The standard enthalpies of formation of , , , and are , , , and respectively. Calculate the enthalpy — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard enthalpies of formation of $CO(g)$, $CO_2(g)$, $N_2O(g)$, and $N_2O_4(g)$ are $-110$, $-393$, $+81$, and $+9.7 \text{ kJ mol}^{-1}$ respectively. Calculate the enthalpy change ($\Delta H$) for the reaction: $N_2O_4(g) + 3CO(g) \rightarrow N_2O(g) + 3CO_2(g)$.
Answer: A
💡 Solution & Explanation
Hess's law dictates that $\Delta H_{reaction} = \sum \Delta H_f^\circ(products) - \sum \Delta H_f^\circ(reactants) = [\Delta H_f^\circ(N_2O) + 3\Delta H_f^\circ(CO_2)] - [\Delta H_f^\circ(N_2O_4) + 3\Delta H_f^\circ(CO)]$. Substituting the given data: $[81 + 3(-393)] - [9.7 + 3(-110)] = - [9.7 - 330] = -1098 - (-320.3) = -1098 + 320.3 = -777.7 \text{ kJ mol}^{-1}$.
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