May/June 2026 Paper 41

Write the equation to represent the lattice energy, $\Delta H_{latt}$, of calcium sulfide, CaS. Include state symbols.

Explain why the value of $\Delta H_{latt}$ of CaS is negative.

State the order of the $\Delta H_{latt}$ values of CaS, MgO and KCl, from most negative to least negative. Explain your answer.

The enthalpy change of formation, $\Delta H_f$, of Cl${}^{-}$(g) or Mg${}^{2+}$(g) is the energy change when one mole of gaseous ions forms from the element in its standard state at 298 K.

enthalpy change of formation of chloride ions, Cl${}^{-}$(g) = $-243$ kJ mol${}^{-1}$ enthalpy change of formation of magnesium ions, Mg${}^{2+}$(g) = $+2336$ kJ mol${}^{-1}$

The enthalpy change of formation of Cl${}^{-}$(g) can be calculated using two energy changes only. Identify the two energy changes.

The lattice energy of magnesium chloride, MgCl${}_2$, is $-2493$ kJ mol${}^{-1}$. Use this value, and the data given in part (d), to calculate the enthalpy change of formation of MgCl${}_2$(s).

Magnesium carbonate, MgCO${}_3$, decomposes when heated.

Predict whether the decomposition of MgCO${}_3$ is feasible at 600 ${}^{\circ }\mathrm{C}$. Use a calculation to explain your answer.

Calcium carbonate, CaCO${}_3$, also decomposes when heated. State whether the decomposition of CaCO${}_3$ occurs at a higher or lower temperature than that of MgCO${}_3$. Explain your answer.