A-Level Edexcel Chemistry Halogen Disproportionation

$\mathbf{2 2 ( c )} \boldsymbol{*}$

This question assesses the student's ability to show a coherent and logically structured answer with linkages and fully sustained reasoning.

Marks are awarded for indicative content and for how the answer is structured and shows lines of reasoning.

The following table shows how the marks should be awarded for indicative content.

Number of indicative marking

points seen in answer

Number of marks awarded for

indicative marking points

6

5-4

3-2

1

0

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The following table shows how the marks should be awarded for structure and lines of reasoning

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Guidance on how the mark scheme should be applied.

The mark for indicative content should be added to the mark for lines of reasoning. For example, a response with five indicative marking points that is partially structured with some linkages and lines of reasoning scores 4 marks (3 marks for indicative content and 1 mark for partial structure and some linkages and lines of reasoning).

If there were no linkages between the points, then the same indicative marking points would yield an overall score of 3 marks (3 marks for indicative content and no marks for linkages).

In general it would be expected that 5 or 6 indicative points would get 2 reasoning marks, and 3 or 4 indicative points would get 1 mark for reasoning, and 0, 1 or 2 indicative points would score zero marks for reasoning.

If there is any incorrect chemistry, deduct mark(s) from the reasoning. If no reasoning mark(s) awarded do not deduct mark(s).

Comment: Look for the indicative marking points first, then consider the mark for the structure of the answer and sustained line of reasoning.

(6)

Indicative content:

IP1 all are disproportionation reactions (because chlorine is both oxidised and reduced)

IP2 chlorine changes/is reduced from 0 to -1 (in $\mathrm{Cl}^{-}$in all reactions)
$$\text { IP3 } \mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCl}+\mathrm{HOCl}$$

IP4 $\mathrm{Cl}_{2}+2 \mathrm{NaOH} \rightarrow \mathrm{NaClO}+\mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}$$$\text { IP5 } 3 \mathrm{Cl}_{2}+6 \mathrm{NaOH} \rightarrow \mathrm{NaClO}_{3}+5 \mathrm{NaCl}+3 \mathrm{H}_{2} \mathrm{O}$$

IP6 chlorine oxidised to +1,+1 and +5 shown in the appropriate reaction

Allow for a description, including the name, of disproportionation if a reaction type is not given for all three equations Allow disproportional/disproportion reaction

Allow $\mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-}+\mathrm{HOCl}$

Allow single arrow instead of equilibrium sign Allow HClO or ClOH

Allow $\mathrm{Cl}_{2}+2 \mathrm{OH}^{-} \rightarrow \mathrm{ClO}^{-}+\mathrm{Cl}^{-}+\mathrm{H}_{2} \mathrm{O}$$$\text { Allow } 3 \mathrm{Cl}_{2}+6 \mathrm{OH}^{-} \rightarrow \mathrm{ClO}_{3}^{-}+5 \mathrm{Cl}^{-}+3 \mathrm{H}_{2} \mathrm{O}$$

Allow oxidation numbers shown next to a combination of correct names and formulae even if the equations are not balanced

Ignore state symbols even if incorrect