4.17 Core practical: electrolysis of copper sulfate
5 exam questions worth 10 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 happens to the mass of a copper anode during the electrolysis of copper sulfate solution?
Choose one answer.
Correct Not quite: the answer is It decreases
Mark scheme
B
Model answer
B: copper atoms at the anode become ions and dissolve, so the anode loses mass.
Explain why the copper sulfate solution stays the same blue colour when copper electrodes are used.
Mark scheme
- copper ions are removed at the cathode (1)
- and replaced at the same rate from the anode (1)
Model answer
Copper ions are deposited at the cathode, but the same number dissolve from the anode, so the concentration of copper ions stays the same.
Give two variables that must be controlled when investigating how current affects the mass change of the electrodes.
Mark scheme
any two from:
- time (1)
- concentration of copper sulfate (1)
- volume of solution / size of electrodes / distance between electrodes / temperature (1)
Model answer
The time the current flows and the concentration of the copper sulfate solution.
In an experiment, the cathode gained 0.08 g when a current of 0.2 A was passed for 20 minutes.
Predict the mass gained with a current of 0.4 A for 20 minutes. Explain your answer.
Mark scheme
- 0.16 g (1)
- mass change is directly proportional to current (1)
Model answer
0.16 g, because doubling the current doubles the mass of copper deposited.
Describe how electrolysis is used to purify copper.
Mark scheme
- impure copper is the anode (1)
- pure copper cathode, where copper is deposited (1)
- impurities fall to the bottom as sludge (1)
Model answer
The impure copper is used as the anode and a thin sheet of pure copper as the cathode. Copper dissolves from the anode and pure copper is deposited on the cathode, while impurities fall to the bottom as sludge.
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