# Relative magnitudes of intermolecular forces from phase diagrams

Is it possible to say whether or not a system A or B has stronger intermolecular forces from a phase diagram according to which:

1. The $T_{\text{b}}$ of A is lower than the $T_{\text{b}}$ of B.
2. The $T_{\text{melting}}$ of A is higher than the $T_{\text{m}}$ of B.
3. The triple points of A and B are the same.

Or is it not possible to compare A and B's intermolecular forces from a phase diagram?

I was sure that B has stronger intermolecular forces, as its boiling point is higher than for A at the same pressure.

## 1 Answer

This is an interesting situation. It would be easier to answer if the mp and bp of A were both either higher or lower than for B.

But your measure of the strength of intermolecular forces will be melting point - and boiling point. And they give different answers, so how do we make a decision?

Consider the boiling point first: the higher boiling point of B suggests higher intermolecular forces, as you have concluded. Heat of vaporization is needed to put a molecule into the vapor phase, and it seems that B needs more energy to go gaseous.

But A melts at a higher temperature, suggesting that its molecules are held together in a solid phase by stronger bonds. Solid A needs more energy to turn it into a liquid; perhaps A can pack into a tighter solid structure. It would then need more heat to melt it, so it has a larger heat of fusion.

Then your answer must have two parts: A is more tightly bound in a solid framework, but after you put enough heat into both A and B to melt them, B has more intermolecular bonding capacity as a liquid than A does. So B boils higher and is more strongly bound as a liquid.

• James, first of all thank you for your answer. Though I'm still not sure how to answer this question. Do you think it would be appropriate to say, that as A and B are in different states of matter, therefore it's impossible to compare their intermolecular forces? – Daniella Jun 14 '18 at 15:51
• Rather, melting and boiling are two different types of strength measurements; each has a heat required, and they are not necessarily related. So you can compare one set of heats (fusion) and decide which of A or B has stronger bonds. Then you compare the heats of evaporation and decide which of A or B seems to show stronger intermolecular strength at the higher temperature. Perhaps strength is a concept that you can measure by the amount of heat required to "break". – James Gaidis Jun 15 '18 at 13:32