A level Chemistry · Edexcel 9CH0 · Topic 8A
Enthalpy changes
Key points
- Standard enthalpy of formation : enthalpy change when one mole of a compound forms from its elements in their standard states. It is zero for elements.
- Standard enthalpy of combustion : one mole of substance burns completely in oxygen.
- Enthalpy of neutralisation: an acid and a base react to form one mole of water.
- Calorimetry: (m of the solution or water in g, c = 4.18 J g⁻¹ K⁻¹), then in kJ mol⁻¹.
- Errors: heat loss to surroundings, incomplete combustion, evaporation. Improve by insulating, using a lid, and extrapolating a temperature–time graph back to the time of mixing.
- Draw enthalpy level diagrams showing reactants, products and the sign of ; activation energy appears on reaction profiles.
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An enthalpy change is the heat energy transferred in a reaction at constant pressure. To compare reactions fairly, chemists define standard enthalpy changes precisely and measure them with simple calorimeters.
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