 83, under the topic "Bridge Bonding" it is given for diborane $(\ce{B2H6}):$

The $d_\ce{B-H}(\text{terminal bonds}) < d_\ce{B-H}(\text{bridge bonds})$
The energy required to replace hydrogen atoms from the bridged position is more than that needed for the terminal position. This is supported by the fact that on reaction with methyl chloride, terminal hydrogen atoms are replaced by methyl group in preference to the bridged hydrogen atoms.
The reason for this could be the fact that to break the bridge bond, the overlapping zone should be broken from both sides and the overall energy required is more as compared to do so for terminal bonds.

Structure of Diborane:

Image Credit: Wikipedia
Based on the second paragraph it can be concluded that terminal bonds are weaker than bridge bonds. But some sources on the internet including Wikipedia state the bridge bonds are weaker than the terminal bonds without any satisfactory reason. I wish to know the reason for this.
Further, kindly explain why the terminal H-B-H bond angle is larger than bridging H-B-H bond angle as evident from the molecular structure as given above. I have searched my book as well as google to my best, but I didn't get any reason for this.
Kindly note, I read the unaccepted answer to this question. But still my doubt persists and makes me type this question by adding relevant details from my book and internet.
Kindly clarify this doubt.


texts: Highly conjugated compounds and the effects of substiuents

texts:
TLDR, what is the effect of electron donating and withdrawing groups on the UV-Vis absorption spectra of pi-conjugated system?

For a conjugated system, my understanding is that as the extent of conjugation increases, the difference in energy between the highest unoccupied molecular orbital and the lowest unoccupied molecular orbital decreases. The delocalization of electrons in a highly conjugated molecule provides the molecule with a greater number of molecular orbitals, decreasing their spacing. Conjugated compounds which absorb visible light do so when the difference in energy of the homo and lumo correspond to a frequency in the visible spectrum.

My question is in regards to the affects of electron donating and withdrawing groups and the absorption of light by these compounds in the visible spectrum.

My intuition is that electron donating groups will cause greater delocalization of electrons within the compound, creating more relevant MO's, decreasing spacing and reducing the HOMO- LUMO energy gap, causing the compound to absorb lower energy (longer wavelength light). Is my intuition correct?


texts: How to draw protonated benzene (horseshoe) carbocation structure with ChemDraw?

texts: Is there a way to draw the aromatic "horseshoe" (forgot the official name) structure of a protonated benzene cation (arenium) with ChemDraw, and not the cation with double bonds?


texts: Why does the mixing of sigma 2s and 2p orbitals lower the energy of the sigma 2s orbital and raise the energy of the sigma 2p orbital?

texts:
My general chemistry textbook (General Chemistry Principles and Modern Applications, Tenth Edition) says that for bonding in diatomic molecules with Z ≤ 7, orbital mixing occurs between the $\sigma_{2s}$ and $\sigma_{2p}$ molecular orbitals because they are close enough in energy. As a result, the energy of the $\sigma_{2s}$ molecular orbital is lowered and the energy of the $\sigma_{2p}$ molecular orbital is raised.

My question is why: if the $\sigma_{2s}$ orbital mixes with the $\sigma_{2p}$ orbital, wouldn't the modified $\sigma_{2s}$ orbital be higher in energy, as a result of mixing with an orbital slightly higher in energy?

Likewise, wouldn't the modified $\sigma_{2p}$ orbital be lower in energy as a result of mixing with an orbital slightly lower in energy? Why is it the other way around?


texts: How to interpret intervals on a FT-IR spectrogram of soil samples?

texts:
I am studying to analyze the IR spectra of soil samples. And I'm interested in the question - how to interpret the intervals in (1750..1820) and (3020 ... 2820) of the two samples on the graph?

The attached file - 1200..600 spectra of soil samples taken on Nikolet 6700 FT-IR Thermo Scientific in DRIFT mode and particular clippings of (1750..1820) and (3020 ... 2820) spectra.

UPDATE
Scan method – FT-IR DRIFT 1 200 cm-1 .. 6 000 cm-1

Red line – paleosol untreated (only dry the samples and sieve it threw a 2 mm mesh)
Blue line – weathering sandy soil untreated (only dry the samples and sieve it threw a 2 mm mesh)


texts: In what ways does the transfer of heat affect bond stability?

texts: It is to my understanding that certain amount of joules may change an element's/compound's physical state given a certain temperature. How does a certain added number of joules affect the orbital of a bonded electron ( I realize my question's ambiguity by not stating a specific type of bond).


texts:
When you mix a solid into a solvent you can see weird patterns right after the mixing.
A common example is mixing sugar in water.

What are those wave-like patterns and why do they happen?


texts: Is Jet Fuel/Kerosene still toxic after drying out?

texts: A friend of mine is currently living in India. The people she is living with just treated the wooden door with either kerosene or jet fuel (its not clear which of the both, as the language barrier makes communication rather difficult). We do now wonder if touching or even being close to this door will be still dangerous after the kerosene/jet fuel dried out.


texts:
Is there an equation to figure out what the temperature of an the area will be a certain distance from the heat source?

