AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: use Hess’s law to perform calculations,…
Full specification wording
Applications of Hess’s law: Students should be able to: use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation.
Hess's law states the enthalpy change of a reaction is independent of the route taken, as enthalpy is a state function.
Checked against the AQA 7405 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Hess's law states the enthalpy change of a reaction is independent of the route taken, as enthalpy is a state function. This allows calculation of values that cannot be measured directly. Draw the target reaction across the top, then build a triangle down to a common set of substances. With enthalpies of formation, arrows point up from elements, so ∆H = Σ∆fH(products) − Σ∆fH(reactants). With enthalpies of combustion, arrows point down to combustion products, so ∆H = Σ∆cH(reactants) − Σ∆cH(products). For example, calculating the enthalpy of formation of ethanol using its combustion data. Multiply each value by the number of moles in the balanced equation, and remember that going against an arrow reverses the sign.
What earns the marks
- The correct cycle or expression, products minus reactants for formation data or reactants minus products for combustion data.
- Multiplying each enthalpy value by its balancing number from the equation.
- Reversing the sign when travelling against the direction of an arrow in the cycle.
- The final answer with correct sign and units kJ mol⁻¹.
Common mistakes and how to put them right
Using the formation rule when the data given are enthalpies of combustion, which reverses the whole answer.
Forgetting that ∆fH of an element in its standard state is zero and leaving it out of the count of substances.
Ignoring the balancing numbers, so 2 CO₂ is counted once rather than twice.
Exam tips
- Draw the cycle as your working; a correct Hess cycle can often earn the method mark on its own even without a written expression.
- Write the balanced equation first and put the number of moles above each substance before you substitute.
Try the lesson
The start of the Applications of Hess’s law: Students should be able to: use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation. lesson, made from checked teaching notes for this specification point.
Have a go first
Use the standard enthalpies of combustion below to calculate the standard enthalpy of formation of methane, ΔfH⦵(CH₄).
ΔcH⦵(C) = −394 kJ mol⁻¹ ΔcH⦵(H₂) = −286 kJ mol⁻¹ ΔcH⦵(CH₄) = −891 kJ mol⁻¹
Listen
Hess's law states the enthalpy change of a reaction is independent of the route taken, as enthalpy is a state function. This allows calculation of values that cannot be measured directly. Draw the target reaction across the top, then build a triangle down to a common set of substances. With enthalpies of formation, arrows point up from elements, so ∆H = Σ∆fH(products) − Σ∆fH(reactants). With enthalpies of combustion, arrows point down to combustion products, so ∆H = Σ∆cH(reactants) − Σ∆cH(products). For example, calculating the enthalpy of formation of ethanol using its combustion data. Multiply each value by the number of moles in the balanced equation, and remember that going against an arrow reverses the sign. A strong answer earns marks for these points. the correct cycle or expression, products minus reactants for formation data or reactants minus products for combustion data. multiplying each enthalpy value by its balancing number from the equation. reversing the sign when travelling against the direction of an arrow in the cycle. the final answer with correct sign and units kJ mol⁻¹. Exam tip: Draw the cycle as your working; a correct Hess cycle can often earn the method mark on its own even without a written expression.
Key words
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Hess's law
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enthalpy cycles
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formation data
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combustion data
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See every definition
- Hess's law
- The enthalpy change of a reaction is independent of the route taken, as enthalpy is a state function.
- enthalpy cycles
- A target reaction is drawn across the top, then a triangle is built down to a common set of substances.
- formation data
- Arrows point up from elements, so the enthalpy change is the sum of the products' enthalpies of formation minus the reactants'.
- combustion data
- Arrows point down to combustion products, so the enthalpy change is the sum of the reactants' enthalpies of combustion minus the products'.
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