Chemical energetics (A Level only): Practice Questions

Which equation defines the first electron affinity of chlorine?

Use the data below to calculate the lattice energy, \(\Delta H_{\text{latt}}^\ominus\), of sodium chloride, \(NaCl(s)\).

\(\Delta H_f^\ominus[NaCl(s)] = -411\text{ kJ mol}^{-1}\)
\(\Delta H_{\text{at}}^\ominus[Na(s)] = +107\text{ kJ mol}^{-1}\)
\(1^{\text{st}}\text{ Ionisation energy of }Na(g) = +496\text{ kJ mol}^{-1}\)
\(\Delta H_{\text{at}}^\ominus[Cl_2(g)] = +122\text{ kJ mol}^{-1}\)
\(1^{\text{st}}\text{ Electron affinity of }Cl(g) = -349\text{ kJ mol}^{-1}\)

The decomposition of calcium carbonate is shown:
\(CaCO_3(s) \rightarrow CaO(s) + CO_2(g)\)
\(\Delta H^\ominus = +178\text{ kJ mol}^{-1}\) and \(\Delta S^\ominus = +160\text{ J K}^{-1}\text{ mol}^{-1}\).
At what minimum temperature, in Kelvin, does this reaction become feasible?

Which statement best defines the standard enthalpy change of formation, \(\Delta H_f^\ominus\)?

The diagram represents the reaction pathway for a reversible reaction.
What is the activation energy, \(E_a\), for the reverse reaction?

Define the term standard enthalpy change of formation, \(\Delta H_f^\ominus\).

Explain why the lattice energy of magnesium oxide, \(MgO\), is significantly more exothermic than the lattice energy of sodium chloride, \(NaCl\).

In the Born-Haber cycle for magnesium oxide, the second electron affinity of oxygen, \(O^-(g) + e^- \rightarrow O^{2-}(g)\), is endothermic. Explain why energy must be supplied for this process.

(a) Define the term bond energy.

(b) Use the average bond energy values provided below to calculate the enthalpy change, $\Delta H$, for the reaction:

$$\text{CH}_4(\text{g}) + \text{Cl}_2(\text{g}) \rightarrow \text{CH}_3\text{Cl}(\text{g}) + \text{HCl}(\text{g})$$

Average bond energies in $\text{kJ mol}^{-1}$:

• $\text{C}-\text{H}$: $413$
• $\text{Cl}-\text{Cl}$: $242$
• $\text{C}-\text{Cl}$: $346$
• $\text{H}-\text{Cl}$: $432$

Magnesium chloride is an ionic compound. The formation of ionic compounds can be understood through Born-Haber cycles.

(a) Define lattice energy (\(\Delta H_{latt}\)) for magnesium chloride.

(b) Construct a Born-Haber cycle for the formation of magnesium chloride (\(MgCl_2\)) from its elements. Clearly label all enthalpy changes involved.

(c) Using the following data, calculate the lattice energy of magnesium chloride:

Standard enthalpy change of formation of \(MgCl_2\) = \(-641 \text{ kJ mol}^{-1}\)

Enthalpy change of atomisation of Mg = \(+148 \text{ kJ mol}^{-1}\)

First ionisation energy of Mg = \(+738 \text{ kJ mol}^{-1}\)

Second ionisation energy of Mg = \(+1451 \text{ kJ mol}^{-1}\)

Enthalpy change of atomisation of Cl = \(+122 \text{ kJ mol}^{-1}\)

First electron affinity of Cl = \(-349 \text{ kJ mol}^{-1}\)

(d) Explain qualitatively how the ionic radius and ionic charge affect the numerical magnitude of lattice energy.