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Therefore we write in general aa pa pa and ab pb

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Unformatted text preview: there is no pressure term in the expression for the Gibbs free energy of mixing, then ΔVmix = 0. Furthermore, we can write ΔUmix = ΔHmix - P ΔVmix since the pressure is constant while we mix two liquids. Therefore, we conclude: ΔUmix = 0, which implies that the sum of the energies of components A and B in an ideal solution is equal to the sum of their energies in the pure state. Obviously, components A and B in a liquid mixture interact with one another, since on the average, molecules in a Marand’s Notes: Chapter 5 - The Properties of Simple Mixtures 169 liquid are very close to each other (remember the discussion of intermolecular interaction potential energy, when we looked at ideal and real gases). So, the final conclusion is that in an ideal solution, the interactions between A and B molecules are the same as those between A molecules or those between B molecules. For this to be true, and therefore for an A/B mixture to form an ideal solution, the molecules A and B must have very similar chemical structure. Indeed, this is verified by experiments. Mixtures of toluene (benzene with a methyl pendant group) and benzene form ideal solutions. Obviously, another conclusion is that when you mix two different kinds of molecules (for example, acetone (CH3- CO- CH3) and chloroform (CHCl3)), these two molecules do not form an ideal solution and the components of the mixture do not obey Raoult’s law over the whole composition range. Indeed, ideal solutions are an exception rather than the rule. However, the con...
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