OCR A-Level Chemistry: Lattice Enthalpy, Entropy & Electrode Potentials

The thermal decomposition of solid strontium carbonate is endothermic and is not feasible at room temperature, yet it proceeds readily in an industrial kiln operated at high temperature.

$\text{SrCO}_3(\text{s}) \rightarrow \text{SrO}(\text{s}) + \text{CO}_2(\text{g})$SrCO3​(s)→SrO(s)+CO2​(g)

Describe and explain, in terms of Gibbs free energy, why this reaction is not feasible at room temperature but becomes feasible when the kiln is heated. Your answer should refer to the sign of the entropy change of this reaction, to the equation $\Delta G = \Delta H - T\Delta S$ΔG=ΔH−TΔS, and should explain why raising the temperature changes the value of $\Delta G$ΔG.

(6 marks)

A student is determining the lattice enthalpy of formation of calcium oxide, CaO, using a Born–Haber cycle. The relevant standard enthalpy changes are listed below.

Enthalpy change	Value / kJ mol⁻¹
Enthalpy of formation of CaO(s), $\Delta_f H^{\circ}$Δf​H∘	−635
Enthalpy of atomisation of Ca(s)	+178
First ionisation energy of Ca	+590
Second ionisation energy of Ca	+1145
Enthalpy of atomisation of O (i.e. $\tfrac{1}{2}\text{O}_2 \rightarrow \text{O}$21​O2​→O)	+249
First electron affinity of O	−141
Second electron affinity of O	+798

Use the data to calculate the lattice enthalpy of formation of calcium oxide, $\Delta_{LE} H^{\circ}$ΔLE​H∘, for the change $\text{Ca}^{2+}(\text{g}) + \text{O}^{2-}(\text{g}) \rightarrow \text{CaO}(\text{s})$Ca2+(g)+O2−(g)→CaO(s). Show your working and pay careful attention to the signs of each term.

(6 marks)

The table shows the lattice enthalpies of formation of four ionic compounds, each formed from a Group 1 or Group 2 cation and the chloride or oxide anion.

Compound	Ions present	Lattice enthalpy of formation / kJ mol⁻¹
NaCl	Na⁺, Cl⁻	−787
KCl	K⁺, Cl⁻	−711
MgCl₂	Mg²⁺, Cl⁻	−2526
MgO	Mg²⁺, O²⁻	−3791

(a) Explain why the lattice enthalpy of formation of KCl is less exothermic than that of NaCl. (2 marks)

(b) The lattice enthalpy of formation of MgO is far more exothermic than that of MgCl₂, even though both contain the Mg²⁺ ion. Explain this difference. (3 marks)

A research team is testing whether acidified vanadium(III) ions can be reduced to vanadium(II) ions by adding zinc metal to the solution. The relevant standard electrode potentials are given below.

Half-equation	$E^{\circ}$E∘ / V
$\text{V}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{V}^{2+}(\text{aq})$V3+(aq)+e−⇌V2+(aq)	−0.26
$\text{Zn}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Zn}(\text{s})$Zn2+(aq)+2e−⇌Zn(s)	−0.76

(a) Use the electrode potentials to predict whether zinc will reduce V³⁺(aq) to V²⁺(aq). Show how you reach your decision, including the value of the standard cell potential. (3 marks)

(b) The team carries out the experiment but observes that the reaction is extremely slow at room temperature. State one reason why a reaction predicted to be feasible from $E^{\circ}$E∘ values may not proceed at a measurable rate. (1 mark)

(c) State one other reason, relating to conditions, why a prediction based on standard electrode potentials might be unreliable for this experiment. (1 mark)

An electrochemical cell is set up by connecting a nickel half-cell to a silver half-cell under standard conditions, joined by a salt bridge. The standard electrode potentials are:

Half-equation	$E^{\circ}$E∘ / V
$\text{Ag}^{+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Ag}(\text{s})$Ag+(aq)+e−⇌Ag(s)	+0.80
$\text{Ni}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Ni}(\text{s})$Ni2+(aq)+2e−⇌Ni(s)	−0.25

(a) Calculate the standard cell potential, $E^{\circ}_{cell}$Ecell∘​, of this cell. (1 mark)

(b) State which electrode is the positive terminal, and write the overall cell reaction that occurs when the cell delivers a current. (3 marks)

This question is about key definitions in energetics and electrochemistry.

(a) State what is meant by the lattice enthalpy of formation of an ionic compound. (1 mark)

(b) State what is meant by the standard electrode potential of a half-cell. (1 mark)

(c) Write the equation that relates the standard Gibbs free-energy change to the standard enthalpy change and the standard entropy change, and state the condition (in terms of $\Delta G$ΔG) for a reaction to be feasible. (1 mark)