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Heat can be evolved or absorbed when two dissimilar liquids are mixed together at constant temperature and pressure. This is called the heat of mixing of the mixture.

I'm trying to understand the molecular "picture" associated with this process. Here is my current attempt:

Initially, there are two pure substances each with its own "like-like" intermolecular forces. During mixing, these forces have to be overcome in order to "break-up" the similar molecules to "make way" for the dissimilar molecules. This requires energy and so is endothermic for both pure substances, correct? It would be like boiling the liquid (energy input) to achieve an ideal gas (no interactions) of each pure substance.

The second component of the net energy change is the release of energy when forming the mixture. Now, we have to accomodate both the "like-like" interaction forces (presumably at different intermolecular distances than the pure substance) and the "like-unlike" interaction forces. Overall, this direction is exothermic since formation of the solution with interaction forces is lower energy overall than the non-interacting ideal gas mixture, right?

So I have two offsetting directions in the two components of energy change associated with mixing. The net effect (endo or exothermic) obviously will depend on the quantitative forces associated with the each component of the process.

Do I have the basic molecular picture correct?

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This picture is generally correct, but do not forget entropy of mixing. :=) – Georg Nov 28 '11 at 20:00
up vote 1 down vote accepted

You can divide the process into 3 virtual subprocesses = evaporation of pure substances, then mixing gases (almost ideal at normal conditions) and then condensation of the mixture. Enthalpy is state variable and therefore does not depend on the path. The process of mixing in gas phase is important only for free energy of mixing (not for enthalpy / heat).

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