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GCSE · Chemistry

IG.C8 Electrolysis of Molten and Aqueous Compounds

EDEXCEL 4CH1 · Calculator allowed · about 50 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Describe, in the context of electrolysis in a chemistry laboratory, what is meant by the term 'electrolysis'.

(Total for Question 1 is 2 marks)

Define the term 'electrolyte' as used when discussing molten or aqueous electrolysis.

(Total for Question 2 is 1 mark)

Describe how inert electrodes differ from reactive electrodes during electrolysis, giving one example of an inert electrode material used in the laboratory.

(Total for Question 3 is 2 marks)

Predict the products formed at the cathode and at the anode during the electrolysis of molten lead(II) bromide, PbBr2, using inert electrodes. Describe briefly what is discharged at each electrode.

(Total for Question 4 is 4 marks)

Write balanced half-equations for the electrode reactions that occur during electrolysis of molten lead(II) bromide, showing electrons explicitly.

(Total for Question 5 is 3 marks)

Predict the products at the electrodes when aqueous sodium chloride solution is electrolysed using inert electrodes. State what is formed at the cathode and at the anode.

(Total for Question 6 is 3 marks)

Write balanced half-equations, including electrons, for the electrode reactions that explain the products seen in Question 6 for aqueous sodium chloride electrolysis with inert electrodes.

(Total for Question 7 is 3 marks)

Predict the products at the electrodes when aqueous copper(II) sulfate is electrolysed using inert electrodes, and state the colour change if any in the solution.

(Total for Question 8 is 2 marks)

Describe briefly how electroplating is carried out using copper electrodes and a copper(II) sulfate solution, and explain why using a copper anode is important in this reactive-electrode method.

(Total for Question 9 is 4 marks)

Explain, with reference to the ions present and the concept of preferential discharge, why electrolysis of molten lead(II) bromide gives different products from electrolysis of an aqueous solution of lead(II) bromide. Refer to what is discharged at each electrode in both molten and aqueous cases.

(Total for Question 10 is 6 marks)

Mark scheme · IG.C8 Electrolysis of Molten and Aqueous Compounds

Question 1

B1 an electrical method that uses an external direct current to cause a chemical change
B1 specifically, it causes ions to move to electrodes and be discharged producing elements or new substances
Answer: Electrolysis is the use of an external direct current to cause a chemical change by moving ions to electrodes where they are discharged to make elements or new substances.

Question 2

B1 a substance that contains ions and conducts electricity when molten or in solution
Answer: A substance that contains ions and conducts electricity when molten or dissolved in water.

Question 3

B1 inert electrodes do not react or dissolve and only provide a surface for electron transfer
B1 example: carbon (graphite) or platinum as an inert electrode material
Answer: Inert electrodes do not chemically react or dissolve, they only provide a surface for electron transfer; example: carbon (graphite) or platinum.

Question 4

B1 at the cathode: Pb2+ ions are discharged to form lead metal
B1 at the anode: Br- ions are discharged to form bromine gas, Br2
B1 states lead metal is produced at the negative electrode and bromine gas at the positive electrode
B1 correct mention that electrodes are inert so products come from the molten ions only
Answer: Cathode: Pb2+ discharged to Pb metal. Anode: Br- discharged to Br2 gas. Inert electrodes mean products are from the ions in the melt.

Question 5

M1 correct cathode half-equation: Pb2+ + 2e- -> Pb
M1 correct anode half-equation: 2Br- -> Br2 + 2e-
A1 both half-equations balanced and electrons shown cao
Answer: Cathode: Pb2+ + 2e- -> Pb. Anode: 2Br- -> Br2 + 2e-.

Question 6

B1 at the cathode: hydrogen gas is produced (H2), from reduction of water, not Na+
B1 at the anode: chlorine gas is produced (Cl2), from oxidation of chloride ions
B1 states sodium remains in solution as Na+ and does not discharge at the cathode
Answer: Cathode: hydrogen gas (H2) from reduction of water. Anode: chlorine gas (Cl2) from oxidation of Cl-; sodium remains as Na+ in solution.

Question 7

M1 correct cathode half-equation: 2H2O + 2e- -> H2 + 2OH- (or 2H+ + 2e- -> H2 with explanation of source)
M1 correct anode half-equation: 2Cl- -> Cl2 + 2e-
A1 equations balanced and consistent with stated products cao
Answer: Cathode: 2H2O + 2e- -> H2 + 2OH-. Anode: 2Cl- -> Cl2 + 2e-.

Question 8

B1 at the cathode: copper metal is deposited from reduction of Cu2+
B1 at the anode: oxygen gas is produced from oxidation of water/ OH-; solution becomes paler as Cu2+ is removed
Answer: Cathode: copper metal deposited. Anode: oxygen gas produced. The blue colour of Cu2+ in solution becomes paler as copper ions are removed.

Question 9

B1 description: the object to be plated is the cathode and a copper block is the anode, both immersed in CuSO4 solution
B1 copper ions are reduced at the cathode to coat the object with copper
B1 the copper anode dissolves producing Cu2+ ions to replace those removed from the solution
B1 explanation: using a copper anode keeps the Cu2+ concentration steady so plating is even and the solution does not become depleted
Answer: The item to be plated is the cathode, a copper block is the anode, and both sit in CuSO4 solution. Cu2+ is reduced at the cathode to form a copper coating; the copper anode dissolves to supply more Cu2+, keeping the solution concentration steady for even plating.

Question 10

Level 1 (1-2): Gives a basic statement about different products in molten and aqueous electrolysis, or lists one correct electrode product for one case.
Level 2 (3-4): Explains the ions present in molten PbBr2 and in aqueous PbBr2 and identifies which ion is discharged at each electrode in both cases, with partial reasoning about preferential discharge.
Level 3 (5-6): Clearly explains that in molten PbBr2 only Pb2+ and Br- are present so Pb2+ is reduced at the cathode to Pb and Br- is oxidised at the anode to Br2, while in aqueous PbBr2 water is also present so at the cathode H+ or metal ion is considered and compares reactivity to show Pb2+ is discharged rather than H+, and at the anode explains that OH- or Br- may be oxidised, giving justified conclusion on products and mentioning how electrode type can affect the outcome.
Indicative content:
- Molten PbBr2 contains only Pb2+ and Br-; no water is present so the ions from the compound are discharged at the electrodes.
- Molten case: at cathode Pb2+ + 2e- -> Pb(s); at anode 2Br- -> Br2(g) + 2e-.
- Aqueous PbBr2 contains Pb2+, Br- and H2O, so water can be involved in electrode reactions.
- Preferential discharge rules: the less reactive positive ion is discharged at the cathode; compare Pb2+ and H+ relative reactivity to explain which is reduced.
- In many aqueous cases Pb2+ is less reactive than hydrogen and so Pb2+ is still reduced to Pb at the cathode, but student should discuss possibility of H2 if metal is more reactive than H.
- At the anode in aqueous solution either Br- or OH- (from water) may be oxidised; halide ions are often oxidised in preference to OH- if halide concentration is significant, producing Br2.
- Mention that inert versus reactive electrode may change observation, for example a reactive anode could dissolve, altering ion concentrations.

Electrolysis of Molten and Aqueous Compounds - Worksheets, Questions and Revision