Introductory organic chemistry and alkanes

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   Methane reacts with chlorine in the presence of ultraviolet light to form chloromethane.

   Give equations for the initiation step, the two propagation steps and one termination step of this free-radical substitution.

   Mark scheme

   • Initiation: $\text{Cl}_2 \rightarrow 2\text{Cl}\bullet$ (1)
   • Propagation: $\text{CH}_4 + \text{Cl}\bullet \rightarrow \text{CH}_3\bullet + \text{HCl}$ (1)
   • Propagation: $\text{CH}_3\bullet + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\bullet$ (1)
   • Termination, e.g. $\text{CH}_3\bullet + \text{Cl}\bullet \rightarrow \text{CH}_3\text{Cl}$ or $2{\text{CH}}_3\bullet \to {\text{C}}_{}$ or (1)
2. Question 23 marks

   Propane, $\text{C}_3\text{H}_8$, is used as a fuel in portable gas stoves.

   (a) Write a balanced equation for the incomplete combustion of propane to form carbon monoxide and water.

   (b) Explain why carbon monoxide is toxic.

   (c) Nitrogen monoxide is also found in the exhaust from car engines that burn hydrocarbons. Explain how it forms.

   Mark scheme

   • (a) ${\text{C}}_3{\text{H}}_{}$ (or doubled) (1)