S4 Chemistry Practice Paper: Reactivity, Extraction and Corrosion of Metals (100 marks)

2026-11-10

S4 Chemistry · Metals and corrosion

S4 Chemistry paper: 6 questions, 30 parts, 100 marks, 75 min. Covers the reactivity series, metal extraction, mole and yield calculations, corrosion and alloys.

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Section A — Reactivity series and displacement

Write full equations or ionic equations where required.

#	Question	Answer and marking notes
1	Arrange magnesium, zinc, copper and silver in order of decreasing reactivity.	Order of reactivity: Mg > Zn > Cu > Ag.
2	Explain why copper does not react with dilute hydrochloric acid.	Copper lies below hydrogen in the reactivity series, so it cannot reduce hydrogen ions to hydrogen gas; it therefore does not react with dilute hydrochloric acid.
3	Write the ionic equation for the reaction when a zinc strip is placed in copper(II) sulphate solution.	Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s); zinc is oxidised and copper(II) ions are reduced.
4	What would be observed when a zinc strip is placed in silver nitrate solution? Write the equation.	A silvery-white deposit of silver forms on the zinc as the silver ions are reduced: Zn(s) + 2Ag+(aq) -> Zn2+(aq) + 2Ag(s).
5	Experimental results: metal X can displace metal Y from a solution of a Y salt, while metal Z cannot displace Y from the same solution. Arrange X, Y and Z in order of decreasing reactivity.	X > Y > Z: X is more reactive than Y, and Y is more reactive than Z because Z cannot displace Y.

Section B — Extraction of metals

Show mole and mass calculations clearly.

#	Question	Answer and marking notes
6	Why cannot aluminium be extracted by reducing its oxide with carbon, as iron is?	Aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide; moreover the oxide has a very high melting point, so electrolysis is used instead.
7	Write the equation for the reduction of iron(III) oxide by carbon monoxide in the blast furnace.	Fe2O3(s) + 3CO(g) -> 2Fe(l) + 3CO2(g).
8	What mass of iron is obtained from the complete reduction of 160 g of iron(III) oxide (Fe2O3, Mr = 160)? Take Fe = 56.	Each mole of Fe2O3 gives 2 moles of Fe; 160 g is 1.00 mol, giving 2.00 mol of Fe, that is 2.00 x 56 = 112 g.
9	Why is limestone added to the blast furnace?	Limestone decomposes on heating to calcium oxide, which reacts with silica impurities in the ore to form slag and so removes the impurities.
10	Why is cryolite added when aluminium is extracted by electrolysis of aluminium oxide?	Cryolite greatly lowers the melting point of the electrolyte so electrolysis can run at a lower temperature and use less energy.

Section C — Mole and yield calculations

Take the molar volume as 24.0 dm3/mol at room conditions.

#	Question	Answer and marking notes
11	How many moles and what mass of iron are obtained from the complete reduction of 0.250 mol of Fe2O3?	2 moles of Fe per mole of Fe2O3: 0.500 mol of Fe, that is 28.0 g.
12	A 100 g sample of iron ore contains 80.0% Fe2O3. What mass of iron could be obtained in theory? If only 42.0 g is obtained in practice, what is the percentage yield?	Mass of Fe2O3 = 80.0 g = 0.500 mol, giving 1.00 mol of Fe = 56.0 g; yield = 42.0 / 56.0 x 100 = 75.0%.
13	0.100 mol of zinc reacts with excess dilute hydrochloric acid. How many moles of hydrogen are produced, and what volume does this occupy at a molar volume of 24.0 dm3/mol?	Zn + 2HCl -> ZnCl2 + H2, so 1 mole of Zn gives 1 mole of H2: 0.100 mol of H2, volume = 2.40 dm3.
14	If only 1.92 dm3 of hydrogen is collected in the previous question, what is the percentage yield?	Yield = 1.92 / 2.40 x 100 = 80.0%.
15	How many moles are there in 2.70 g of aluminium (Mr = 27.0)?	Moles = 2.70 / 27.0 = 0.100 mol.

Section D — Corrosion and rust prevention

Answers should refer to the underlying chemistry.

#	Question	Answer and marking notes
16	Which two conditions are needed for iron to rust? What is the main constituent of rust?	Oxygen (air) and water are both required; rust is mainly hydrated iron(III) oxide (Fe2O3.xH2O).
17	Explain how zinc protects iron by sacrificial protection.	Zinc is more reactive than iron and is oxidised first, acting as a sacrificial anode; the electrons it releases keep the iron from being oxidised even when the coating is scratched.
18	Why does iron rust faster than bare iron once the tin coating of a tin-plated can is scratched?	Tin is less reactive than iron, so once the coating is broken a tin-iron couple forms, iron becomes the anode and corrodes faster.
19	Describe one physical method of preventing iron from rusting and explain how it works.	Painting or greasing the iron surface (or coating it with plastic) keeps air and water away from the iron, removing the two conditions needed for rusting.
20	Why is stainless steel more resistant to corrosion than pure iron?	Stainless steel contains chromium, which forms a thin adherent chromium oxide layer that prevents oxygen and water from reaching the iron.

Section E — Alloys and properties of metals

Explain in terms of changes in structure.

#	Question	Answer and marking notes
21	Define an alloy.	An alloy is a mixture with metallic properties made by melting a metal together with other elements, metallic or non-metallic.
22	Explain why brass (a copper-zinc alloy) is harder than pure copper.	Zinc atoms differ in size from copper atoms, so layers of copper atoms can no longer slide over one another easily; slip is blocked and the alloy is harder.
23	How do steel and cast iron differ in carbon content, and how does this affect their properties?	Cast iron has a higher carbon content (about 3% to 4%) and is hard but brittle; steel has less carbon (usually under 2%), is tougher and can be forged.
24	Aluminium is fairly reactive, yet aluminium articles resist corrosion well. Why?	A thin adherent layer of aluminium oxide forms on the surface and protects the metal beneath from air and water.
25	Give one example of a light alloy used in transport and state its advantage.	Aluminium-magnesium alloys are used for car bodies and aircraft structures; their low density reduces weight and saves fuel.

Section F — Practical work and safety

Answers should be specific and include observations.

#	Question	Answer and marking notes
26	In the experiment on the reduction of copper(II) oxide by carbon, what is the purpose of passing the gas through limewater, and what is observed?	The limewater tests for the carbon dioxide produced; it turns milky because a white precipitate forms.
27	Describe how copper(II) oxide can be obtained from a copper(II) hydroxide precipitate.	Filter the precipitate, wash it with distilled water, then heat it to dryness; copper(II) hydroxide decomposes into black copper(II) oxide and water.
28	Why is sodium metal stored under paraffin oil?	Sodium is very reactive and reacts with oxygen and water vapour in the air; paraffin oil excludes air and moisture.
29	What safety precautions are needed when using metal powders such as magnesium powder?	Metal powders have a large surface area and are flammable: keep them away from flames, take only small amounts with a spatula, wear goggles and do not heap up large quantities.
30	Why should aluminium containers not be used to store acidic food?	Aluminium reacts with the acid in food, corroding the container and possibly contaminating the food with aluminium salts.

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