The standard enthalpies of formation of , and are –46, –155 and –285 kJ/mol, respectively. The entha — Thermodynamics and Thermochemistry Chemistry Question
Question
The standard enthalpies of formation of $\text{NH}_3\text{(g)}$, $\text{CuO(s)}$ and $\text{H}_2\text{O(l)}$ are –46, –155 and –285 kJ/mol, respectively. The enthalpy change when 6.80 g of $\text{NH}_3$ is passed over cupric oxide is
Answer: A
💡 Solution & Explanation
Reaction: $2\text{NH}_3 + 3\text{CuO} \to \text{N}_2 + 3\text{Cu} + 3\text{H}_2\text{O}$. $\Delta H = 3(-285) - [2(-46) + 3(-155)] = -855 - (-92 - 465) = -855 + 557 = -298\text{ kJ}$ for 2 moles of $\text{NH}_3$. Moles of $\text{NH}_3$ used = $6.80 / 17 = 0.4\text{ mol}$. Heat change = $-298 \times (0.4 / 2) = -59.6\text{ kJ}$.
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