Reversible reactions and equilibrium
6.3What the syllabus asks of you
- State that some chemical reactions are reversible as shown by the symbol ⇌
- Describe how changing the conditions can change the direction of a reversible reaction for: (a) the effect of heat on hydrated compounds (b) the addition of water to anhydrous compounds including copper(II) sulfate and cobalt(II) chloride
- State that a reversible reaction in a closed system is at equilibrium when: (a) the rate of the forward reaction is equal to the rate of the reverse reaction (b) the concentrations of reactants and products are no longer changing
- Predict and explain, for a reversible reaction, how the position of equilibrium is affected by: (a) changing temperature (b) changing pressure (c) changing concentration (d) using a catalyst using information provided
- State the symbol equation for the production of ammonia in the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g)
- State the sources of the hydrogen (methane) and nitrogen (air) in the Haber process
- State the typical conditions in the Haber process as 450 °C, 20 000 kPa / 200 atm and an iron catalyst
- State the symbol equation for the conversion of sulfur dioxide to sulfur trioxide in the Contact process, 2SO2(g) + O2(g) ⇌ 2SO3(g)
- State the sources of the sulfur dioxide (burning sulfur or roasting sulfide ores) and oxygen (air) in the Contact process
- State the typical conditions for the conversion of sulfur dioxide to sulfur trioxide in the Contact process as 450 °C, 200 kPa / 2 atm and a vanadium(V) oxide catalyst
- Explain, in terms of rate of reaction and position of equilibrium, why the typical conditions stated are used in the Haber process and in the Contact process, including safety considerations and economics
Read the notes first
Chapter 6: Reversible reactions and equilibrium
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Every question on Reversible reactions and equilibrium, untimed, marked as you go.
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63 questions
1May / June 2026 · Paper 11 · Q16Show mark scheme →
A1 and 2
B1 and 3
C1 only
D2 and 3
Mark scheme
2May / June 2026 · Paper 11 · Q17Show mark scheme →
Airon
Bnickel
Cplatinum
Dvanadium(V) oxide
Mark scheme
3May / June 2026 · Paper 12 · Q15Show mark scheme →
A1, 2 and 3
B2 and 3 only
C2 only
D3 only
Mark scheme
4Oct / Nov 2025 · Paper 11 · Q16Show mark scheme →
AThe activation energy increases.
BThe equilibrium shifts to form more ammonia.
CThe rate of the forward reaction decreases.
DThe rate at which ammonia decomposes increases.
Mark scheme
5Oct / Nov 2025 · Paper 12 · Q17Show mark scheme →
A1, 2 and 3
B1 and 2 only
C1 and 3 only
D2 and 3 only
Mark scheme
6May / June 2025 · Paper 11 · Q18Also set in Oct / Nov 2018 Paper 12Show mark scheme →
Aadding a catalyst
Bdecreasing the temperature
Cincreasing the pressure
Dincreasing the volume of the flask
Mark scheme
7May / June 2025 · Paper 11 · Q19Show mark scheme →
Airon, 20 000
Biron, 200
Cvanadium(V) oxide, 20 000
Dvanadium(V) oxide, 200
Mark scheme
8May / June 2025 · Paper 12 · Q19Show mark scheme →
AContact process: iron; Haber process: iron
BContact process: iron; Haber process: nickel
CContact process: vanadium(V) oxide; Haber process: iron
DContact process: vanadium(V) oxide; Haber process: nickel
Mark scheme
9Oct / Nov 2024 · Paper 11 · Q16Show mark scheme →
Aactivation energy
Bbackward reaction
Cequilibrium
Dneutralisation
Mark scheme
10Oct / Nov 2024 · Paper 11 · Q17Show mark scheme →
A100, 200, V2O5
B300, 200, Fe
C450, 2, Fe
D450, 2, V2O5
Mark scheme
11Oct / Nov 2024 · Paper 12 · Q18Show mark scheme →
AThe amount of product increases.
BThe amount of product decreases.
CThe rate of the forward reaction is greater than the rate of the reverse reaction.
DThe rate of the forward reaction is less than the rate of the reverse reaction.
Mark scheme
12Oct / Nov 2024 · Paper 12 · Q19Show mark scheme →
AGas Z is SO2.
BIn R, an iron catalyst speeds up the reaction.
CIn R, the pressure is approximately 200 atm.
DIn R, the temperature is approximately 45 °C.
