IB Chemistry: Hess's Law & Energy Cycles

Draw the cycle first: reactants and products at the top, then a route via elements (for enthalpy of formation) or via combustion products (for enthalpy of combustion) underneath. Label every arrow with its enthalpy value and direction, then apply the rule that ΔH is the same regardless of route — sum the arrows you actually travel along.

Hess's law states that the total enthalpy change for a reaction is the same whether it happens in one step or several steps, provided the starting reactants and final products are identical. It's a direct consequence of enthalpy being a state function — it depends only on initial and final states, not the path taken.

Some enthalpy changes can't be measured directly in a lab — combustion of graphite to diamond, or the enthalpy of formation of methane, for example, because side reactions or impurities get in the way. An energy cycle lets you calculate that unmeasurable value from other enthalpies you can measure, like combustion or formation data.

A Born-Haber cycle is a specific type of Hess's law energy cycle used for ionic compounds, linking lattice enthalpy to ionisation energy, electron affinity, atomisation enthalpy and enthalpy of formation. It's HL-only content and always involves gaseous ions — Hess's law itself is the general principle both SL and HL students use.

Use the cycle: reactants and products at the top connected directly by ΔHf(reaction), with a route via combustion products at the bottom. Sum the combustion enthalpies of the reactants, subtract the sum of combustion enthalpies of the products, and that gives you ΔHf.

Build the cycle from elements in their standard states through atomisation, ionisation energies and electron affinities to the gaseous ions, then down to the solid ionic lattice. Lattice enthalpy is the missing arrow — rearrange so all known values sum to equal the enthalpy of formation.

The biggest one is sign errors from not flipping ΔH when an arrow is reversed on the cycle. Second most common: mixing up formation cycles (route via elements) with combustion cycles (route via combustion products) — they use opposite arithmetic. Third: forgetting units, kJ/mol matters in longer calculations.

Look at what data you're given. If you have ΔHf values, the route goes reactants → elements → products, with elements as the base. If you're given ΔHc values, the route goes reactants → combustion products → back up, since combustion products (usually CO2 and H2O) sit at the bottom of that cycle.

It's conceptually simple but exam technique is where marks are lost — students understand the theory but mess up cycle diagrams under time pressure. In my experience marking mock papers, most errors are arithmetic sign mistakes, not misunderstanding the law itself. Practise the diagram-drawing until it's automatic.

Yes — basic Hess's law cycles using formation and combustion data appear on both SL and HL papers. Born-Haber cycles for ionic lattice enthalpy, and cycles involving entropy and Gibbs free energy, are HL-only extensions under Reactivity 1.

Hess's law and energy cycle questions typically appear as 3-6 mark structured questions in Paper 2, sometimes combined with bond enthalpy or entropy calculations for a longer 8-10 mark question at HL. Method marks are awarded even without a correct final numerical answer, so showing the cycle clearly matters.

You'll use section 12 (average bond enthalpies) and section 13 (specific enthalpies including standard enthalpies of formation, atomisation, ionisation energy and electron affinity) of the IB Chemistry data booklet. Everything you need for a Born-Haber cycle or standard Hess's law calculation is listed there — you don't need to memorise values.

This is one of the most common gaps I see — students genuinely understand the law but haven't practised drawing the cycle diagram enough times to do it accurately under exam time pressure. Sign errors from reversed arrows are the single biggest cause of lost marks, not a lack of conceptual understanding.

The fastest fix is repeated practice drawing energy cycles from data booklet values until the arrow-and-sign logic becomes automatic, not something they have to reason through under time pressure. On RevisionPrep, Topical Worksheets group these questions by cycle type so your child can drill formation cycles, combustion cycles, and Born-Haber cycles separately.