Chemical Energetics (A Level)

• Enthalpy change of atomisation, lattice energy and first electron affinity; trends in electron affinity for Groups 16 and 17.
• Born–Haber cycles for ionic solids and the effect of ionic charge and radius on lattice energy.
• Enthalpy changes of hydration and solution: ΔH_sol = −ΔH_latt + ΣΔH_hyd.
• Entropy S, the sign of ΔS, and ΔS^⦵ = ΣS^⦵(products) − ΣS^⦵(reactants).
• ΔG^⦵ = ΔH^⦵ − TΔS^⦵; a reaction is feasible when ΔG ≤ 0.

Definitions

Enthalpy change of atomisation, ΔH_at: the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions. Always endothermic. Na(s) → Na(g); ½Cl₂(g) → Cl(g).

Lattice energy, ΔH_latt: the enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions. Always exothermic. Na⁺(g) + Cl⁻(g) → NaCl(s).

First electron affinity, EA: the enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1− ions under standard conditions. Cl(g) + e⁻ → Cl⁻(g).

Electron affinity trends

First EA is usually exothermic: the incoming electron is attracted by the nucleus. It is more exothermic when the nuclear charge is greater, the atom is smaller (electron enters closer to the nucleus) and there is less shielding.

• From Cl downwards, EA becomes less exothermic: the atoms get larger and shielding increases, so the added electron is less strongly attracted.
• F and O are anomalies: less exothermic than Cl and S. They are very small, so the added electron is crowded into a small 2p region and is repelled by the electrons already there.
• Group 17 values are more exothermic than Group 16 in the same period: greater nuclear charge, similar shielding.
• The second electron affinity (e.g. O⁻(g) + e⁻ → O²⁻(g), +798 kJ mol⁻¹) is always endothermic: an electron is being added to a negative ion, which repels it.

Born–Haber cycle

Two routes from the elements to the ionic solid have the same total enthalpy change (Hess’s law):

What controls the size of the lattice energy

Lattice energy is more exothermic (larger in magnitude) when the electrostatic attraction between the ions is stronger:

• Higher ionic charge: MgO (2+/2−, about −3800 kJ mol⁻¹) is far more exothermic than NaCl (1+/1−, −787).
• Smaller ionic radius

Definitions

_hyd ^{+ +}

_sol ^{+ −}