4.15 Half equations at the electrodes
5 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What type of reaction happens at the cathode?
Choose one answer.
Correct Not quite: the answer is Reduction
Mark scheme
B
Model answer
B: positive ions gain electrons at the cathode, which is reduction.
Write the half equation for the formation of bromine at the anode.
Mark scheme
- 2\mathrm{Br^-} → Br2 (1)
- + 2\mathrm{e^-} on the right (1)
Model answer
2\mathrm{Br^-} → Br2 + 2\mathrm{e^-}
Write the half equation for the formation of hydrogen at the cathode.
Mark scheme
- 2\mathrm{H^+} → H2 (1)
- + 2\mathrm{e^-} on the left (1)
Model answer
2\mathrm{H^+} + 2\mathrm{e^-} → H2
Explain why the reaction at the anode is described as oxidation.
Mark scheme
- negative ions lose electrons at the anode (1)
- loss of electrons is oxidation (1)
Model answer
Negative ions lose electrons at the anode, and loss of electrons is oxidation.
Complete and balance this half equation: \mathrm{Zn^{2+}} + ... → Zn
Mark scheme
- \mathrm{Zn^{2+}} + 2\mathrm{e^-} → Zn (1)
Model answer
\mathrm{Zn^{2+}} + 2\mathrm{e^-} → Zn
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