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Hess's Law Example 3

Worked example: enthalpy change of reaction from formation dataWhen enthalpy changes of formation are given, the Hess cycle is built with the elements at the bottom, because every compound in the equation can be made from its elements.Find the enthalpy change of combustion of propane:
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l)
Data: ΔfHθ[C3H8(g)] = -104.5, ΔfHθ[CO2(g)] = -393.5, ΔfHθ[H2O(l)] = -285.8 kJ mol-1Step 1. The arrows point up from the elements (3C, 4H2 and 5O2) to the reactants and to the products.Step 2. Going round the cycle gives:
ΔrHθ = ΣΔfHθ(products) - ΣΔfHθ(reactants)
Step 3. O2(g) is an element in its standard state, so its ΔfHθ is zero.Step 4. Substitute:
Products: 3(-393.5) + 4(-285.8) = -1180.5 + (-1143.2) = -2323.7

Reactants: -104.5 + 5(0) = -104.5

ΔcHθ = -2323.7 - (-104.5) = -2219.2 kJ mol-1
Notice that the order is the reverse of the combustion-data rule: with formation data it is products minus reactants.

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Specification: Edexcel Chemistry (8CH0), AS-level, Topic 8: Energetics I. Topic: Enthalpy.

Key terms in this lesson

enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
enthalpy change of combustion
The enthalpy change when one mole of a substance is completely burned in oxygen, with all reactants and products in standard states.
hess's law
The principle that the total enthalpy change for a reaction is the same regardless of the route taken, provided the starting and finishing conditions are the same.

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