Calculation of enthalpy changes of reaction using bond energies
2 marks · EasyExplain why a reaction is exothermic in terms of bond energies.
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Exothermic reactions occur when more energy is released by forming new bonds in the products than is required to break bonds in the reactants.
Also accept: The total energy released making bonds in products is greater than the total energy required to break bonds in the reactants.
Common mistakes: Mixing up bond making and bond breaking; Omitting a comparison of energies
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Marking points
M1 · 1 mark · Mentions bond breaking and bond making.
M2 · 1 mark · States that more energy is released in bond making than is absorbed in bond breaking.
Worked solution
A reaction is exothermic if the energy released by bond formation in the products is greater than the energy absorbed to break bonds in the reactants.