Topic 6 Practice Paper: Chemical reactions — mark scheme (print)

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QQ/IGCSE-CHEM/T6

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

Mark scheme

Chemical reactions

Topic 6 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	CHeating copper carbonate makes new substances, copper oxide and carbon dioxide, so this is a chemical change. The other changes only alter the state or form of a substance.	1
2	AA catalyst opens a route with a lower energy barrier, so more collisions have enough energy to react. It does not change the equilibrium yield.	1
3	DHeating gives the particles more kinetic energy, so they move faster and a greater fraction of their collisions reach the activation energy.	1
4	BA powder has a larger surface area and a warmer acid gives faster particles, and both changes increase the rate. The other pairs slow the reaction down.	1
5	CAmmonium chloride breaks down on heating and re-forms when the gases cool, so the products can react to give back the reactant.	1
6	AThere are fewer gas molecules on the right, so a higher pressure shifts the equilibrium towards the ammonia side. A catalyst changes the rate but not the position.	1
7	DA catalyst lowers the activation energy for both directions, so forward and reverse reactions speed up equally and the equilibrium position is unchanged.	1
8	BCarbon monoxide gains oxygen to become carbon dioxide, so CO is oxidised. The iron(III) oxide loses oxygen and is reduced.	1
9	DIn terms of electrons, oxidation is the loss of electrons. Gain of electrons is reduction.	1
10	BThe manganate(VII) ion has been reduced, which means it oxidised something in the solution. A substance that reduces another is itself a reducing agent.	1

Section B: Structured questions[20 marks]

Question	Answer and marking points	Marks
11(a)	(ii) burning magnesium and (iv) rusting. (ii) and (iv) [1]	1
11(b)	A white solid (magnesium oxide) is formed, and light and heat are given out. a new white solid is formed / colour change [1] light or heat is given out [1]	2
11(c)	Only the state changes and no new substance is formed. only the state changes / no new substance is formed [1]	1
	Total for Question 11	4
12(a)	Carbon dioxide; bubble it through limewater, which turns milky (cloudy). carbon dioxide [1] turns limewater milky / cloudy [1]	2
12(b)	The rate increases. The powder has a larger surface area, so the acid particles collide with the marble more frequently. the rate increases [1] larger surface area gives more frequent collisions [1]	2
12(c)	The particles have more kinetic energy and move faster, so collisions are more frequent and a larger fraction of them have at least the activation energy. particles have more kinetic energy / move faster [1] more collisions have at least the activation energy [1]	2
	Total for Question 12	6
13(a)	N₂ + 3H₂ ⇌ 2NH₃ [1]	1
13(b)	Natural gas (methane), which is reacted with steam. natural gas / methane [1]	1
13(c)	A higher temperature speeds up both reactions, so the rate in both directions increases. The equilibrium shifts in the endothermic (backward) direction, so the yield of ammonia decreases. a higher temperature increases the rate of both reactions [1] the equilibrium shifts in the endothermic / backward direction [1] so the yield of ammonia falls [1]	3
	Total for Question 13	5
14(a)	+3 (III). +3 / III [1]	1
14(b)	The iron changes from +3 in Fe₂O₃ to 0 in Fe, so it is reduced. The carbon changes from +2 in CO to +4 in CO₂, so it is oxidised. iron changes from +3 to 0, so it is reduced [1] carbon changes from +2 to +4, so it is oxidised [1]	2
14(c)	Carbon monoxide is the reducing agent. It reduces the iron(III) oxide and is itself oxidised. carbon monoxide [1] it reduces the iron(III) oxide / is itself oxidised [1]	2
	Total for Question 14	5

— End of mark scheme —