Count the bonds in displayed formulas, then work out the energy taken in, the energy given out and the overall energy change. Every reaction uses the given bond energies below.
The rules
- Breaking bonds takes in energy. Making bonds gives out energy. All the bonds in the reactants break, and all the bonds in the products form.
- Count every bond in the displayed formula. CH4 has 4 C–H bonds. H2O has 2 O–H bonds.
- Multiply by the big number in front of the formula. 2O2 means 2 O=O bonds to break.
- Double and triple bonds have their own values. C=O is 805 kJ/mol, not the C–O value of 358 kJ/mol.
- Energy change = bonds broken − bonds made. Always this way round.
- Negative means exothermic: more energy is given out making bonds than is taken in breaking them. Positive means endothermic.
Bond energy table
These are given values in kJ/mol, as you would see them in an exam question. Different data books give slightly different values, so always use the ones in the question.
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