Edexcel A-Level Chemistry: Energetics & Thermodynamics

When an ionic solid dissolves in water, the standard enthalpy of solution can be either exothermic or endothermic. It is linked to the lattice energy and the enthalpies of hydration of the ions by an energy cycle.

For a 1:1 ionic solid $\text{MX}(s)$MX(s) dissolving as $\text{MX}(s) \rightarrow \text{M}^{+}(aq) + \text{X}^{-}(aq)$MX(s)→M+(aq)+X−(aq), describe and explain what determines the sign of the standard enthalpy of solution, $\Delta_{\text{sol}}H^{\circ}$Δsol​H∘. In your answer you should:

• state the energy-cycle relationship between $\Delta_{\text{sol}}H^{\circ}$Δsol​H∘, the lattice energy and the enthalpies of hydration, taking care with the sign of the lattice term;
• explain, in terms of the bonding involved, why the lattice-breaking step is endothermic and why hydration is exothermic;
• explain how the relative magnitudes of these contributions decide whether dissolving is overall endothermic or exothermic.

(6 marks)

Calcium sulfide is an ionic solid that contains $\text{Ca}^{2+}$Ca2+ and $\text{S}^{2-}$S2− ions. Its formation from its elements can be represented by a Born-Haber cycle:

$\text{Ca}(s) + \text{S}(s) \rightarrow \text{CaS}(s)$Ca(s)+S(s)→CaS(s)

Use the data below to calculate the lattice energy of formation of calcium sulfide, $\Delta_{\text{latt}}H^{\circ}$Δlatt​H∘, in kJ mol⁻¹. Note that the second electron affinity of sulfur is endothermic.

Enthalpy change	Value / kJ mol⁻¹
Standard enthalpy of formation of CaS, $\Delta_{f}H^{\circ}$Δf​H∘	−482
Enthalpy of atomisation of calcium, $\Delta_{\text{at}}H^{\circ}$Δat​H∘	+178
First ionisation energy of calcium, $\text{IE}_1$IE1​	+590
Second ionisation energy of calcium, $\text{IE}_2$IE2​	+1145
Enthalpy of atomisation of sulfur, $\Delta_{\text{at}}H^{\circ}$Δat​H∘	+279
First electron affinity of sulfur, $\text{EA}_1$EA1​	−200
Second electron affinity of sulfur, $\text{EA}_2$EA2​	+640

Show your working and give the correct sign.

(6 marks)

A student determined the enthalpy of solution of ammonium nitrate by dissolving it in water in a polystyrene cup:

$\text{NH}_4\text{NO}_3(s) \rightarrow \text{NH}_4^{+}(aq) + \text{NO}_3^{-}(aq)$NH4​NO3​(s)→NH4+​(aq)+NO3−​(aq)

The student added 8.00 g of solid ammonium nitrate (molar mass = 80.0 g mol⁻¹) to 100.0 g of water. The temperature of the water fell by 7.5 °C.

Quantity	Value
Mass of water	100.0 g
Specific heat capacity, $c$c	4.18 J g⁻¹ K⁻¹
Temperature change, $\Delta T$ΔT	−7.5 K

Assume all the heat is exchanged with the water and that the solution has the same specific heat capacity as water. Calculate the standard enthalpy of solution of ammonium nitrate, $\Delta_{\text{sol}}H^{\circ}$Δsol​H∘, in kJ mol⁻¹, giving the correct sign and units.

(5 marks)

A metal ore is processed by reducing the metal oxide with carbon inside a furnace:

$\text{MO}(s) + \text{C}(s) \rightarrow \text{M}(s) + \text{CO}(g)$MO(s)+C(s)→M(s)+CO(g)

For this reaction, $\Delta H^{\circ} = +492 \text{ kJ mol}^{-1}$ΔH∘=+492 kJ mol−1 and $\Delta S^{\circ} = +542 \text{ J K}^{-1}\text{ mol}^{-1}$ΔS∘=+542 J K−1 mol−1.

(a) Using $\Delta G^{\circ} = \Delta H^{\circ} - T\Delta S^{\circ}$ΔG∘=ΔH∘−TΔS∘, calculate the minimum furnace temperature at which this reduction becomes thermodynamically feasible. Show your working and take care with units. (3 marks)

(b) Explain, with reference to your answer, why this process is run at high temperature rather than at 298 K. (2 marks)

Chloromethane can be converted to methane in the gas phase by reaction with hydrogen:

$\text{CH}_3\text{Cl}(g) + \text{H}_2(g) \rightarrow \text{CH}_4(g) + \text{HCl}(g)$CH3​Cl(g)+H2​(g)→CH4​(g)+HCl(g)

Using the mean bond enthalpies below, calculate the enthalpy change of this reaction, $\Delta H$ΔH, in kJ mol⁻¹. Show your working.

Bond	Mean bond enthalpy / kJ mol⁻¹
C–H	413
C–Cl	346
H–H	436
H–Cl	432

(4 marks)

(a) Define the term lattice energy (lattice enthalpy of formation) and state whether it is exothermic or endothermic. (2 marks)

(b) Write a balanced equation, including state symbols, that represents the lattice energy of formation of calcium oxide, CaO(s). (1 mark)