Edexcel AS Chemistry Unit 1 Question?

Question

I was doing the January 2007 Paper (Unit 1) when I realised that I got a question wrong:

State the strongest intermolecular bond in each of the following:
HF
HCl
HBr
HI

I said HF is hydrogen bonding
and the rest are dipole-dipole forces.

The mark scheme said that HF was correct but HCl, HBr and HI were Van der Waals forces or dispersion forces or induced dipole forces. It clearly said "reject dipole-dipole forces"

I do not understand why?
1. Firstly note it said state the strongest intermolecular force.
2. Also dipole-dipole forces are a type of van der Waals, so how could they accept Van der Waals but not dipole-dipole forces.

Any help would be widely appreciated.

Answer 1

Right, Ive got notes on this (i answered the same question) but basically down the group te halogens become less electronegative and so the polarity decreases, to the point where the van der waals forces are stronger. The van der waals get stronger because of the increased number of electrons in the atom also.

secondly van der waals are not a type of dipole-dipole force. dipole-dipole attractions (also known as permenant dipole attractions) occor between molecules that are dipolar ONLY. van der waals forces occor between non polar molecules, caused by a shift in the electron cloud density. (also known as temporary dipole)

generally, temporary dipole attractions are weaker than permenant dipole attractions, just not in this case.

i hope this helps, my exact notes are as follows:

"as you move down the table F-> I the tempory dipole increases in strenght until its greater than the permenant dipole.
H-Cl, H-Br, H-I
Although permenant dipole atractions decrease down the group (as the halogen is less electronegative) the temporary dipole attractions are stronger than the permenent ones IN THIS CASE"

i dont know how much sense my actual notes make... but i hope that helps answer you're question.

do you know where i can find the mark schemes off the internet? i was looking for them when i found this.

Answer 2

I very much sympathise. We teach H-bonding>dipole-dipole>vdW year after year, but that is not always the correct order, as this questions shows up.