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 cycles use the elements as the common state. Combustion cycles use common combustion products. Choose the route that matches the data you have been given.

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

Draw the route before you calculate

Formation data: I’d put the elements at the common point and draw both formation arrows first. Following that route gives ΣΔH_f(products) − ΣΔH_f(reactants).

Combustion data: draw both substances burning to the same combustion products. Following the arrows gives ΣΔH_c(reactants) − ΣΔH_c(products).

If the signs feel uncertain, follow the arrows one at a time and change a sign whenever you travel against an equation. Following the arrows keeps the sign choice tied to the actual route.

Formation routeelements↑reactants / productsarrows rise from the common state

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

Four checks before you finish

1 Include every equation coefficient in the enthalpy sum. 2 Use ΔH_f° = 0 for elements in their standard states. 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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