Chromatography Forum

Can any body tell me why heat formation happens during mixing of water and methanol?

Hi,
Heat formation (MeOH-H2O) or absorption (ACN-H2O) are related with Gibbs free energy of mixing which you may remember from thermodynamics.
As I recall, entropy of mixing is equals to sum of entropies of the substances if they were expanded to the same extent.
You may google for more detailed (and comprehensible than mine ) answer.
Good luck,
Bulent

The delta G is positive for the rxn of MeOH + H2O because the system is gaining 'entropy'. Eventually your solution will cool to room temp ~25C

You may also notice that the volume of the MeOH:H2O mixture is less than the sum of the individual components. Think of the heat of mixing as an adiabatic compression.

It is so much "cooler" to mix acetonitrile and water.

Don´t forget enthalpy.

I like Mark's answer. I might also suggest that the reason for this compression is hydrogen bonding between water and methanol

But unfortunately it's a misleading use of terminology; by definition all systems undergoing an "adiabatic compression" will increase in temperature. The volume of a MeCN:H2O mixture is less than the sum of the individual components yet this one gets colder.

A compression doesn't really take place and I believe Mark was using an analogy. Molecules of the two liquids (when miscible) can interdisperse as things are less ordered (more spaced out) in this phase.

JA- methanol and water must be very able to hydrogen bond with one another, whereas an acetonitrile /water mix is a very different situation. I cannot see how your line of thought explains the differences between methanol/water and ACN/water. Perhaps you could rationalise further?

I haven't tried to explain the differences between the two mixtures as I am unable to do so; I had only commented on how I felt a previous post was inconsistent with the known results. Using terminology introduced in this thread both mixtures are compressed, yet hydrogen bonding between the two solutes is only possible in the case of MeOH:H2O.

I think that delta G is negative for the formation of both MeOH:H2O and MeCN:H2O mixtures as they are spontaneous. I think it follows that delta H is -ve for MeOH:H2O and +ve for MeCN:H2O as they are exo- and endothermic, respectively. I read that the entropy decreases in MeOH:H2O due to formation of more ordered clusters compared to water alone, though I don't know how much importance this holds.

To me, HW Mueller's contribution regarding enthalpy is a bit like your old high school teacher giving you a clue to jog the memory - is this the only factor in explaining the temperature differences? I'm hoping he will come in and answer fully

yet hydrogen bonding between the two solutes is only possible in the case of MeOH:H2O

By definition, hydrogen bonding is possible between H and O as well as H and N, thus hydrogen bonding is completely feasible in the case of the water/acetonitrile mixture.

Best Regards

Thank you for that; I was thrown off by earlier inference that H-bonding only occured for MeOH:H2O and it was this which resulted in heating...

The formula of acetonitrile is CH3CN. For effective hydrogen bonding to take place, the hydrogens must be attached to nitrogen or oxygen. Acetonitrile has no hydrogen atom attached to nitrogen......

I think acetonitrile could act as a hydrogen bond acceptor due to the polarity of the CN group. However, as I said previously, I think this is a very different situation to that between methanol and water which can both act as hydrogen bond donors and acceptors.

If there is a hydrogen bond to the CN it is so weak as to be neglected by all the literature on H-bonds which I have seen. You have to have a fairly active (basic) electron pair. The CN group is termed a pseudo halogen. Real halogens have several unpaired electrons, yet one doesn´t usually postulate a hydrogen bond to them.
To get a definitive explanation of the temp changes it would be advisable that one of us hits the books. No time right now, out of memory it seems mostly a entropy effect with a little bit of enthalpy due to the H-bonding in the case of methanol.
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?