energy changes

Energy Transfer during reactions

  • when chemical reactions occur, energy is conserved

- if a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred

Exothermic Reactions

  • transfers energy to surroundings so temperature of surroundings increases; product molecules have less energy than the reactants

  • eg. combustion, neutralisation, self-heating cans, hand warmers

Endothermic Reactions

  • takes in energy from surroundings so temperature of surroundings decreases; product molecules have more energy than the reactants

  • eg. thermal decomposition, citric acid and sodium hydrogencarbonate. sports injury packs

Reaction Profiles

  • can be used to show the relative energies of reactants and products, the activation energy and overall energy change of a reaction

    • activation energy = the minimum amount of energy colliding particles need to cause a reaction

Energy Change of Reactions

  • breaking bonds in reactants requires energy

  • forming bonds in products releases energy

→ can be calculated from bond energies:

energy taken in to break < energy released when formed = exothermic

energy taken in to break > energy released when formed = endothermic

  • energy change = sum of bonds broken - sum of bonds made

Worked example