State the equation for the reaction between methane and bromine to form bromomethane.

Explain, using equations, the complete free-radical mechanism for the reaction of methane with bromine, including necessary reaction conditions.

Bromine can be produced by the electrolysis of molten sodium bromide. Deduce the half-equation for the reaction at each electrode.

Positive electrode (anode):

Negative electrode (cathode):

Bromine reacts with aqueous sodium iodide:

\[{\text{B}}{{\text{r}}_{\text{2}}}{\text{(aq)}} + {\text{2NaI(aq)}} \to {{\text{I}}_{\text{2}}}{\text{(aq)}} + {\text{2NaBr(aq)}}\]

Identify the oxidizing agent in this reaction.

\({\text{C}}{{\text{H}}_{\text{4}}} + {\text{B}}{{\text{r}}_{\text{2}}} \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{Br}} + {\text{HBr }}\);

Initiation:

\({\text{B}}{{\text{r}}_2}\xrightarrow{{UV/hf/hv}}2{\text{BR}} \bullet \);

Reference to UV/light or high temperatures must be included.

Propagation:

\({\text{Br}} \bullet  + {\text{C}}{{\text{H}}_4} \to {\text{C}}{{\text{H}}_3} \bullet  + {\text{HBr}}\);

\({\text{C}}{{\text{H}}_3} \bullet  + {\text{B}}{{\text{r}}_2} \to {\text{C}}{{\text{H}}_3}{\text{Br}} + {\text{Br}} \bullet \);

Termination:

Award [1 max] for any one of:

\({\text{Br}} \bullet  + {\text{Br}} \bullet  \to {\text{B}}{{\text{r}}_2}\);

\({\text{C}}{{\text{H}}_3} \bullet  + {\text{Br}} \bullet  \to {\text{C}}{{\text{H}}_3}{\text{Br}}\);

\({\text{C}}{{\text{H}}_3} \bullet  + {\text{C}}{{\text{H}}_3} \bullet  \to {{\text{C}}_2}{{\text{H}}_6}\);

Accept representation of radical without \( \bullet \) (eg Br, CH₃) if consistent throughout mechanism.

Award [3 max] if initiation, propagation and termination are not stated or are incorrectly labelled for equations.

Positive electrode (anode):

\({\text{2B}}{{\text{r}}^ - } \to {\text{B}}{{\text{r}}_2}{\text{(g)}} + {\text{2}}{{\text{e}}^ - }/{\text{B}}{{\text{r}}^ - } \to \frac{1}{2}{\text{B}}{{\text{r}}_2}{\text{(g)}} + {{\text{e}}^ - }\);

Negative electrode (cathode):

\({\text{N}}{{\text{a}}^ + } + {{\text{e}}^ - } \to {\text{Na(l)}}\);

Award [1 max] for correct equations at incorrect electrodes.

Ignore state symbols.

Accept e instead of \({e^ - }\).

Penalize use of equilibrium signs once only.

bromine/\({\text{B}}{{\text{r}}_{\text{2}}}\);

Do not accept bromide/\(B{r^ - }\).

Part (a) and (b) of the question were based on the free radical substitution of methane and typical marks were 0/5 or 5/5, as it required the stating of a mechanism that has been on the syllabus and on examination papers for many years. Students generally had little idea but some had obviously learned it well. The students who gained some marks often lost marks for creating hydrogen radicals. The rest of the question was based on redox and (c) which asked for the equations at the electrodes during the electrolysis of sodium bromide was done very badly indeed. These half-equations are in the data booklet and yet very few students could come up with the correct equations. Some students had NaBr at both electrodes. However many students did correctly identify bromine as the oxidizing agent in (d).

Part (a) and (b) of the question were based on the free radical substitution of methane and typical marks were 0/5 or 5/5, as it required the stating of a mechanism that has been on the syllabus and on examination papers for many years. Students generally had little idea but some had obviously learned it well. The students who gained some marks often lost marks for creating hydrogen radicals. The rest of the question was based on redox and (c) which asked for the equations at the electrodes during the electrolysis of sodium bromide was done very badly indeed. These half-equations are in the data booklet and yet very few students could come up with the correct equations. Some students had NaBr at both electrodes. However many students did correctly identify bromine as the oxidizing agent in (d).

Part (a) and (b) of the question were based on the free radical substitution of methane and typical marks were 0/5 or 5/5, as it required the stating of a mechanism that has been on the syllabus and on examination papers for many years. Students generally had little idea but some had obviously learned it well. The students who gained some marks often lost marks for creating hydrogen radicals. The rest of the question was based on redox and (c) which asked for the equations at the electrodes during the electrolysis of sodium bromide was done very badly indeed. These half-equations are in the data booklet and yet very few students could come up with the correct equations. Some students had NaBr at both electrodes. However many students did correctly identify bromine as the oxidizing agent in (d).

Part (a) and (b) of the question were based on the free radical substitution of methane and typical marks were 0/5 or 5/5, as it required the stating of a mechanism that has been on the syllabus and on examination papers for many years. Students generally had little idea but some had obviously learned it well. The students who gained some marks often lost marks for creating hydrogen radicals. The rest of the question was based on redox and (c) which asked for the equations at the electrodes during the electrolysis of sodium bromide was done very badly indeed. These half-equations are in the data booklet and yet very few students could come up with the correct equations. Some students had NaBr at both electrodes. However many students did correctly identify bromine as the oxidizing agent in (d).