Intermolecular Forces

When I run face to face workshops looking at the changes to the new guide I tend to classify changes to the syllabus:

1, Movable content (eg, standard enthalpy changes of reaction, moved from AHL to SL)

2, Deletions (eg, operation of the mass spectrometer)

3, New content (eg, formal charge)

4, Subtle changes (eg, molar volume at STP now being 22.7dm³)

5, New ways of thinking

I intend covering one of the examples of new ways of thinking in this blog post – new ways of thinking with respect to intermolecular forces.

Intermolecular forces can be found on page 48 of the new guide (section 4.4 under topic 4: Chemical bonding and structure). The essential idea for this topic states:

“The physical properties of molecular substances result from different types of forces between their molecules.” ¹

I hope, after reading this you are beginning to think there is nothing new …. but read on!

For this particular topic the ‘understanding’ states:

“Intermolecular forces include London (dispersion) forces, dipole-dipole forces and hydrogen bonding.”

At this point you may be asking yourself, what about van der Waals forces – has this been renamed as a London (dispersion) force?

It is only really when we study the guidance that the ‘new way of thinking’ becomes apparent.

“The term ‘London (dispersion) forces’ refers to instantaneous induced dipole- induced dipole forces that exist between any atoms or groups of atoms and should be used for non-polar entities. The term “van der Waals” is an inclusive term, which includes dipole–dipole, dipole-induced dipole and London (dispersion) forces.”

So from what I can make out, in the past I used to teach intermolecular forces (IMF’s) as being Hydrogen bonds, dipole – dipoles (which are still there and haven’t changed – phew!) and the third IMF would have been what I used to call ‘van der Waal’s forces’. When I taught van der Waal’s forces, I would also tell students that it was called an ‘instantaneous dipole induced dipole’.

What I now need to do it to stop calling the weakest IMF a ‘van der Waal’s’ force and instead call it a London (dispersion) force. London (dispersion) forces can also be known as ‘instantaneous dipole induced dipoles’.

Wan der Waal’s forces can now be a type of either dipole – dipole or London (dispersion) force.

It appears that we can now omit van der Waal’s forces from our teaching altogether – although I think it would be sensible to keep it in for  the moment as well. Van der Waal’s forces are not mentioned in the understandings, only the guidance which implies van der Waal’s won’t come up in exams ….. but you never know!

Are there any clarifications of this point you need? Are there any other ‘subtle changes’ you have picked up on or would like clarification of?

If so, please feel free to post your questions below.

References:

¹ IB subject guide – page 48