1.8.1Physical Chemistry
Enthalpy Changes — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 33 of 42
Part 1 of 6Enthalpy and Heat
Enthalpy change
- ΔH is the heat energy change at constant pressure, measured in kJ mol⁻¹.
Exothermic
- Heat is released to the surroundings, the temperature rises and ΔH is negative.
Endothermic
- Heat is absorbed from the surroundings, the temperature falls and ΔH is positive.
Bonds and energy
- Bond breaking absorbs energy; bond forming releases energy.
Part 2 of 6Standard Conditions
Standard Conditions
Pressure
- The standard pressure is 100 kPa for all gases and for measuring enthalpy changes.
Temperature
- 298 K is the conventional temperature quoted, though conditions should be stated.
Solutions
- Any dissolved reactant must have a concentration of 1.00 mol dm⁻³ under standard conditions.
Standard states
- Each substance must be in its normal physical state under standard conditions;
- the standard symbol is a small circle added to ΔH.
Part 3 of 6Standard Enthalpy Definitions
Enthalpy change of reaction
- Enthalpy change when the amounts shown in the balanced equation react under standard conditions.
Enthalpy change of formation
- Enthalpy change when one mole of a compound forms from its elements in their standard states.
Enthalpy change of combustion
- Enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
Elements in standard state
- The enthalpy change of formation of an element in its standard state is zero.
| Type | Symbol | Example equation |
|---|---|---|
| Formation | ΔfH | C(s, graphite) + O₂(g) → CO₂(g) |
| Combustion | ΔcH | CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) |
| Reaction | ΔrH | Any balanced equation as written |
Part 4 of 6Writing Enthalpy Equations
1
Identify which standard enthalpy change is described and write its definition in words.
2
For formation, put one mole of the product on the right and the elements in their standard states on the left.
3
For combustion, put one mole of the fuel on the left and burn it in excess oxygen.
4
Use fractions in front of formulae if needed to keep exactly one mole of the key substance.
5
Add state symbols to every species, because the physical state changes the energy value.
Part 5 of 6Worked Example
Question
- 1.00 g of methane releases 55.6 kJ on complete combustion.
- Calculate the enthalpy change of combustion per mole.
- (Mr CH₄ = 16.0)
- Moles of CH₄ = 1.00 ÷ 16.0 = 0.0625 mol.
- Heat released per mole = 55.6 ÷ 0.0625 = 890 kJ.
- Combustion releases heat, so ΔH is negative.
Answer ΔcH ≈ −890 kJ mol⁻¹
Part 6 of 6Exam Checklist & Tips
1.8.1 – Key Points for the Exam
- Enthalpy change Heat energy change at constant pressure.
- Exothermic
- Reaction that releases heat;
- ΔH is negative.
- Reaction that releases heat;
- Endothermic
- Reaction that absorbs heat;
- ΔH is positive.
- Reaction that absorbs heat;
- Standard state The physical form of a substance at 100 kPa.
- Standard enthalpy of formation Enthalpy change when one mole of a compound forms from its elements in standard states.
Common Mistakes
- Giving a positive ΔH for combustion; burning always releases heat.
- Forgetting that formation equations must form exactly one mole of product.
- Writing H₂O(g) as the product of standard combustion instead of H₂O(l).
- Omitting the units kJ mol⁻¹.
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