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Thermodynamics
Enthalpy Changes and Reaction Energy Profiles
1 question from 1 paper, each with its answer.
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- years asked, 2017–2026
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- of Thermodynamics
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Concepts tested
- Exothermic Reaction Energy Diagram
All Questions
2025 1 question
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$\text{C}(s) + 2\text{H}_2(g) \rightarrow \text{CH}_4(g); \Delta H = -74.8\text{ kJ mol}^{-1}$. Which of the following diagrams gives an accurate representation of the above reaction? [R $\rightarrow$ reactants; P $\rightarrow$ products]
Since $\Delta H = -74.8\text{ kJ mol}^{-1}$ is exothermic, products have lower potential energy than reactants by $74.8\text{ kJ mol}^{-1}$.
The formation of methane from carbon and hydrogen is exothermic ($\Delta H = -74.8\text{ kJ mol}^{-1}$). For an exothermic reaction, the enthalpy of products is lower than the enthalpy of reactants: $H_P < H_R$, with $\Delta H = H_P - H_R = -74.8\text{ kJ mol}^{-1}$. Thus, the potential energy profile shows an initial activation barrier followed by a product level lying $74.8\text{ kJ mol}^{-1}$ below the reactant level, represented accurately by diagram (1).