How are lattice, hydration and solution enthalpy linked?

For an ionic solid, reverse the lattice-formation step to form gaseous ions, then hydrate the ions. The sum of those steps gives the enthalpy of solution.

What makes hydration enthalpy more exothermic?

Greater ionic charge and smaller ionic radius increase charge density and strengthen ion–dipole attraction between the ion and water molecules.

What does disagreement with a perfect ionic model mean?

For AQA, a substantial difference between a Born–Haber lattice enthalpy and a perfect-ionic-model value is evidence that the real bonding has more covalent character than the perfect ionic model assumes.

Year 13 · AQA & OCR A

Lattice, hydration & solution enthalpy

Practise lattice enthalpy, hydration enthalpy and enthalpy of solution with calculations, rearrangements and ionic-trend questions.

Draw the solution cycle before rearranging it

I’d draw one mole of solid, the gaseous ions and the aqueous ions before putting any values in. The direct solid-to-aqueous route is ΔH_sol.

Compound Lattice data

Use ionic charge and ionic radius

Lattice and hydration trends depend on ionic charge and ionic radius. For example, compare the radii of F⁻ and I⁻ when explaining their ionic enthalpy trends.

Greater charge strengthens attraction

Larger ionic charges strengthen electrostatic attraction in a lattice and ion–dipole attraction during hydration. That usually makes the corresponding enthalpy more exothermic.

Smaller ions strengthen attraction

For ions with the same charge, a smaller ionic radius lets opposite charges approach more closely. Hydration also becomes more exothermic because water can interact more strongly with the concentrated ionic charge.

Count every hydrated ion

One mole of MgCl₂ gives one Mg²⁺ and two Cl⁻ ions. The chloride hydration enthalpy therefore appears twice in the solution cycle.

AQA: compare with the perfect ionic model

Use the size of the discrepancy, then interpret it in the right direction. A Born–Haber value comes from experimental thermochemical data. A perfect ionic model assumes completely ionic bonding. Close agreement supports that model; a substantial discrepancy points to extra covalent character in the real bonding.

Data set

Use Born–Haber if the lattice step is the weak point

Practise Born–Haber next if the lattice term is where you get stuck. The full A-level bonding lab also has a shorter conceptual ionic-energetics route alongside complex ions and delocalisation.

Practise Born–Haber Open bonding lab

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