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Energy changes in reactions
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Every reaction involves an energy change. Some heat their surroundings, others cool them down.
Exothermic and endothermic
Exothermic: gives out heat; the temperature of the surroundings rises. Combustion, neutralisation.
Endothermic: takes in heat; the temperature falls. Thermal decomposition, dissolving ammonium nitrate.
Endothermic: takes in heat; the temperature falls. Thermal decomposition, dissolving ammonium nitrate.
Reaction profiles
Exothermic: products lower than reactants. Endothermic: products higher.
The hump shows the activation energy.
The hump shows the activation energy.
Bond energies (Higher)
Breaking bonds takes in energy; making bonds releases energy.
Energy change = energy to break bonds − energy released making bonds
A negative answer means exothermic.
Energy change = energy to break bonds − energy released making bonds
A negative answer means exothermic.
Breaking bonds needs 2648 kJ and making bonds releases 3466 kJ. Find the energy change.
- 2648 − 3466
Answer: −818 kJ (exothermic)
Exothermic reactions release heat (temperature rises); endothermic reactions absorb it (temperature falls). Energy change = bonds broken − bonds made; negative means exothermic.
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