Topic 4 Practice Paper: Electrochemistry — mark scheme (print)

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QQ/IGCSE-CHEM/T4

IGCSE · Cambridge IGCSE Chemistry (Extended (Core + Supplement))

Mark scheme

Electrochemistry

Topic 4 Practice Paper · Maximum mark: 30

Notes for markers

Award one mark for each point marked [1], up to the marks shown for the part.
Accept answers worded differently that show the same understanding.
Units are required where the answer line shows one. ecf: allow an error carried forward.

Section A: Multiple-choice questions[10 marks]

Question	Answer
1	CAn ionic compound must be molten or dissolved for its ions to move, so only molten sodium chloride conducts and can be electrolysed.	1
2	APositive ions (cations) are attracted to the negative electrode, which is the cathode.	1
3	DMolten zinc chloride contains Zn²⁺ and Cl⁻ ions. Chloride ions are discharged at the anode, giving off chlorine gas.	1
4	BGraphite conducts electricity and is inert, so it does not interfere with the products forming at the electrodes.	1
5	CIodide ions are attracted to the anode and lose electrons, forming iodine. Hydrogen would form at the cathode.	1
6	AChloride ions are lost at the positive anode during oxidation, so chlorine gas is produced there.	1
7	DDilute sulfuric acid gives hydrogen at the cathode and oxygen at the anode, and the volume of hydrogen is twice that of oxygen.	1
8	CThe electrolyte must supply ions of the plating metal, so a silver salt solution such as silver nitrate is used.	1
9	BThe plating metal forms the anode, and it slowly dissolves, releasing metal ions that are deposited on the cathode (the object being plated).	1
10	AHydrogen and oxygen combine in the fuel cell, and the only product formed is water.	1

Section B: Structured questions[20 marks]

Question	Answer and marking points	Marks
11(a)	To improve its appearance by giving a shiny finish, and to resist corrosion so that it does not tarnish or rust. improve appearance / shiny finish [1] resist corrosion / stop it rusting [1]	2
11(b)	The negative terminal, so the cutlery forms the cathode. negative terminal [1] cathode [1]	2
11(c)	A solution containing chromium ions, such as chromium sulfate or another chromium salt solution. a solution containing chromium ions / a chromium salt solution [1]	1
	Total for Question 11	5
12(a)	Electrons carry the charge through the metal wires of the external circuit, while ions carry the charge through the electrolyte. electrons carry charge through the metal wires [1] ions carry charge through the electrolyte [1]	2
12(b)	Na⁺ + e⁻ → Na [1]	1
12(c)	2Cl⁻ → Cl₂ + 2e⁻ [2]	2
	Total for Question 12	5
13(a)	hydrogen [1]	1
13(b)	chlorine [1]	1
13(c)	oxygen [1]	1
13(d)	The concentrated solution contains enough chloride ions for them to be discharged at the anode, releasing chlorine. In the dilute solution there are too few chloride ions, so water is oxidised instead and oxygen is released. the concentrated solution has enough chloride ions to be discharged [1] in the dilute solution water is oxidised, so oxygen is produced [1]	2
	Total for Question 13	5
14(a)	The reactants are hydrogen and oxygen, and the only product is water. hydrogen and oxygen [1] water [1]	2
14(b)	Advantage: the only product is water, so no carbon dioxide or pollutant gases are released. Disadvantage: hydrogen is flammable and difficult to store safely (or the cells are expensive). advantage: only product is water / no carbon dioxide or pollutant gases [1] disadvantage: hydrogen is flammable / hard to store / cells are expensive [1]	2
14(c)	hydrogen + oxygen → water [1]	1
	Total for Question 14	5

— End of mark scheme —