Chemguide: Support for CIE A level Chemistry

Learning outcome 3.3(d)

This statement deals with van der Waals forces.

Before you go on, you should find and read the statement in your copy of the syllabus.

van der Waals forces

First, read the page about van der Waals forces.

That will give you all that you need (plus a bit more) to satisfy this syllabus statement, apart from the bit about liquid bromine.

Chloromethane isn't mentioned specifically on the page, but there is a model of fluoromethane. Chloromethane looks just like this, but will have a smaller permanent dipole because chlorine is less electronegative than fluorine.

You will find an explanation of how van der Waals forces affect the halogens (including bromine) on a page about the physical properties of Group 7. You just need the section titled "Trends in Melting Point and Boiling Point" just over half-way down that page.

Important: If you have read the follow-on page from the van der Waals page about the strengths of dispersion forces, you will know that dipole-dipole attractions contribute less to overall intermolecular attractions than dispersion forces do in the great majority of cases, and that dispersion forces can be quite strong.

But a lot of textbooks and web sources still quote the traditional view that dispersion forces are the weakest of the intermolecular forces.

What should you do if this comes up in an exam? Up to the time of writing (November 2013), CIE have never asked a question about the relative sizes of the various intermolecular attractions. This is almost certainly something you don't need to worry about.

They might, of course, expect you to know that a molecule which has dipole-dipole attractions as well as dispersion forces will probably have a boiling point greater than a similarly sized molecule with only dispersion forces. That is perfectly reasonable.

Hydrogen bonding

If you delayed reading about hydrogen bonding in section 3.3(a), go back and do it now before you forget!

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© Jim Clark 2010 (last modified March 2014)