For example, if I know there is a heat source at $0~\mathrm{m}$, and the temperature is $100~\mathrm{^\circ C}$, is there a way to find out what the temperature will be at $50~\mathrm{m}$ away from the heat source? Is there some sort of equation for this? All of this is happening in oxygen.


texts: HCl synthesis from sulfuric acid and sodium salt

texts:
I'd like know if this is right. The reaction between $\ce{NaCl}$ and $\ce{H_2SO_4}$ should yield $\ce{HCl}$ and sodium sulfate. This is a double substitution, true? No oxidation took place here. The thing is, when I tried the reaction in the lab, the resulting solution (gas bubbled into water) seemed to be sulfuric acid. Simply enough, the solution didn't react with aluminum. I presume that $\ce{SO_2}$ was formed from the oxidation the sulfur (it a little bit like it).

What am I doing wrong? Shouldn't the substitution take place instead?

Process:

$
\ce{NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl}\\
\ce{NaCl + NaHSO_4 \rightarrow Na_2SO_4 + HCl}
$

Adding:

$
\ce{2\space NaCl + H_2SO_4 \rightarrow Na_2SO_4 + 2\space HCl }
$

Is this right?


texts: Is enthalpy of activation the same as activation energy? Is there a purely mathematical way to find that value?

texts:
I asked about the effect of pressure on a reaction, and since then I learned a lot more. One of the things I learned was the Eyring equation:

$$\kappa = \frac{\kappa_B T}{h} e^\frac{\Delta^\ddagger S°}{R} e^\frac{- \Delta^\ddagger H°}{R}$$

where:

$\begin{multline}
\begin{split}
\kappa &= \text{reaction rate} \\
\kappa_B &= \text{Boltzmann constant} \\
T &= \text{temperature} \\
h &= \text{Planck's constant} \\
R &= \text{gas constant} \\
\Delta^\ddagger S° &= \text{entropy of activation} \\
\Delta^\ddagger H° &= \text{enthalpy of activation} \\
\end{split}
\end{multline}$

My questions:

Is the enthalpy of activation the same thing as the activation
energy?
Is there a purely mathematical way to determine both $\Delta^\ddagger S°$ and $\Delta^\ddagger H°$? (without having a laboratory to use to observe the reaction)


texts:
A recent question (on $\ce{CI}$ v $\ce{Cl}$) reminded of this question: are there any real (as opposed to contrived) formulae which would be the same if case was ignored?

One certain example is $\ce{Co}$, Cobalt, and $\ce{CO}$, Carbon Monoxide.  Of course, confusing Cobalt and Carbon Monoxide is rather unlikely.

In my search for others, the best example I could find was $\ce{BaS}$, Barium Sulphide, and $\ce{BAs}$, Boron Arsenide.  Are these real substances and is the stoichiometry just $1:1$?  I had trouble verifying that.

Are there any more interesting examples?

One real example that I saw once was a poster in a university chemistry lab showing some complex organic molecules.  $\ce{Ac}$ appeared at many points but it seemed very unlikely that these compounds contained Actinium.  I then noticed $\ce{Me}$ and $\ce{Et}$ and guessed that they were organic units e.g. methyl and ethyl so $\ce{Ac}$ may have been acetyl.


texts:
I've just learnt about both types of NMR, and I haven't ever used NMR in real life, so anything I say may be wrong.

The crux of this question is that, in my own opinion, proton NMR gives you more information than carbon NMR, which I'd imagine to be much more helpful when analysing compounds. Because of this, can anyone tell me whether carbon NMR is ever used instead of proton NMR (which seems to be more 'powerful')? If it is, when?


texts:
The question is asking which reagents can be used to carry out this reaction. I get the L (PCC) is used to turn the primary alcohol into an aldehyde. However, I believe that is not the only reagent needed as the tertiary alcohol -OH must become -O- for the reaction to be SN2. A strong nucleophile is needed to carry out the formation of the SN2 reaction that forms the ring as the aldehyde is only primary (thus SN2 needed). I think that J, the tertbutoxide or S (NaOH) will deprotonate the OH group on the tertiary alcohol for the reaction to proceed.

Why is only L needed and not L and J (or S)?


texts:
Is this compound chiral?

I can't tell where the chiral centres are because this polycyclic backbone is too confusing. I tried to draw a mirror image, but I got really confused and couldn't tell whether it was superimposable. And I don't have a molecular modelling set so I can't build a model.


texts: Differences between miscible and soluble

texts:
What are the differences between soluble and miscible? So far I found this (page 3 http://faculty.sites.uci.edu/chem1l/files/2011/03/D01MANmiscibility.pdf):

soluble=solid dissolves into a liquid

miscible=two liquid give rise to a homogeneous solution

Are there other differences?

Thank you.


texts: Why does stale bread go soft in the microwave?

texts:
It's fairly well known that stale bread will go soft in the microwave. It makes hard bread edible (albeit a little bit chewy sometimes.)

It's obvious that moisture is being released from somewhere in the bread, and this is what causes the softness.

