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2022 O Level Chem P1 Q15 — Energetics / Hess's Law Solution

Energetics / Hess's Law 2022 O Level Pure Chemistry · Paper 1 · Q15

Question

The enthalpy change for the reaction shown in this equation is X kJ. 2C(s) + 2H₂(g) → 3O₂(g) → 2CO₂(g) + 2H₂O(l). The enthalpy changes for two other reactions are shown. C(s) + O₂(g) → CO₂(g) ΔH = −394 kJ. H₂(g) + ½O₂(g) → H₂O(l) ΔH = −286 kJ. What is the value of X?

A−1360
B−1074
C−680
D−216

Correct answer: A

Video Solution

Explanation

Answer: A. Use Hess's law: the target reaction can be built from two copies of the carbon-combustion equation and two copies of the hydrogen-combustion equation.

Target: \(2\text{C(s)} + 2\text{H}_2(\text{g}) + 3\text{O}_2(\text{g}) \rightarrow 2\text{CO}_2(\text{g}) + 2\text{H}_2\text{O(l)}\).

Combine: \(2 \times \bigl(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\bigr)\) gives \(2\text{C} + 2\text{O}_2 \rightarrow 2\text{CO}_2\), enthalpy \(2 \times (-394) = -788\) kJ. \(2 \times \bigl(\text{H}_2 + \tfrac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{O}\bigr)\) gives \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\), enthalpy \(2 \times (-286) = -572\) kJ.

Sum: enthalpy = \(-788 + (-572) = -1360\) kJ → option A.

Options B, C and D forget one of the doublings or get the sign convention wrong.

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