The symbol ⇌ in a chemical equation means:
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These Reversible reactions and equilibrium Topical Past Paper Questions are written in the style of recent Cambridge IGCSE Chemistry 0620 Extended papers and grouped by the Chemical reactions (C6.3) section of the 2025–2027 syllabus. Use them to revise the exact skills examiners test in this part of the course.
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Describe a reversible reaction and the ⇌ symbol. State the conditions for a dynamic equilibrium. Extended candidates predict the effect of changing temperature, pressure and concentration on the position of equilibrium (Le Chatelier-style reasoning) and apply this to the Haber and Contact processes.
The symbol ⇌ in a chemical equation means:
When blue copper(II) sulfate crystals (CuSO₄·5H₂O) are heated, they turn into white anhydrous copper(II) sulfate (CuSO₄). When water is added back, the blue colour returns.
Write a balanced equation (using ⇌) for this reversible reaction.
[1 mark]State which direction is exothermic and which is endothermic. Justify your answer briefly.
[1 mark]Which statement is correct about a reversible reaction at dynamic equilibrium in a closed container?
The Haber process is the industrial production of ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol.
Predict and explain the effect of INCREASING the pressure on the equilibrium position.
[1 mark]Predict and explain the effect of INCREASING the temperature on the equilibrium yield of NH₃.
[1 mark]Despite the lower yield, the Haber process is run at about 450 °C, not at room temperature. Why?
[1 mark]Which of the following will NOT change the position of equilibrium in a reversible reaction?
The Contact process is used to manufacture sulfuric acid. The key reversible step is: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −197 kJ/mol.
Predict and explain the effect of INCREASING the pressure on the equilibrium yield of SO₃.
[1 mark]Predict and explain the effect of INCREASING the temperature on the equilibrium yield of SO₃.
[1 mark]The Contact process is actually run at MODERATE pressure (about 2 atm), not at very high pressure. Suggest why.
[1 mark]A catalyst (V₂O₅) is also used in the Contact process. Why?
[1 mark]Hydrogen iodide decomposes reversibly: 2HI(g) ⇌ H₂(g) + I₂(g) ΔH = +52 kJ/mol.
Predict and explain the effect of INCREASING the pressure on the position of equilibrium.
[1 mark]Predict and explain the effect of INCREASING the temperature on the equilibrium concentration of H₂.
[1 mark]Suggest, in terms of rates, what happens at the instant the temperature is suddenly raised, before equilibrium is re-established.
[1 mark]An exothermic reversible reaction X ⇌ Y (both in solution) is at equilibrium. A small amount of catalyst is added. What happens to (i) the rate of forward reaction, (ii) the rate of reverse reaction, and (iii) the position of equilibrium?