Edexcel International A Level Chemistry

Topic 7: Intermolecular Forces

Molecules attract each other with three kinds of force, in order of strength. This topic names them, shows how to spot each one, and uses them to explain boiling temperatures, the strange properties of water, and what dissolves in what.

The one idea

When a liquid boils, its molecules are pulled apart from each other. The covalent bonds inside each molecule stay as they are. So the boiling temperature tells you how strong the forces between molecules are. There are three:

1. London forces are in every molecule. They get stronger with more electrons and more surface contact.
2. Permanent dipole–permanent dipole interactions are in molecules with a net dipole, and add to London forces.
3. Hydrogen bonds need an H on N, O or F pointing at a lone pair on N, O or F in another molecule. They are the strongest, and add to London forces.

Almost every question in this topic is one of three moves:

1. Name the forces. Decide which of the three each substance has, from its structure.
2. Compare for boiling. Strongest type of force first, then electrons, then surface contact; finish with "more energy is needed to overcome the forces between molecules".
3. Compare for solubility. Ask which attractions are broken and which are formed, and whether the new ones are about as strong as the old ones.

Where to find things

How to use these notes

• Read the pages in order the first time. Pages 1 to 6 give you the three forces. Pages 7 to 10 use them to explain boiling and melting temperatures and the properties of water. Pages 11 to 14 use them for solubility and bring everything together.
• Each page opens with its one idea in a sentence. If you can explain it to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

This topic has no core practical. The boiling temperatures and densities quoted are standard values, rounded to the nearest degree or to three significant figures, and any data that is made up for an example is marked illustrative.

Constants used in this topic: density of liquid water 1.00 g cm⁻³ and of ice 0.917 g cm⁻³ at 0 °C. Relative atomic masses are the Edexcel IAL values to one decimal place: H 1.0, C 12.0, O 16.0. To count electrons, add up the atomic numbers (H 1, C 6, N 7, O 8, F 9, S 16, Cl 17, Br 35, I 53).

The skills that earn most marks here are naming the forces in each substance, counting electrons before you compare London forces, drawing a hydrogen bond with the lone pair, partial charges and a straight dashed line, never mentioning covalent bonds breaking on boiling, and writing "forces broken against forces formed" for every solubility question.

Pages

1. Forces between molecules

   Boiling separates whole molecules from each other and leaves the atoms inside them joined. So boiling temperature measures the forces between molecules, and there are three kinds of them, in order of strength.

   10 min 7.1

2. London forces

   Electrons keep moving, so for an instant one end of a molecule is slightly negative and the other slightly positive. That instantaneous dipole induces a dipole in the next molecule, and the two attract. Every molecule has London forces.

   12 min 7.17.1(i)

3. Alkane chains and branching

   Longer alkane chains have more electrons, stronger London forces and higher boiling temperatures. Branching keeps the electron count the same but reduces surface contact, so a branched isomer boils lower than the straight chain.

   14 min 7.57.5(i)7.5(ii)

4. Permanent dipole–permanent dipole interactions

   A molecule with polar bonds that do not cancel has a permanent dipole. The δ+ end of one molecule attracts the δ− end of the next. This force acts in addition to London forces.

   12 min 7.17.1(ii)

5. Hydrogen bonds

   A hydrogen bond needs a δ+ hydrogen on N, O or F pointing at a lone pair on a N, O or F atom of another molecule. It is the strongest intermolecular force and holds together liquid water, ammonia and hydrogen fluoride.

   15 min 7.17.1(iii)7.2

6. Predicting hydrogen bonding

   To decide whether a substance has hydrogen bonds, check for an H atom bonded directly to N, O or F, and for a lone pair on N, O or F. Molecules with an O–H or N–H bond, such as alcohols and amines, pass both tests.

   12 min 7.4

7. Why water melts and boils so high

   Down a group, the hydrides boil higher as London forces grow. The first member of groups 15, 16 and 17 breaks the pattern because its molecules form hydrogen bonds. Water is the most extreme, because it forms the most hydrogen bonds per molecule.

   14 min 7.37.3(i)

8. Why ice floats

   In ice, hydrogen bonds hold each water molecule in an open lattice with empty space inside it. When ice melts, some hydrogen bonds break, the lattice collapses and the molecules pack closer, so liquid water is denser than ice.

   12 min 7.37.3(ii)

9. Alcohols against alkanes

   An alcohol boils much higher than an alkane with the same number of electrons. The electrons give similar London forces, and the O–H group adds hydrogen bonds, so the alcohol is less volatile.

   12 min 7.57.5(iii)

10. The hydrogen halides

    From HCl to HI the boiling temperature rises because London forces grow with the number of electrons. HF breaks the pattern and boils highest, because it forms hydrogen bonds. A three-step method ranks any set of molecules.

    13 min 7.57.5(iv)

11. Water as a solvent for ionic compounds

    Water molecules are polar. They surround each ion of an ionic solid, with the δ− oxygen towards positive ions and the δ+ hydrogens towards negative ions. This is hydration, and the energy released pays for breaking up the lattice.

    12 min 7.67.6(i)

12. Alcohols and halogenoalkanes in water

    Simple alcohols dissolve in water because they form hydrogen bonds with it, which replace the hydrogen bonds between water molecules. Halogenoalkanes are polar but cannot form hydrogen bonds with water, so water is a poor solvent for them.

    14 min 7.67.6(ii)7.6(iii)

13. Non-aqueous solvents

    A solute dissolves in a solvent when the attractions formed between solute and solvent are about as strong as the attractions broken in each. Substances with London forces dissolve in hexane, not water. Ionic compounds do the opposite.

    12 min 7.67.6(iv)

14. Putting the forces together

    One table of the three forces, one method for boiling temperatures and one for solubility. Use this page to practise mixed questions, where you must choose which force and which argument to use.

    15 min 7.17.57.6

Also in this topic: Topic 7 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.