Energy cycles in reactions: notes and practice questions

For which reactions can the enthalpy change be calculated using only average bond enthalpy data?

I. $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$

II. $2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O(l)$

III. $CH_4(g) + Cl_2(g) \rightarrow CH_3Cl(g) + HCl(g)$

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

The standard enthalpy of hydrogenation of cyclohexene is $-120 \text{ kJ mol}^{-1}$.

$C_6H_{10}(l) + H_2(g) \rightarrow C_6H_{12}(l) \quad \Delta H^{\ominus} = -120 \text{ kJ mol}^{-1}$

Based on this value, what would be the predicted standard enthalpy of hydrogenation for the Kekulé structure of benzene, and what does the experimental value being significantly less exothermic indicate about the stability of benzene?

A. Predicted $\Delta H^{\ominus} = -360 \text{ kJ mol}^{-1}$; Benzene is less stable than the Kekulé structure.

B. Predicted $\Delta H^{\ominus} = -360 \text{ kJ mol}^{-1}$; Benzene is more stable than the Kekulé structure.

C. Predicted $\Delta H^{\ominus} = -240 \text{ kJ mol}^{-1}$; Benzene is less stable than the Kekulé structure.

D. Predicted $\Delta H^{\ominus} = -120 \text{ kJ mol}^{-1}$; Benzene is more stable than the Kekulé structure.

Which equation represents the standard enthalpy change of atomisation, $\Delta H_{at}^{\ominus}$, of phosphorus?

A. $P_4(s) \rightarrow 4P(g)$

B. $P(s) \rightarrow P(g)$

C. $\frac{1}{4} P_4(s) \rightarrow P(g)$

D. $P_4(s) \rightarrow P_4(g)$

Consider the following thermochemical equations related to the formation of nitrogen oxides:

$\frac{1}{2}N_2(g) + O_2(g) \rightarrow NO_2(g)$ $\Delta H^\ominus = a$ kJ mol⁻¹

$N_2(g) + 2O_2(g) \rightarrow N_2O_4(g)$ $\Delta H^\ominus = b$ kJ mol⁻¹

What is the standard enthalpy change, $\Delta H^\ominus$, in kJ, for the dimerization of nitrogen dioxide?

$2NO_2(g) \rightarrow N_2O_4(g)$

A. $2a + b$

B. $2a - b$

C. $b - 2a$

D. $b - a$

The hydrogenation of ethene is an important industrial process to produce ethane. Use the provided standard enthalpy of combustion data to calculate the enthalpy change for the hydrogenation of ethene in $kJ\ mol^{-1}$.

$C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)$

Given standard enthalpy of combustion values:

I. $C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) + 2H_2O(l)$ $\Delta H^{\ominus} = -1410\ kJ\ mol^{-1}$

II. $C_2H_6(g) + 3.5O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$ $\Delta H^{\ominus} = -1560\ kJ\ mol^{-1}$

III. $H_2(g) + 0.5O_2(g) \rightarrow H_2O(l)$ $\Delta H^{\ominus} = -285\ kJ\ mol^{-1}$

A. $-3255$

B. $-135$

C. $+135$

D. $+2685$

The Contact Process is a key industrial method for producing sulfuric acid. One crucial step involves the oxidation of sulfur dioxide to sulfur trioxide. Consider the following enthalpy changes related to the formation of sulfur trioxide:

$S(s) + O_2(g) \rightarrow SO_2(g) \quad \Delta H_1 = -296.8 \text{ kJ mol}^{-1}$

$SO_2(g) + \frac{1}{2}O_2(g) \rightarrow SO_3(g) \quad \Delta H_2 = -98.9 \text{ kJ mol}^{-1}$

What is the enthalpy change, $\Delta H$, in $kJ \text{ mol}^{-1}$, for the direct formation of sulfur trioxide from its elements, $S(s) + \frac{3}{2}O_2(g) \rightarrow SO_3(g)$?

A. $-197.9$

B. $-395.7$

C. $+197.9$

D. $+395.7$

Consider the following thermochemical equations:

$2C(s) + O_2(g) \rightarrow 2CO(g) \quad \Delta H^\ominus = m \text{ kJ mol}^{-1}$

$CO_2(g) \rightarrow C(s) + O_2(g) \quad \Delta H^\ominus = n \text{ kJ mol}^{-1}$

What is the value of $\Delta H^\ominus$, in $\text{kJ mol}^{-1}$, for the following reaction?

$2CO(g) + O_2(g) \rightarrow 2CO_2(g)$

A. $m + 2n$

B. $m - 2n$

C. $-m + 2n$

D. $-m - 2n$