IB Chemistry Born–Haber Cycles (HL): The Complete FAQ

You need to construct and use a Born–Haber cycle to find an unknown enthalpy change — usually lattice enthalpy — by applying Hess's Law to a sequence of steps: atomisation, ionisation, electron affinity, and formation. According to the IB Chemistry guide (first exams 2025), this sits under Reactivity 3.1 at HL only.

Lattice enthalpy of formation is gaseous ions combining into a solid lattice — always exothermic, negative value. Lattice dissociation enthalpy is the reverse — solid lattice breaking into gaseous ions — always endothermic, positive value. IB data booklets and exam questions sometimes give one, sometimes ask you to find the other, so check the sign every time.

Start with the elements in their standard states at the bottom left, draw a direct arrow to the compound (enthalpy of formation), then build an indirect path up through atomisation, ionisation, electron affinity and lattice formation back down to the compound. Hess's Law then lets you equate the two routes.

Add up every known enthalpy change around the indirect route, set that sum equal to the enthalpy of formation (the direct route), then solve algebraically for the missing value — usually lattice enthalpy. Get your signs right first; the arithmetic itself is simple GCSE-level addition and subtraction.

The three biggest losses I see marking mocks are: wrong sign on electron affinity or lattice enthalpy, forgetting to double an ionisation energy or atomisation value for a formula with a coefficient of two, and drawing arrows in the wrong direction so the algebra falls apart. None of these are conceptual gaps — they're carelessness under time pressure.

Theoretical lattice enthalpy (from an ionic model, purely electrostatic) assumes perfectly spherical ions with no covalent character. The Born–Haber experimental value includes real polarisation effects, so a big gap between the two — as with silver halides — signals covalent character in what you'd otherwise call an ionic bond.

No — Born–Haber cycles are HL-only content under Reactivity 3.1 in the current IB Chemistry guide. SL students study Hess's Law and enthalpy cycles more generally but are never examined on the full ionic-compound cycle with ionisation energy and electron affinity terms.

Expect 'construct', 'calculate', 'define', 'explain' and 'compare' — construct asks you to build the actual cycle diagram or equation, calculate wants a numerical answer with correct units and sign, and explain/compare usually targets the theoretical-versus-experimental lattice enthalpy discussion involving ionic versus covalent character.

Practise drawing the cycle from a blank page for three or four different compounds — NaCl, MgO, CaBr2 — until the arrow directions and mole ratios are automatic, then move to past-paper calculations. Timed practice matters more than re-reading notes; this is an arithmetic-under-pressure topic, not a memorisation one.

It's more mechanical than conceptually hard — most students who struggle are making sign or arithmetic slips, not misunderstanding the chemistry. Compared to something like acid-base equilibria or organic mechanisms, Born–Haber cycles reward careful, repeatable practice rather than deep theoretical insight, which makes them a genuinely gettable set of marks with enough drilling.

Yes — Born–Haber questions typically appear in Paper 2 as a structured 6-9 mark question, sometimes paired with a theoretical-versus-experimental lattice enthalpy discussion worth further marks. For a student aiming at grade 6 or 7 at HL, this is a reliably practisable topic rather than one where marks are unpredictable.