Chemistry 9701 · AS & A Level · Hydrocarbons

Hydrocarbons — practice question

The halogens chlorine and bromine react quickly with hydrogen. $X_2(g) + H_2(g) \rightarrow 2HX(g)$ where $X = \text{Cl or Br}$.
(a(i))

Describe the way you could carry out this reaction using chlorine.

(a(ii))

Describe two observations you would make if bromine were used in this reaction.

(a(iii))

Use bond energy data from the Data Booklet to work out the $\Delta H^\circ$ for this reaction when $X = \text{Cl}$.\n\n$\Delta H^\circ = \ldots\ldots\ldots\ldots\ldots\ \text{kJ mol}^{-1}$\n\n$X = \text{Br}$.\n\n$\Delta H^\circ = \ldots\ldots\ldots\ldots\ldots\ \text{kJ mol}^{-1}$

(a(iv))[8]

What is the main reason for the gap between these two $\Delta H^\circ$ values?

(b(i))

Some halogens also react readily with methane. $\text{CH}_4(g) + X_2(g) \rightarrow \text{CH}_3X(g) + HX(g)$ What conditions are required to carry out this reaction when $X$ is bromine, $\text{Br}$?

(b(ii))

Use bond energy data from the Data Booklet to work out the $\Delta H^\circ$ of this reaction for the situation where $X$ is iodine, $\text{I}$.\n\n$\Delta H^\circ = \ldots\ldots\ldots\ldots\ldots\ \text{kJ mol}^{-1}$

(b(iii))[4]

Hence suggest why it is not possible to make iodomethane, $\text{CH}_3\text{I}$, by this reaction.

(c(i))

Halogenoalkanes can undergo homolytic fission in the upper atmosphere. Explain the term homolytic fission.

(c(ii))[3]

Suggest the most likely organic radical formed by homolytic fission of bromochloromethane, $\text{CH}_2\text{BrCl}$. Explain your answer.

(d)[3]

The reaction between propane and chlorine gives a mixture of many compounds, four of which are structural isomers with the molecular formula $\text{C}_3\text{H}_6\text{Cl}_2$. Draw the structural or skeletal formulae of these isomers, and show any chiral atoms with an asterisk $(*)$.

Worked solution & mark scheme

This 18-mark question has a full step-by-step worked solution and mark scheme. One marking point: Either ignite/burn it or expose the $\mathrm{H_2 + Cl_2}$ mixture to light/UV, but do NOT heat it

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