Mark scheme
13May / June 2024 · Paper 11 · Q19Show mark scheme →
A1, 2 and 3
B1 and 2 only
C1 and 3 only
D2 and 3 only
Mark scheme
14May / June 2024 · Paper 11 · Q20Show mark scheme →
Acracking; long-chain alkanes; nitrogen
Bcracking; long-chain alkenes; oxygen
Cfractional distillation; long-chain alkanes; nitrogen
Dfractional distillation; long-chain alkenes; oxygen
Mark scheme
15May / June 2024 · Paper 12 · Q18Show mark scheme →
ALess calcium carbonate is produced if the pressure in the flask increases.
BMore calcium carbonate is produced when the temperature is increased.
CThe flask must be sealed if equilibrium is to be reached.
DWhen equilibrium is reached, the reaction stops.
Mark scheme
16May / June 2024 · Paper 12 · Q19Show mark scheme →
Ahydrogen, nickel
Bhydrogen, iron
Cnitrogen, nickel
Dnitrogen, iron
Mark scheme
17Oct / Nov 2023 · Paper 11 · Q18Show mark scheme →
Aadding sulfuric acid: darker orange; adding solid sodium bromide: darker orange; adding water: darker orange
Badding sulfuric acid: darker orange; adding solid sodium bromide: darker orange; adding water: paler orange
Cadding sulfuric acid: darker orange; adding solid sodium bromide: paler orange; adding water: darker orange
Dadding sulfuric acid: paler orange; adding solid sodium bromide: darker orange; adding water: paler orange
Mark scheme
18Oct / Nov 2023 · Paper 12 · Q17Show mark scheme →
Airon / alkane
Biron / alcohol
Cnickel / alkane
Dnickel / alcohol
Mark scheme
19May / June 2023 · Paper 11 · Q15Show mark scheme →
Adecrease / no change
Bdecrease / decrease
Cincrease / increase
Dincrease / decrease
Mark scheme
20May / June 2023 · Paper 12 · Q17Show mark scheme →
A1, 2 and 3
B1 and 3 only
C1 only
D2 and 4
Mark scheme
21May / June 2023 · Paper 12 · Q18Show mark scheme →
Areaction 1: no, reaction 2: no
Breaction 1: no, reaction 2: yes
Creaction 1: yes, reaction 2: no
Dreaction 1: yes, reaction 2: yes
Mark scheme
22Oct / Nov 2022 · Paper 11 · Q30Show mark scheme →
AIn the manufacture of sulfuric acid, iron is used as the catalyst in the Contact process.
BSulfuric acid is used in some batteries.
CSulfuric acid is used as a fertiliser.
DSulfuric acid is used as a food preservative.
Mark scheme
23Oct / Nov 2022 · Paper 11 · Q32Show mark scheme →
Aboth aluminium and iron; moves to the right
Bboth aluminium and iron; no change
Ciron only; moves to the right
Diron only; no change
Mark scheme
24Oct / Nov 2022 · Paper 12 · Q26Also set in Oct / Nov 2022 Paper 11Show mark scheme →
Aadding more O2
Bincreasing the pressure
Cincreasing the temperature
Dremoving SO3 from the reacting mixture
Mark scheme
25Oct / Nov 2022 · Paper 12 · Q31Show mark scheme →
A100–200 / 200 / V2O5
B100–200 / 1–2 / Fe
C400–500 / 1–2 / V2O5
D400–500 / 200 / V2O5
Mark scheme
26May / June 2022 · Paper 11 · Q29Show mark scheme →
AFood can be preserved by using sulfur dioxide.
BIn the Contact process oxygen reacts with sulfur to make sulfur trioxide.
CSulfur dioxide is used to kill bacteria present in wood pulp.
DSulfuric acid is used as a bleach.
Mark scheme
27May / June 2022 · Paper 12 · Q22Also set in May / June 2022 Paper 11Show mark scheme →
AIt becomes a darker brown.
BIt becomes a paler brown.
CIt turns colourless.
DThere is no change.
Mark scheme
28May / June 2022 · Paper 12 · Q29Show mark scheme →
A1 nitrogen, 2 oxygen, 3 hydrogen, 4 sulfur
B1 nitrogen, 2 oxygen, 3 hydrogen, 4 oxygen
C1 oxygen, 2 nitrogen, 3 hydrogen, 4 sulfur
D1 oxygen, 2 nitrogen, 3 sulfur, 4 hydrogen
Mark scheme
29May / June 2022 · Paper 12 · Q30Show mark scheme →
Asulfur dioxide: as a bleach; sulfuric acid: as battery acid
Bsulfur dioxide: killing bacteria in food; sulfuric acid: as a bleach
Csulfur dioxide: making detergents; sulfuric acid: as battery acid
Dsulfur dioxide: making fertilisers; sulfuric acid: making fertilisers
Mark scheme
30Oct / Nov 2021 · Paper 11 · Q21Show mark scheme →
A1, 2 and 3
B2 only
C3 only
D2 and 3 only
Mark scheme
31Oct / Nov 2021 · Paper 12 · Q21Show mark scheme →
AIntroduction of a catalyst changes the position of the equilibrium.
