Use ions, not atoms
Lattice and hydration trends depend on ionic charge and ionic radius. For example, F− is smaller than I−; comparing fluorine and iodine atoms answers a different question.
Year 13 · AQA & OCR A
Lattice, hydration & solution enthalpy
Start with the ionic solid, separate it into gaseous ions, then hydrate those ions. The practice keeps the direction, sign and ion coefficients visible so the Hess cycle does the chemistry rather than becoming a sign-guessing exercise.
Calculate, rearrange or explain the trend
The harder mix removes some of the scaffolding and adds AQA perfect-ionic-model questions where they belong.
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See which direction each enthalpy term must run
Choose a salt and follow one mole of solid through gaseous ions to aqueous ions. The direct route is ΔHsol.
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.