Enthalpy changes; Hess’s Law; energy cycles: Practice Questions

Which of the following conditions represents standard state for enthalpy changes?

Consider the following enthalpy changes of combustion:
\(\text{C(graphite)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} \text{     } \text{Δ}H = -393.5\thinspace\text{kJ mol}^{-1}\)
\(\text{C(diamond)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} \text{     } \text{Δ}H = -395.4\thinspace\text{kJ mol}^{-1}\)

What is the enthalpy change for the conversion of graphite to diamond: \(\text{C(graphite)} \rightarrow \text{C(diamond)}\)?

Estimate the standard enthalpy change of formation of acetylene gas, \(C_2H_2(g)\), given the following information:

Standard enthalpy of atomisation of \(C(s) = +715 \text{ kJ mol}^{-1}\)
Bond enthalpy of \(H-H = 436 \text{ kJ mol}^{-1}\)
Bond enthalpy of \(C\equiv C = 839 \text{ kJ mol}^{-1}\)
Average bond enthalpy of \(C-H = 413 \text{ kJ mol}^{-1}\)

For an endothermic reaction, which statement best describes the energy changes involved?

Calculate the standard enthalpy change for the combustion of carbon monoxide based on the following standard enthalpy changes of formation at \( 298\thinspace\text{K} \):
\( \Delta H_f^\circ [\text{CO(g)}] = -110.5\thinspace\text{kJ mol}^{-1} \)
\( \Delta H_f^\circ [\text{CO}_2\text{(g)}] = -393.5\thinspace\text{kJ mol}^{-1} \)

What is an exothermic reaction, and what is the sign of its enthalpy change ($$\Delta H$$)?

Given the following average bond enthalpies:

\(\text{C-C} = 347 \text{ kJ mol}^{-1}\)
\(\text{C=O} = 745 \text{ kJ mol}^{-1}\)
\(\text{O-H} = 464 \text{ kJ mol}^{-1}\)
\(\text{C-H} = 413 \text{ kJ mol}^{-1}\)

Using these values, estimate the enthalpy of combustion of methanal, \(\text{CH}_2\text{O(g)}\):

\(\text{CH}_2\text{O(g)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + \text{H}_2\text{O(g)}\)

The bond enthalpy of \(\text{O=O}\) is \(498 \text{ kJ mol}^{-1}\).

Using the following data and a Born-Haber cycle, calculate the lattice enthalpy of sodium oxide ($$\text{Na}_2\text{O(s)}$$):

Standard enthalpy of formation of $$\text{Na}_2\text{O(s)}$$: $$-414 \text{ kJ mol}^{-1}$$
Enthalpy of atomisation of Na(s): $$+107 \text{ kJ mol}^{-1}$$
First ionisation energy of Na(g): $$+496 \text{ kJ mol}^{-1}$$
Enthalpy of atomisation of $$\text{O}_2\text{(g)}$$: $$+249 \text{ kJ mol}^{-1}$$
First electron affinity of O(g): $$-141 \text{ kJ mol}^{-1}$$
Second electron affinity of O(g): $$+790 \text{ kJ mol}^{-1}$$

Sulphur dioxide ($$SO_2$$) reacts with oxygen ($$O_2$$) to form sulphur trioxide ($$SO_3$$), a key step in the Contact Process for sulphuric acid production.

$$2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)$$

Given the standard enthalpies of formation:

• $$\Delta H_f^\circ [SO_2(g)] = -296.8 \text{ kJ mol}^{-1}$$
• $$\Delta H_f^\circ [SO_3(g)] = -395.7 \text{ kJ mol}^{-1}$$

Calculate the standard enthalpy change ($$\Delta H^\circ$$) for the reaction shown above.

Ethanol ($$C_2H_5OH$$) is a widely used organic compound, serving as a solvent, fuel, and raw material in various industries. The direct experimental determination of its standard enthalpy of formation is not practical.

(a) Using the standard enthalpy of combustion data provided below, calculate the standard enthalpy of formation of liquid ethanol, $$\Delta H_f^\circ [C_2H_5OH(l)]$$.

• Standard enthalpy of combustion of liquid ethanol, $$\Delta H_c^\circ [C_2H_5OH(l)] = -1367 \text{ kJ mol}^{-1}$$
• Standard enthalpy of combustion of solid carbon (graphite), $$\Delta H_c^\circ [C(s)] = -393.5 \text{ kJ mol}^{-1}$$
• Standard enthalpy of combustion of hydrogen gas, $$\Delta H_c^\circ [H_2(g)] = -285.8 \text{ kJ mol}^{-1}$$

(b) Explain briefly why it is difficult to determine the standard enthalpy of formation of ethanol directly by simply reacting its constituent elements (solid carbon, hydrogen gas, and oxygen gas) under standard conditions.