AQA · OCR A · Year 12

Hess law questions and practice

Choose the route you want to practise: formation data, combustion data, missing enthalpies, equation combinations or experimental Hess cycles. Questions change each time, and worked answers show the arrow route before the arithmetic.

Start with formation data Try hard mixed questions

Reactantsroute 1

ΔH?

Productsroute 2

known ΔH

Common statesame start or finish

Pick the part that causes trouble

Formation and combustion cycles look similar but the arrows run in different directions. The separate tools make that distinction hard to miss.

Formation dataUse standard enthalpies of formation, equation coefficients and state symbols to calculate reaction enthalpies. ΣΔHf products − reactantscoefficientsstate symbols Practise Combustion dataBuild the route through common combustion products and keep the direction of each arrow under control. combustion cyclesarrow directioncoefficients Practise Missing dataRearrange a Hess cycle to find a missing formation, combustion or reaction enthalpy. rearrangementunknown ΔHsigns Practise Combine equationsReverse, scale and add equations to reach a target reaction without relying on a memorised formula. reverse equationsscale ΔHcancel species Practise Experimental HessUse q = mcΔT to obtain experimental reaction enthalpies, then combine the measured routes with Hess law. q = mcΔTmolar ΔHexperimental cycle Practise Hard mixed HessUnstructured mixed questions with missing data, awkward coefficients and experimental routes. choose the routemulti-step Hessexam reasoning Practise

Two reliable routes

Formation data: imagine both sides being formed from their elements. Following the arrows gives ΣΔH_f(products) − ΣΔH_f(reactants).

Combustion data: imagine both sides burning to the same final products. The route is reversed, so the useful calculation is ΣΔH_c(reactants) − ΣΔH_c(products).

If you forget which subtraction to use, draw the arrows. Hess's law only says the overall enthalpy change is independent of route; the signs come from the direction you actually travel.

Formation routeelements↑reactants / productsarrows rise from the common state

Combustion routereactants / products↓CO₂ + H₂Oarrows fall to the common state

What the exam questions keep testing

1 Include every equation coefficient in the enthalpy sum. 2 Treat elements in their standard states as ΔH_f° = 0, not as missing data. 3 Reverse an equation → reverse the sign. Multiply an equation → multiply ΔH by the same factor. 4 In calorimetry, decide the sign of ΔH only after deciding whether the reaction warmed or cooled the surroundings.

Related Year 12 energetics

Hess law often sits beside calorimetry and bond enthalpy calculations. The experimental Hess tool includes the calorimetry route.

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