Chemistry 9701 · AS & A Level · States of matter

States of matter — practice question

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

Describe how this reaction could be carried out using chlorine.

(a(ii))

Describe two observations you would make if this reaction were done with bromine.

(a(iii))

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

(a(iv))[8]

What is the main cause of the difference 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 needed 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 case where $X$ is iodine, $\text{I}$. Give your answer in $\text{kJ mol}^{-1}$.

(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 that would be formed by the 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 the mixture of $H_2 + Cl_2$ or expose it to light/UV, but NOT heat

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