Explain, in some detail, why in mixtures of A and B, the compounds A and B would very likely have different infinite dilution activity coefficients

Question

What will be an ideal response?

Answer 1

In a solution with two compounds, (1) and (2), there will be three types of interactions between the molecules: 1-1, 1-2, and 2-2. Each of these interactions contributes to the numerical value of an activity coefficient.

The activity coefficient essentially quantifies departure from ideal solution volatility.

For a solution of (1) that contains an infinitely dilute amount of (2), there will be 1-1 interactions and 1-2 interactions, but essentially no 2-2 interactions. It can be assumed that the solution is so dilute, compound (2) will not be able to interact with other molecules of the same type, therefore no 2-2 interactions are present. Similarly, for a solution of (2) that contains an infinitely dilute amount of (1), there will be 2-2 and 1-2 interactions, but no 1-1 interactions.

So in both cases, “1-2 interactions” are present, but the systems being modeled are fundamentally different. In one case, the system essentially consists of a single “1” molecule completely surrounded by “2” molecules, and the activity coefficient quantifies whether “1” in this circumstance is more or less volatile than pure 1 would be. In the other case, a single “2” molecule is completely surrounded by “1” molecules.
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