Intermolecular forces

WCH12Topic 2.23 questions

London forces, permanent dipoles and hydrogen bonding, and their effect on physical properties.

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Question 13 marks
Pentane (boiling temperature $36\,^\circ\text{C}$ ) and 2,2-dimethylpropane (boiling temperature $10\,^\circ\text{C}$ ) are isomers.

Explain the difference in their boiling temperatures.
Mark scheme
- Both have only London forces between molecules and the same number of electrons (1)
- Pentane molecules are unbranched so have a larger surface area of contact between molecules (1)
- So pentane has stronger London forces, needing more energy to separate the molecules (1)
Question 23 marks
Water, $\text{H}_2\text{O}$ , boils at $100\,^\circ\text{C}$ , whereas hydrogen sulfide, $\text{H}_2\text{S}$ , boils at , even though has more electrons.

H_{2} S

-60\,^\circ\text{C}

\text{H}_2\text{S}

Explain this difference.

Mark scheme

Water forms hydrogen bonds between molecules because oxygen is highly electronegative and has lone pairs (1)
Hydrogen sulfide has only London forces and permanent dipole–dipole forces, as sulfur is not electronegative enough to form hydrogen bonds (1)
Hydrogen bonds are much stronger, so more energy is needed to separate water molecules (1)