Study notes for Unit 2 Topic 5 - Intermolecular Forces for Chemistry Edexcel A Level 2015. Focusing on the three different types of intermolecular forces, London forces, permanent dipoles and hydrogen bonds, as well as the properties of each one with specific examples referring to ice/water, alkane...
Intermolecular forces:
● They are the forces between molecules.
● They are very weak, much weaker than covalent, ionic or metallic bonds.
● There are 3 types resulting from these interactions; London forces, Permanent
dipoles, Hydrogen bonds.
London forces:
All atoms and molecules form London forces (also called instantaneous dipole-induced
dipole bonds).
They cause all atoms and molecules to be attracted to each other.
● Electrons in an atom move quickly and are likely to be more to one side than the
other, and this creates a temporary dipole in the atom.
● This dipole can induce another temporary dipole in the opposite direction on a
neighbouring atom. The two dipoles are then attracted to each other.
● The second dipole can then induce another dipole in a third atom, and so on.
● The electrons are constantly moving so dipoles are being created and destroyed all
the time, and overall the atoms are attracted to each other.
Properties of London forces:
● London forces can hold molecules in a lattice - simple molecular structure simply
means covalent molecules held together in a weak lattice by London forces.
● Liquids and solids with stronger London forces will have higher melting/boiling points,
as more energy will be needed to overcome the stronger intermolecular forces.
● More electrons = more London forces
● (Not all London forces are the same strength. Larger molecules or molecules
with greater surface areas, for example, will have stronger London forces)
Permanent dipoles:
● Polar molecules have permanent dipole-permanent dipole bonds.
● The positive and negative charges on polar molecules cause weak electrostatic
forces of attraction between molecules.
● These are known as permanent dipole-permanent dipole bonds.
● They can happen as well as London forces, so molecules that can form permanent
dipole-permanent dipole bonds and London forces will generally have higher
melting/boiling points.
Hydrogen bonds:
● This is the strongest type of intermolecular force.
● It only happens when hydrogen is covalently bonded to fluorine, nitrogen or
oxygen.
● F, N and O are very electronegative so they draw the bonding electrons away from
the H atom.
● The bond is very polarised and hydrogen has a very high charge density (because
it's so small) so the hydrogen atoms form weak bonds with lone pairs of
electrons on the F, N or O atoms.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller einavd-l. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £5.49. You're not tied to anything after your purchase.