But the bread doesn't blacken as it would if the carbohydrates were decomposing to carbon, so where (chemically) does this moisture come from?


texts: Hard and Soft Acid and Bases in terms of HOMO and LUMO

texts:
It has been given in a book and on Wikipedia that:

hard bases have highest-occupied molecular orbitals (HOMO) of low energy, and hard acids have lowest-unoccupied molecular orbitals (LUMO) of high energy.[7][8]
soft bases have HOMO of higher energy than hard bases, and soft acids have LUMO of lower energy than hard acids. (However the soft-base HOMO energies are still lower than the soft-acid LUMO energies.)[7][8]

Let us consider the energy difference between HOMO and LUMO is given by $\Delta E$. Thus,
$$\Delta E (S-S)<\Delta E(H-S) < \Delta E (H-H)$$
Hence the hard-hard interaction should have been weaker than hard-soft, which is not the case. Please explain.
P.S.: I went through the reference (Meissler and Tarr) and this question but could not find a satisfying explanation.


texts: Can I use an electrolyte containing a different cation from the anode when doing an electroplating experiment?

texts:
For example, I want to purify copper. So the  anode will be impure copper and cathode will be pure copper. However, instead of using copper sulphate/halides/nitrate solution, can I use magnesium chloride?

My thought process: Magnesium is more reactive than copper, so copper from the anode that are oxidised into Cu2+ ions will still be preferentially discharged, therefore not affecting the reaction(?).

Sorry if there are conceptual errors here and there, Im just really confused about this topic. T_T


texts:
Why are the viscosities of noble gases higher than almost all other gases? This seems counter-intuitive since they have much weaker inter-molecular interactions and lower boiling points.

Gas Viscosity Table:
http://www.engineeringtoolbox.com/gases-absolute-dynamic-viscosity-d_1888.html


texts: How to reduce the turbidity of a water sample?

texts:
A task requires us to experiment water quality.
How would you reduce turbidity of a clay water sample with an effective and cost-effective method?


texts: Which white powder first "boils off" and then catches fire at high temperatures?

texts:
I'm a high school student doing a project where we need to identify chemicals and one of the substances had a very unique change when heated.

Before heating it was an extremely fine, flour-like white powder. I placed it on a hotplate at 350 degrees Celsius in its solid form and it "boiled off", I presume a hydrate evaporating. Before the hydrate appeared to evaporate, the powder had small pink dots appear all over it. This also happened when exposed to $\ce{NaClO}$ and $\ce{NaOH}$.

Once the temperature was raised to 400 degrees Celsius, the remaining solid turned yellow and then spontaneously caught fire, turning it all black before the fire ran out.

I had ran other tests on this substance, being insoluble in water, ethanol, and hexane. It also produced no apparent flame color.

My initial thought was that this compound was starch, however now that I think about it, it may be an insoluble magnesium compound, due to the lack of flame color? Just curious as to if anyone knows what substances display these properties when heated.

Here is a list of potential substances based off of a predetermined list.

Aluminum nitrate
Ammonium chloride
Ammonium nitrate
Ammonium sulfate
Barium acetate
Barium nitrate
Barium oxide
Barium sulfate
Calcium carbonate
Calcium chloride
Calcium oxide
Calcium nitrate
Calcium sulfate
Iron(II) sulfate
Iron(III) sulfate
Lauric acid
Lithium carbonate
Lithium nitrate
Magnesium carbonate
Magnesium chloride
Magnesium hydroxide
Magnesium oxide
Magnesium sulfate
Potassium bromate
Potassium carbonate
Potassium chloride
Potassium iodide
Potassium metabisulfite
Potassium nitrate
Potassium persulfate
Potassium phosphate
Potassium sodium tartrate
Potassium thiocyanate
Sodium acetate
Sodium bisulfate
Sodium borate
Sodium carbonate
Sodium citrate
Sodium chloride
Sodium fluoride
Sodium nitrate
Sodium nitrite
Sodium oxalate
Sodium sulfate
Sodium sulfite
Sodium thiosulfate
Starch
Strontium carbonate
Strontium hydroxide
Strontium nitrate
Sucrose
Tin(II) chloride
Urea
Zinc nitrate
Zinc sulfate

Of these, as stated earlier, an Mg compound (due to insolubility) or starch is most likely.


texts: Water molecule bent shape and electron configuration

texts:
The hydrogen atoms in a water molecule are negatively charged, so they should repulse each other completely due to the electromagnetic force, forcing the hydrogen atoms to be on opposite sides of the oxygen atom. However, this is not the case, as the H-O-H bond angle is $104.5 ^\circ$.

Why does the lone electron pairs repulse the hydrogen atoms to an angle of $104.5 ^\circ$ in $\ce{H2O}$ instead of $180 ^\circ$?


texts:
Question: Why do we insert a numeric multiplier after group positions when naming an isomer?

Take 2,3 dimethylbutane for example. Why do we add the numerical multiplier di when we've already explicitly mentioned two locations? Why don't we just use 2,3 methylbutane? This just doesn't feel intuitive.