Chromatography Forum

The "big three" all contract (we use this slide in our Troubleshooting courses):

There is a problem of logic here. I claimed (without stating it formally):
if exothermic then contracts
If this is true, then the logical converse must also be true:
if expands then endothermic
Testing this second proposition is a test of the first also. But the rules of propositional logic tell us that the reverse:
if contracts then exothermic
does not need to be true neither does the inverse:
if endothermic then expands
need to be true. The falsity of the inverse or reverse tells nothing about the validity of the original proposition.

Now, it would be a different proposition were I to claim if and only if the mixing were exothermic would the mixture contract. That claim would be refuted by the observation that the reverse and inverse are false for MeCN:H2O.

I remember being taught this as a freshman in college because the faculty thought the students lacked critical thinking skills. I was mildly offended at the time, but in retrospect, I suppose they were right.

If you want freely available data to make an Excel spreadsheet for Acetonitrile - water density, then:-
http://www.scielo.org.ar/scielo.php?scr ... en&nrm=iso
or the pdf version at...
http://www.scielo.org.ar/pdf/jacs/v92n4 ... 4-6a12.pdf

The Excel curve from their Table One shows maximum ( ~4.7% ) decrease in expected density at 1:1 mixture. I assume that the mixture would then have a larger volume, not smaller... I assume that the smaller volume may be due to cooling when mixing, but the final solution volume should be larger when returned to initial temperature.

I need to have a play sometime...

Also, truly eye glazing material would be...

"Density functional study of hydrogen-bonded acetonitrile-water complex "
Ajay Chaudhari, Shyi-Long Lee
Int J Quantum Chem, 2005

Abstract
We report the interaction of acetonitrile with one, two, and three water molecules using the Density Functional Theory method and the 6-31+G* basis set. Different conformers were studied and the most stable conformer of acetonitrile-(water)n complex has total energies -209.1922504, -285.6224478, and -362.068728 hartrees with one, two, and three water molecules, respectively. The corresponding binding energy for these three structures is 4.52, 8.34, and 22.48 kcal/mol. The hydrogen-bonding results in blue, blue, and redshift in CN stretching mode in acetonitrile with one, two, and three water molecules, respectively, whereas there was a redshift in OH symmetric stretching mode of water. © 2004 Wiley Periodicals, Inc.

Please keep having fun,

Bruce Hamilton

Ja, are you quite sure about the decrease of volume when CH3CN is mixed with water? I seem to recall that some endothermic mixers rather expand?

Yes, this afternoon when the lab was quiet I mixed equal volumes of MeCN:H2O and MeOH:H2O and observed the following; NB (1) refers to measurement immediately after mixing & a quick stir, (2) upon temperature equilibration with the lab at 22C. I know it's pretty crude and the results aren't stated with any certainty of the accuracy, however the same equipment was used in both cases with the MeOH mixture clearly displaying greater volume. We also recall that retention times increase due to lower effective flow rate when water and either of these two organics are mixed on-line in a high-pressure mixing system as compared to a premixed MP or use of a low-pressure mixing system.

JA,
Hooray for real data! I was too lazy to be quantitative, but I tried the same thing with MeCN:H2O and saw about the same results to eyeball precision.

I know it's pretty crude and the results aren't stated with any certainty of the accuracy, however the same equipment was used in both cases with the MeOH mixture clearly displaying greater volume. We also recall that retention times increase due to lower effective flow rate when water and either of these two organics are mixed on-line in a high-pressure mixing system as compared to a premixed MP or use of a low-pressure mixing system.

My data and interpretation were inconsistent; I should of said that the acetonitrile-water combination ended up with the greater mixed volume

JA, that makes me feel better. Given "eyeball" precision, your numbers for 50/50 ratios are about the same as shown in the graph from the Foley et al paper. I'll second Mark's "hooray for real data!" comment.

If you want freely available data to make an Excel spreadsheet for Acetonitrile - water density, then:-
http://www.scielo.org.ar/scielo.php?scr ... en&nrm=iso
or the pdf version at...
http://www.scielo.org.ar/pdf/jacs/v92n4 ... 4-6a12.pdf

The Excel curve from their Table One shows maximum ( ~4.7% ) decrease in expected density at 1:1 mixture. I assume that the mixture would then have a larger volume, not smaller...

Apologies for the confusion caused by the above on-line reference that I posted. I've no idea what the values in their Table One represent, but clearly they are not density, as labelled, or as we know it.

Thanks to JA for the experimental data that demonstrates what really happens.

Please keep having fun,

Bruce Hamilton

I (also going on the basis of vague, ancient recollection) am pretty sure that the change in volume is independent of the change in entropy, which is usually the larger, temperature steering factor in these things. If there's no change in entropy, enthalpy and volume (not independent) will dominate the direction and magnitude of temperature change (which will probably be small enough as to go undetected by placing a human hand on the outside of a beaker).

This discussion is making me think subversive thoughts - like how little we would know about these temperature changes and such if the safety people had been around 100 years ago, making us all wear alternating layers of kevlar and nitrile gloves so that we couldn't expose ourselves to chemicals, or cut ourselves etc. etc.

Richard Feynman said unless he could explain a process so it could be understood by an intelligent layman he had not properly understood it himself. So far I don't think we have had a satisfactory explanation.

The first point is that if a reaction is exothermic - (giving out heat) more disorder is being introduced, if it is endothermic - (requires energy) more order is being created.

In the case of methanol, the reference below is interesting showing that Methanol chains are disrupted when it is mixed with water, the breaking of these weak bonds presumably generates heat.

http://www.isis.rl.ac.uk/isis2001/pdf/h6.pdf

It is a long time since I studied physical chemistry, perhaps a physical chemist out there could give us a better explanation

Bruce, the strongest H-bonds which I have in memory are around 7kcal/mole. Is the 28kcal in your article maybe cal/mole instead?

Abstract
We report the interaction of acetonitrile with one, two, and three water molecules using the Density Functional Theory method and the 6-31+G* basis set. Different conformers were studied and the most stable conformer of acetonitrile-(water)n complex has total energies -209.1922504, -285.6224478, and -362.068728 hartrees with one, two, and three water molecules, respectively. The corresponding binding energy for these three structures is 4.52, 8.34, and 22.48 kcal/mol.

I think the binding energies referred to correspond to 1, 2 and 3 bonds.ie the last figure is the energy of three bonds.

This paper seems to confirm my last comment that this an endothermic reaction - more ordered -because acetonitrile in water is creating a hydogen bonded complex which needs energy to form

Even at 22kcal for 3 bonds it has to be way to high for ACN. Prior to this chain I have never heard or seen anything about needing to consider H-bonding for ACN in wet chemistry.
The formation of a H-bond represents a stabilization, thus it is an exothermic process, not endothermic. If it were endothermic it would have to represent a repulsion, not an attracion.

Bruce, the strongest H-bonds which I have in memory are around 7kcal/mole. Is the 28kcal in your article maybe cal/mole instead?

I'll try to find out next time I'm in the library, but I may not have free access to that journal. My guess is that the abstract may have misrelated some data, and the full paper should clarify.

Bruce Hamilton

...The formation of a H-bond represents a stabilization, thus it is an exothermic process, not endothermic. If it were endothermic it would have to represent a repulsion, not an attracion.

If all stabilizing (net lowering of order, increase in entropy) were exothermic, there would be no such thing as an endothermic reaction. As this is not the case, I suspect that heat is not the only germane measure of a system's entropy.


(Having never personally taken P-Chem, I'm way out of my element here. Feel free to shoo me away from this thread)