Hess's law Knowledge-Based Practice

Questions

1. 1.

   Given the thermochemical equations: $2\text{Al}(s) + \frac{3}{2}\text{O}_2(g) \rightarrow \text{Al}_2\text{O}_3(s)\quad \Delta H^\circ = -1676\text{ kJ}$ $2\text{Fe}(s) + \frac{3}{2}\text{O}_2(g) \rightarrow \text{Fe}_2\text{O}_3(s)\quad \Delta H^\circ = -824\text{ kJ}$ What is the standard enthalpy change, $\Delta H^\circ$, for the reaction $2\text{Al}(s) + \text{Fe}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + 2\text{Fe}(s)$?

   • A) $+852\text{ kJ}$
   • B) $-852\text{ kJ}$
   • C) $-2500\text{ kJ}$
   • D) $+2500\text{ kJ}$
2. 2.

   Given the standard enthalpies of combustion: $\text{C}_2\text{H}_4(g) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 2\text{H}_2\text{O}(l)$ ($\Delta H^\circ = -1411.0\text{ kJ}$), $\text{C}_2\text{H}_6(g) + \frac{7}{2}\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(l)$ ($\Delta H^\circ = -1560.0\text{ kJ}$), and $\text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l)$ ($\Delta H^\circ = -285.8\text{ kJ}$), what is the standard enthalpy change for the hydrogenation reaction $\text{C}_2\text{H}_4(g) + \text{H}_2(g) \rightarrow \text{C}_2\text{H6}(g)$?

   • A) $-3256.8\text{ kJ}$
   • B) $+136.8\text{ kJ}$
   • C) $-136.8\text{ kJ}$
   • D) $-434.8\text{ kJ}$
3. 3.

   Given the following thermochemical equations: $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)\quad \Delta H^\circ = -92.2\text{ kJ}$ $\text{N}_2(g) + \text{O}_2(g) \rightarrow 2\text{NO}(g)\quad \Delta H^\circ = +180.5\text{ kJ}$ $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(g)\quad \Delta H^\circ = -483.6\text{ kJ}$ What is the standard enthalpy change, $\Delta H^\circ$, for the reaction $4\text{NH}_3(g) + 5\text{O}_2(g) \rightarrow 4\text{NO}(g) + 6\text{H}_2\text{O}(g)$?

   • A) $-905.4\text{ kJ}$
   • B) $-1274.2\text{ kJ}$
   • C) $+905.4\text{ kJ}$
   • D) $-721.0\text{ kJ}$
4. 4.

   Given the thermochemical equations $\text{S}(s) + \text{O}_2(g) \rightarrow \text{SO}_2(g)$ ($\Delta H^\circ = -296.8\text{ kJ}$) and $2\text{S}(s) + 3\text{O}_2(g) \rightarrow 2\text{SO}_3(g)$ ($\Delta H^\circ = -791.4\text{ kJ}$), what is the standard enthalpy change, $\Delta H^\circ$, for the reaction $2\text{SO}_2(g) + \text{O}_2(g) \rightarrow 2\text{SO}_3(g)$?

   • A) $+197.8\text{ kJ}$
   • B) $-494.6\text{ kJ}$
   • C) $-1385.0\text{ kJ}$
   • D) $-197.8\text{ kJ}$
5. 5.

   Given the following thermochemical equations: 1) $\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$ $\Delta H^\circ = -393.5\text{ kJ}$ 2) $\text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l)$ $\Delta H^\circ = -285.8\text{ kJ}$ 3) $\text{C}_2\text{H}_2(g) + \frac{5}{2}\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + \text{H}_2\text{O}(l)$ $\Delta H^\circ = -1299.4\text{ kJ}$ What is the standard enthalpy change, $\Delta H^\circ$, for the reaction $2\text{C}(s) + \text{H}_2(g) \rightarrow \text{C}_2\text{H}_2(g)$?

   • A) $-226.6\text{ kJ}$
   • B) $-1978.7\text{ kJ}$
   • C) $+226.6\text{ kJ}$
   • D) $+620.1\text{ kJ}$

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