MadSci Network: Chemistry |
Why does 100 ml MeOH + 100 ml H20 = < 200 ml soln?
I understand that this must go hand-in-hand with the exothermic reaction that it is -- something to do with hydrogen bonding (must be more favorable) -- but why? Why is MeOH H-bonding with H20 molecules better than with other MeOH molecules? An increase in entropy doesn't seem realistic given the negative volume change... All very confusing, if you ask me! There are two things to sort out here. First, the mixing of methanol with water is observably exothermic, that is, the enthalpy change is negative (DH < 0). The problem is that the enthalpy change is favorable for mixing almost any liquid with water! This is because water can almost always form stronger intermolecular attractions, even with non-polar molecules, than the other molecules can form with themselves. A large enough, negative DH will overcome a reduction in entropy every time, and as you say, the reduction in volume leads us to suspect that DS < 0. But entropy typically controls mixing, and therefore I believe that in this case - as with other miscible liquids - the net entropy of mixing is favorable, that is, DS > 0. Here's a hand-waving explanation: Ahem.
As for the volume change, I suspect that's because of the fact that methanol is not able to form three hydrogen bonds the way a molecule of water is. This might result in a slightly less open and structured H-bonding network, which would both increase the entropy and decrease the volume. But this is just a guess!
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