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Three thermochemical equations are given below: (i) C(graphite) + O2(g) -> CO2(g), ΔH = x kJ mol⁻¹ (ii) C(graphite) + 1/2 O2(g) -> CO(g), ΔH = y kJ mol⁻¹ (iii) CO(g) + 1/2 O2(g) -> CO2(g), ΔH = z kJ mol⁻¹ Based on the above equations, find out which of the relationship given below is correct?

1. x = y - z
2. z = x + y
3. x = y + z
4. y = z - x

Correct answer: x = y + z

Solution

Using Hess's law, the enthalpy change for the reaction C(graphite) + O2(g) -> CO2(g) (ΔH = x) is the sum of the enthalpy changes for the reactions C(graphite) + 1/2 O2(g) -> CO(g) (ΔH = y) and CO(g) + 1/2 O2(g) -> CO2(g) (ΔH = z). Thus, x = y + z.

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