The standard enthalpy of formation of solid potassium fluoride, KF(s)
CHEMISTRY · Multi-part · CBSE · Chemistry · Chemical Bonding and Molecular Structure · Chemical Bonding and Molecular Structure
Problem
The standard enthalpy of formation of solid potassium fluoride, KF(s), is −567 kJ mol⁻¹. The following additional thermochemical data are provided:
Process ΔH (kJ mol⁻¹)
K(s) → K(g) +89
K(g) → K⁺(g) + e⁻ +419
½ F₂(g) → F(g) +79.5
K⁺(g) + F⁻(g) → KF(s) −808
Using these data, determine the enthalpy change for the process F(g) + e⁻ → F⁻(g) and evaluate whether the calculated value is consistent with periodic trends.
(a) Construct a thermochemical pathway that converts the given reactants, K(s) + ½ F₂(g), into KF(s). Clearly identify each step and its associated enthalpy change.
(b) Calculate the enthalpy change for F(g) + e⁻ → F⁻(g). Show all work and report the value with appropriate sign and units.
(c) Compare your calculated value with the qualitative trend in electron affinity across Period 2. Discuss effective nuclear charge and shielding to account for any agreement or discrepancy.
(d) If the lattice energy were underestimated by 5 %, state qualitatively how the calculated electron affinity would be affected and explain your reasoning.