Why are reaction-energy values calculated from mean bond energies approximate?
The activation energy of the reaction is not included in the sum
A bond's energy varies slightly from one molecule to another
Some of the energy is lost as heat to the surroundings
The numbers of bonds broken and made can only be estimated
94 exam-style questions on Edexcel GCSE Chemistry 8.2 Heat energy changes in chemical reactions, covering 8.2.1 Exothermic and endothermic changes, 8.2.2 Bond breaking, bond making and overall energy change, 8.2.3 Calculating energy change from bond energies, and 8.2.4 Activation energy and reaction profiles. Each one has a worked solution and a mark scheme showing where the marks go.