Hess's Law of Constant Heat Summation — Prelims Questions

Given the following thermochemical equations: 1. $C(s) + O_2(g) \to CO_2(g)$, $\Delta H_1 = -393.5 \text{ kJ}$ 2. $H_2(g) + \frac{1}{2}O_2(g) \to H_2O(l)$, $\Delta H_2 = -285.8 \text{ kJ}$ 3. $CH_4(g) + 2O_2(g) \to CO_2(g) + 2H_2O(l)$, $\Delta H_3 = -890.3 \text{ kJ}$ Calculate the standard enthalpy of formation of methane, $CH_4(g)$, i.e., $\Delta H_f^\circ$ for the reaction: $C(s) + 2H_2(g) \to CH_4(g)$.

Which of the following statements about Hess's Law is INCORRECT?

Given the following standard enthalpy of combustion values: $\Delta H_c^\circ (C_2H_6(g)) = -1560 \text{ kJ/mol}$ $\Delta H_c^\circ (C(s)) = -393.5 \text{ kJ/mol}$ $\Delta H_c^\circ (H_2(g)) = -285.8 \text{ kJ/mol}$ Calculate the standard enthalpy of formation of ethane, $C_2H_6(g)$.

Given the following reactions: 1. $S(s) + O_2(g) \to SO_2(g)$, $\Delta H_1 = -296.8 \text{ kJ}$ 2. $SO_2(g) + \frac{1}{2}O_2(g) \to SO_3(g)$, $\Delta H_2 = -98.9 \text{ kJ}$ Calculate the enthalpy change for the reaction: $2S(s) + 3O_2(g) \to 2SO_3(g)$.

The standard enthalpy of formation of $NH_3(g)$ is $-46.1 \text{ kJ/mol}$. What is the enthalpy change for the decomposition of $2 \text{ moles}$ of $NH_3(g)$ into its elements?

Given: $C_2H_4(g) + 3O_2(g) \to 2CO_2(g) + 2H_2O(l)$, $\Delta H = -1411 \text{ kJ}$ $C_2H_6(g) + \frac{7}{2}O_2(g) \to 2CO_2(g) + 3H_2O(l)$, $\Delta H = -1560 \text{ kJ}$ $H_2(g) + \frac{1}{2}O_2(g) \to H_2O(l)$, $\Delta H = -285.8 \text{ kJ}$ Calculate the enthalpy change for the hydrogenation of ethene: $C_2H_4(g) + H_2(g) \to C_2H_6(g)$.

The enthalpy of combustion of $C(s)$ is $-393.5 \text{ kJ/mol}$. The enthalpy of combustion of $CO(g)$ is $-283.0 \text{ kJ/mol}$. What is the enthalpy of formation of $CO(g)$?