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Consider the following reactions: (i) H⁺(aq) + OH⁻(aq) → H₂O(l), ΔH = −x₁ kJ mol⁻¹ (ii) H₂(g) + 1/2 O₂(g) → H₂O(l), ΔH = −x₂ kJ mol⁻¹ (iii) CO₂(g) + H₂(g) → CO(g) + H₂O, ΔH = −x₃ kJ mol⁻¹ (iv) C₂H₂(g) + 5/2 O₂(g) → 2CO₂(g) + H₂O(l), ΔH = +4x₄ kJ mol⁻¹

1. x₁
2. x₂
3. x₃
4. 4x₄

Correct answer: x₂

Solution

The reaction (ii) represents the enthalpy change for the formation of water from its elements in their standard states, which is a standard enthalpy of formation. This makes x₂ the most relevant enthalpy term.

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