Organic Chemistry: How does the molecular size of a molecule affect its boiling and melting points?

Question

I've noticed that with organic compounds, as a molecule gets bigger (more carbon atoms), its melting and boiling point increases. This has to do with intermolecular forces.

Why does it require more energy to break the intermolecular bonds of a bigger molecule? Aren't the dispersion (intermolecular fores) the same for both molecules?

C-C-C-C --- C-C-C-C (dispersion between 2 butane molecules)

C-C-C-C-C-C-C-C --- C-C-C-C-C-C-C-C (dispersion between 2 octane molecules)

Can someone please explain?

Gervald, can you explain why bigger molecules can induce bigger the temporary dipoles?

Answer 1

The bigger the (sideways) surface area of contact, the bigger the temporary dipoles that can be induced, and the bigger the intermolecular forces.

Small molecules have small forces. Big molecules have big ones. Look at the boiling point of the halogens, if you want another example.

Answer 2

In compounds incapable of hydrogen-bonding, the foremost factors affecting boiling element are molecular weight and shape of the molecule. to illustrate, the hexane and a pair of-hexanone could be predicted to boil with regard to a similar,because they're the two bascally 6-carbon chains. the two-methylpentane, even nevertheless a hydrocarbon, has a similar molecular weight as hexane. in spite of the shown fact that, by using fact it has the methyl branching, it finally ends up having a greater around shape, so it may be predicted to boil a sprint decrease. --- gazing the BPs, the oxygen is approximately akin to a distinctive methyl group.