CAIE IGCSE Chemistry 6.3 Reversible reactions & equilibrium Question Bank

Practise reversible reactions, equilibrium ideas and industrial conditions for ammonia and sulfuric acid manufacture.

Syllabus
2026–2028
Topic
6.3
Level
—

Exam points

  • interpret ⇌ equations and colour changes to decide whether a reaction is reversible
  • explain equilibrium using equal forward and reverse rates and constant concentrations
  • predict shifts in yield from pressure or temperature for Haber and Contact process equations

Covered syllabus points

6.3.1—Some chemical reactions are reversible6.3.2—Changing the conditions can change the6.3.3—Reversible reaction in a closed system6.3.4—Predict and explain, for a reversible6.3.5—Symbol equation for the production of6.3.6—Sources of the hydrogen (methane) and6.3.7—Typical conditions in the Haber6.3.8—Symbol equation for the conversion of6.3.9—Sources of the sulfur dioxide (burning6.3.10—Typical conditions for the conversion6.3.11—Typical Haber and Contact process

6.3 Reversible reactions and equilibrium question 1

[Maximum number: 8]

Sulfuric acid can be manufactured from the raw materials sulfur, air and water. The process can be divided into four stages.
stage 1 converting sulfur into sulfur dioxide
stage 2 converting sulfur dioxide into sulfur trioxide
stage 3 converting sulfur trioxide into oleum, H2 S2O7\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7}
stage 4 converting oleum into sulfuric acid
stage 1

Question (a)

(a)

Describe how sulfur is converted into sulfur dioxide.

[ 1 ]

Question (b)

(b)

Sulfur dioxide is converted into sulfur trioxide according to the following equation.

2SO2+O2⇌2SO32 \mathrm{SO}_{2}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{SO}_{3}

The reaction is carried out at a temperature of 450∘C450^{\circ} \mathrm{C} and a pressure of 1-2 atmospheres using a catalyst. The energy change, ΔH\Delta H, for the reaction is −196 kJ/mol-196 \mathrm{~kJ} / \mathrm{mol}.

[ 7 ]

Question (i)

(i)

What is the meaning of the symbol ⇌?

[ 1 ]

Question (ii)

(ii)

Name the catalyst used in this reaction.

[ 1 ]

Question (iii)

(iii)

If a temperature higher than 450∘C450^{\circ} \mathrm{C} were used, what would happen to the amount of sulfur trioxide produced? Give a reason for your answer.

[ 2 ]

Question (iv)

(iv)

Suggest a reason why a temperature lower than 450∘C450^{\circ} \mathrm{C} is not used.

[ 1 ]

Question (v)

(v)

If a pressure higher than 1-2 atmospheres were used, what would happen to the amount of sulfur trioxide produced? Give a reason for your answer.
stage 3

[ 2 ]

6.3 Reversible reactions and equilibrium question 2

[Maximum number: 1]

Cobalt and copper are transition elements.

Both copper and cobalt can form coloured compounds. Some of these compounds contain water of crystallisation.

State how this colour change can be reversed.
[Total: 14]

6.3 Reversible reactions and equilibrium question 3

[Maximum number: 6]

The Periodic Table is a method of classifying elements.

Question (a)

(a)

PCl3\mathrm{PCl}_{3} reacts with chlorine, Cl2\mathrm{Cl}_{2}, to form PCl5\mathrm{PCl}_{5}. This reaction is exothermic and reaches an equilibrium.

PCl3( g)+Cl2( g)⇌PCl5( g)\mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{5}(\mathrm{~g})
[ 6 ]

Question (i)

(i)

Describe two features of an equilibrium.

[ 2 ]

Question (ii)

(ii)

State the effect, if any, on the position of this equilibrium when the following changes are made.
Explain your answers.
temperature is increased
pressure is increased

[ 4 ]
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