2.7 Simple molecules and polymers
5 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What happens when water boils?
Choose one answer.
Correct Not quite: the answer is Intermolecular forces are overcome
Mark scheme
A
Model answer
A: only the weak forces between molecules are overcome; the covalent bonds stay intact.
Explain why methane, CH4, has a low boiling point.
Mark scheme
- weak intermolecular forces between molecules (1)
- need little energy to overcome (1)
Model answer
Methane is made of small molecules with only weak intermolecular forces between them, which need little energy to overcome.
Explain why simple molecular substances do not conduct electricity.
Mark scheme
- molecules have no overall charge (1)
- no free electrons or ions to carry charge (1)
Model answer
The molecules have no overall electric charge and there are no free ions or electrons to carry charge.
Bromine (Br2) is a liquid at room temperature but chlorine (Cl2) is a gas.
Explain why.
Mark scheme
- bromine molecules are larger / have a higher relative molecular mass (1)
- so stronger intermolecular forces, needing more energy to overcome (1)
Model answer
Bromine molecules are larger, so the intermolecular forces between them are stronger and more energy is needed to separate them.
Explain why poly(ethene) is a solid at room temperature, but ethene (C2H4) is a gas.
Mark scheme
- poly(ethene) molecules are very long chains / very large (1)
- so the intermolecular forces between them are stronger (1)
- more energy needed to overcome them, so higher melting point (1)
Model answer
Poly(ethene) is made of very long chain molecules, so the intermolecular forces between the chains are much stronger than between small ethene molecules. More energy is needed to overcome them, so poly(ethene) melts at a higher temperature.
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