Hess's law Knowledge-Based Download PDF

Questions

1. 1.

   Given the thermochemical equations $\text{N}_2\text{H}_4(\text{l}) + \text{O}_2(\text{g}) \rightarrow \text{N}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l})$ with $\Delta H^\circ = -622.2\text{ kJ}$ and $\text{H}_2(\text{g}) + \frac{1}{2}\text{O}_2(\text{g}) \rightarrow \text{H}_2\text{O}(\text{l})$ with $\Delta H^\circ = -285.8\text{ kJ}$, what is the standard enthalpy change, $\Delta H^\circ$, for the reaction $\text{N}_2(\text{g}) + 2\text{H}_2(\text{g}) \rightarrow \text{N}_2\text{H}_4(\text{l})$?

   • A) $-50.6\text{ kJ}$
   • B) $+50.6\text{ kJ}$
   • C) $+336.4\text{ kJ}$
   • D) $-1193.8\text{ kJ}$
2. 2.

   Consider the following thermochemical equations: $$2\text{N}_2\text{O}(\text{g}) \rightarrow 2\text{N}_2(\text{g}) + \text{O}_2(\text{g}) \quad \Delta H_1^\circ = -164.2\text{ kJ}$$$$\text{N}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{NO}(\text{g}) \quad \Delta H_2^\circ = +180.5\text{ kJ}$$$$2\text{NO}(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{NO}_2(\text{g}) \quad \Delta H_3^\circ = -114.1\text{ kJ}$$According to Hess's law, what is the standard enthalpy change, $\Delta H^\circ$, for the reaction $\text{N}_2\text{O}(\text{g}) + \frac{3}{2}\text{O}_2(\text{g}) \rightarrow 2\text{NO}_2(\text{g})$?

   • A) $-97.8\text{ kJ}$
   • B) $-15.7\text{ kJ}$
   • C) $+148.5\text{ kJ}$
   • D) $+212.5\text{ kJ}$
3. 3.

   Given the thermochemical equations: $4\text{Al}(\text{s}) + 3\text{O}_2(\text{g}) \rightarrow 2\text{Al}_2\text{O}_3(\text{s}) \quad \Delta H^\circ = -3352\text{ kJ}$ $4\text{Fe}(\text{s}) + 3\text{O}_2(\text{g}) \rightarrow 2\text{Fe}_2\text{O}_3(\text{s}) \quad \Delta H^\circ = -1648\text{ kJ}$ What is the standard enthalpy change, $\Delta H^\circ$, for the reaction $2\text{Al}(\text{s}) + \text{Fe}_2\text{O}_3(\text{s}) \rightarrow \text{Al}_2\text{O}_3(\text{s}) + 2\text{Fe}(\text{s})$?

   • A) $-852\text{ kJ}$
   • B) $-1704\text{ kJ}$
   • C) $-2500\text{ kJ}$
   • D) $+852\text{ kJ}$
4. 4.

   Given the following thermochemical equations: $$2\text{P}(\text{s}) + 3\text{Cl}_2(\text{g}) \rightarrow 2\text{PCl}_3(\text{l}) \quad \Delta H^\circ = -640\text{ kJ}$$$$2\text{P}(\text{s}) + 5\text{Cl}_2(\text{g}) \rightarrow 2\text{PCl}_5(\text{s}) \quad \Delta H^\circ = -886\text{ kJ}$$What is the standard enthalpy change, $\Delta H^\circ$, for the reaction $\text{PCl}_3(\text{l}) + \text{Cl}_2(\text{g}) \rightarrow \text{PCl}_5(\text{s})$?

   • A) $-246\text{ kJ}$
   • B) $-763\text{ kJ}$
   • C) $+123\text{ kJ}$
   • D) $-123\text{ kJ}$
5. 5.

   Given the standard enthalpies of combustion for $\text{C}(\text{graphite})$ ($-393.5\text{ kJ/mol}$), $\text{H}_2(\text{g})$ ($-285.8\text{ kJ/mol}$), and $\text{C}_2\text{H}_2(\text{g})$ ($-1299.5\text{ kJ/mol}$), what is the standard enthalpy of formation of ethyne according to the reaction $2\text{C}(\text{graphite}) + \text{H}_2(\text{g}) \rightarrow \text{C}_2\text{H}_2(\text{g})$?

   • A) $+226.7\text{ kJ/mol}$
   • B) $-226.7\text{ kJ/mol}$
   • C) $+620.2\text{ kJ/mol}$
   • D) $-2372.3\text{ kJ/mol}$