BThe number of moles of products equals the number of moles of reactants.
CThe rate of the forward reaction equals the rate of the reverse reaction.
DWhen the reaction reaches the position of equilibrium the reaction stops.
Mark scheme
32Oct / Nov 2021 · Paper 12 · Q25Show mark scheme →
A% NH3 at equilibrium vs pressure; 350 °C curve above 250 °C curve
B% NH3 at equilibrium vs pressure; 250 °C curve above 350 °C curve
C% NH3 at equilibrium vs pressure; 350 °C curve above 250 °C curve, both decreasing
D% NH3 at equilibrium vs pressure; 250 °C curve above 350 °C curve, both decreasing
Mark scheme
33Oct / Nov 2021 · Paper 12 · Q26Show mark scheme →
AIt is dissolved in water to make sulfuric acid for car batteries.
BIt is the final product of the Contact process.
CIt is used as a food preservative.
DIt turns aqueous potassium iodide brown.
Mark scheme
34May / June 2021 · Paper 11 · Q21Show mark scheme →
Apressure is increased / moves to the left
Bpressure is reduced / no change
Ccatalyst present / moves to the right
Dcatalyst present / no change
Mark scheme
35May / June 2021 · Paper 11 · Q25Show mark scheme →
AIt is made at an increased rate and the position of the equilibrium moves to the left.
BIt is made at an increased rate and the position of the equilibrium moves to the right.
CIt is made at a decreased rate and the position of the equilibrium moves to the left.
DIt is made at a decreased rate and the position of the equilibrium moves to the right.
Mark scheme
36May / June 2021 · Paper 12 · Q21Show mark scheme →
Atemperature decrease, pressure decrease
Btemperature decrease, pressure increase
Ctemperature increase, pressure decrease
Dtemperature increase, pressure increase
Mark scheme
37May / June 2021 · Paper 12 · Q25Show mark scheme →
Avanadium(V) oxide: catalyst, water: reactant
Bvanadium(V) oxide: catalyst, water: solvent
Cvanadium(V) oxide: reactant, water: reactant
Dvanadium(V) oxide: reactant, water: solvent
Mark scheme
38Oct / Nov 2020 · Paper 11 · Q25Show mark scheme →
Airon
Bnickel
Cplatinum
Dvanadium(V) oxide
Mark scheme
39Oct / Nov 2020 · Paper 12 · Q22Also set in Oct / Nov 2020 Paper 11, Oct / Nov 2016 Paper 12Show mark scheme →
A1 and 3
B1 only
C2 and 3
D3 only
Mark scheme
40Oct / Nov 2020 · Paper 12 · Q25Show mark scheme →
AA low pressure is used to shift the position of the equilibrium to the right.
BA high temperature is used to shift the position of the equilibrium to the right.
CAn iron catalyst is used to shift the position of the equilibrium to the right.
DThe nitrogen used is obtained from the air.
Mark scheme
41May / June 2020 · Paper 12 · Q23Show mark scheme →
A1 and 2
B1 and 3
C1 and 4
D2 and 4
Mark scheme
42Oct / Nov 2019 · Paper 11 · Q22Show mark scheme →
A1 and 2 only
B1 and 3 only
C2 and 3 only
D1, 2 and 3
Mark scheme
43May / June 2019 · Paper 11 · Q19Show mark scheme →
AH2(g) + I2(g) ⇌ 2HI(g) — exothermic
B4NO(g) + 6H2O(g) ⇌ 4NH3(g) + 5O2(g) — endothermic
CPCl5(g) ⇌ PCl3(g) + Cl2(g) — endothermic
D2SO2(g) + O2(g) ⇌ 2SO3(g) — exothermic
Mark scheme
44May / June 2019 · Paper 11 · Q24Show mark scheme →
A1 and 2
B1 and 3
C2 only
D3 only
Mark scheme
45May / June 2019 · Paper 12 · Q24Show mark scheme →
A1 and 2 only
B2 and 3 only
C3 and 4 only
D1, 2 and 3 only
Mark scheme
46May / June 2019 · Paper 12 · Q25Also set in May / June 2019 Paper 11, May / June 2016 Paper 11Show mark scheme →
A1 and 3
B2 and 4
C2 only
D3 only
Mark scheme
47Oct / Nov 2018 · Paper 11 · Q17Show mark scheme →
A1 and 2
B1 and 3
C1 only
D2 and 3
Mark scheme
48Oct / Nov 2018 · Paper 11 · Q22Show mark scheme →
AAmmonia is returned to the start of the process to shift the equilibrium towards the product.
