© Oxford University Press 2011 Unit C2 Part 2 Upgrade: Model answers 15 Electrolysis Foundation...

© Oxford University Press 2011

Unit C2 Part 2 Upgrade: Model answers 15

Electrolysis Foundation tier question

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Question

Explanation of question

Answer

Question (part 1)

A teacher passes electricity through liquid lead bromide.Lead bromide contains lead ions (Pb2+) and bromide ions (Br−). She uses the electrolysis cell below.



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Answer

Question (part 2)

(a) Name the ions that move to the positive electrode.

[1]

(b) Explain why a silver-coloured substance forms around the negative electrode.

[2]

(c) Name the substance that is reduced in the electrolysis cell.

Give a reason for your decision.

[2]



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Answer

Question (part 3)

(d) The teacher tries to pass electricity through solid lead bromide.

Predict what will happen.

Give a reason for your prediction.

[2]



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Answer


This question tests specification statements on electrolysis.

In (a), you are simply asked to name the ions that move to the positive electrode. In (c), you are asked for an explanation. Think about why a silver-coloured substance forms around the negative electrode.

Both (c) and (d) ask for reasons. Again, you need to explain your answers carefully.



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Answer

G–E answer

(a) Bromine

(b) Lead metal forms at the negative electrode.

comment

No marks are gained for this answer to (a). The candidate has identified the ions as bromine ions instead of bromide ions.

In (b), the candidate gains one mark out of two because the explanation is incomplete.

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Answer

G–E answer (continued)

(c) Lead ions.

(d) Nothing will happen because only liquid lead bromide conducts because the ions in it can move but they can’t in the solid.

comment

The answer to (c) is correct. However, no explanation is included so the candidate can only be awarded one mark.

The answer to (d) gains two marks. The explanation is correct even though it is not clearly expressed.

This answer is worth a total of four marks out of seven.

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Answer

D–C answer

(a) Bromine

comment

The response to (a) gains no marks. The candidate has identified the ions as bromine ions. The correct answer is bromide ions

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Answer

D–C answer (continued)

(b) Positive lead ions are attracted to the negative electrodes and form lead metal which is silver-coloured.

(c) Lead ions are reduced because they gain electrons.

(d) The solid cannot conduct the electricity. This is because the ions are not free to move to carry the current.

comment

The candidate has included careful explanations when answering (b), (c), and (d). The word ‘because’ is used twice to link answers to the candidate’s reasoning.

In total, this candidate has gained six marks out of seven.

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© Oxford University Press 2011 Unit C2 Part 2 Upgrade: Model answers 15 Electrolysis Foundation...

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