BThe gases are cooled to cause ammonia to form a liquid.
CUnreacted hydrogen only is recycled.
DUnreacted hydrogen and nitrogen are recycled.
Mark scheme
49Oct / Nov 2018 · Paper 12 · Q17Show mark scheme →
ABoth reaction 1 and reaction 2 are exothermic.
BReaction 2 is reversible.
CThe equation for reaction 1 is NH5Cl → NH4 + HCl.
DThe three substances involved in each reaction all have a simple molecular structure.
Mark scheme
50Oct / Nov 2018 · Paper 12 · Q23Show mark scheme →
A100, 1, V2O5
B300, 1000, Fe
C450, 1, Fe
D450, 1, V2O5
Mark scheme
51May / June 2018 · Paper 12 · Q21Also set in May / June 2018 Paper 11Show mark scheme →
AThe concentration of nitrogen present will change with time.
BThe forward and backward reactions are taking place at the same rate.
CThe forward reaction releases heat energy.
DThere are more molecules on the left hand side of the equation than on the right.
Mark scheme
52May / June 2018 · Paper 12 · Q25Also set in May / June 2018 Paper 11Show mark scheme →
AP, Q and R
BQ and R only
CQ only
DR only
Mark scheme
53Oct / Nov 2017 · Paper 11 · Q24Show mark scheme →
A→ and cat
B→ and cat
C⇌ and cat
D⇌ and cat
Mark scheme
54Oct / Nov 2017 · Paper 12 · Q20Also set in Oct / Nov 2017 Paper 11Show mark scheme →
Aadding a catalyst
Bincreasing the pressure
Cincreasing the temperature
Dremoving some SO2 and O2
Mark scheme
55Oct / Nov 2017 · Paper 12 · Q29Also set in Oct / Nov 2017 Paper 11Show mark scheme →
Anickel | iron | vanadium(V) oxide
Bnickel | vanadium(V) oxide | iron
Cvanadium(V) oxide | iron | nickel
Dvanadium(V) oxide | nickel | iron
Mark scheme
56May / June 2017 · Paper 11 · Q28Show mark scheme →
Acontact process — vanadium(V) oxide
BHaber process — manganese(IV) oxide
Chydrogenation of alkenes — iron
Dphotosynthesis — glucose
Mark scheme
57May / June 2017 · Paper 12 · Q17Also set in May / June 2017 Paper 11Show mark scheme →
Aaddition of more oxygen
Ban increase in pressure
Can increase in temperature
Dremoval of sulfur trioxide from the reaction chamber
Mark scheme
58May / June 2017 · Paper 12 · Q30Show mark scheme →
Aa displacement reaction
Ba neutralisation reaction
Ca precipitation reaction
Da reversible reaction
Mark scheme
59Oct / Nov 2016 · Paper 11 · Q30Show mark scheme →
Ahydrogen / iron
Bhydrogen / nickel
Cnitrogen / iron
Dnitrogen / nickel
Mark scheme
60Oct / Nov 2016 · Paper 12 · Q29Show mark scheme →
Aa bleach
Ba catalyst
Can oxidising agent
Da solvent
Mark scheme
61May / June 2016 · Paper 12 · Q14Show mark scheme →
Aadding a catalyst
Bdecreasing the pressure
Cdecreasing the temperature
Dremoving some ammonia during the reaction
Mark scheme
62May / June 2016 · Paper 12 · Q17Also set in May / June 2016 Paper 11Show mark scheme →
Athe extraction of iron from haematite in a blast furnace
Bthe manufacture of sulfur trioxide
Cthe production of ammonia from nitrogen and hydrogen
Dthe redox reactions that remove combustion pollutants from car exhausts
Mark scheme
63May / June 2016 · Paper 12 · Q26Also set in May / June 2016 Paper 11Show mark scheme →
AW = H₂, X = N₂, Y = high, Z = NH₃
BW = O₂, X = SO₂, Y = high, Z = SO₃
CW = O₂, X = SO₂, Y = low, Z = SO₃
DW = N₂, X = H₂, Y = high, Z = NH₃
Mark scheme