Heat Energy Changes in Chemical Reactions

7.9 Recall that changes in heat energy accompany the following changes: salts dissolving in water, neutralisation reactions, displacement reactions, and precipitation reactions and that, when these reactions take place in solution, temperature changes can be measured to reflect the heat changes  
  • Salts dissolving in water is either exothermic or endothermic
  • Neutralisation reaction is exothermic
  • Displacement is an exothermic or endothermic reaction
  • Precipitation is an exothermic reaction
7.10 Describe an exothermic change or reaction as one in which heat energy is given out
  • An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
    • Product molecules must have less energy than the reactants, by the amount transferred.
  • Examples of exothermic reactions include; combustion, many oxidisation reactions and neutralisation.
  • Everyday examples of exothermic reactions include; self-heating cans (e.g for coffee) and hand warmers.

7.11 Describe an endothermic change or reaction as one in which heat energy is taken in
  • An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
  • Examples of endothermic reactions are thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.
  • Some sports injury packs are based on endothermic reactions.
7.12 Recall that the breaking of bonds is endothermic and the making of bonds is exothermic
  • During a chemical reaction:
    • Energy must be SUPPLIED to BREAK bonds in the reactants
    • Energy is RELEASED when bonds in the products are FORMED
      • Think about how you require energy to break a stick in half.
    • Energy needed to BREAK bonds and energy RELEASED when bonds are formed can both be calculated from bond energies
    • Sum of energy to BREAK – sum of energy RELEASED = overall energy change
  • Energy needed to BREAK > energy RELEASED ENDOTHERMIC
  • Energy needed to BREAK < energy RELEASED EXOTHERMIC
7.13 Recall that the overall heat energy change for a reaction is: exothermic if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants and endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants

*see 7.12

7.14 (HT only) Calculate the energy change in a reaction given the energies of bonds (in kJ mol^-1)
  1. Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
  2. Add together the bond energies for all the bonds in the products – this is the ‘energy out’
  3. Calculate the energy change: energy in – energy out

Source: http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/calculating_energy_changes/energy_from_reactions/revision/7/

Therefore, if the energy out > energy in, the energy change will be negative showing an exothermic reaction and if the energy out < energy in, the energy change will be positive showing an endothermic reaction

7.15 Explain the term activation energy
  • Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
    • Activation energy = minimum amount of energy that particles must have to react
7.16 Draw and label reaction profiles for endothermic and exothermic reactions, identifying activation energy
  • Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
  • The arrow shows overall energy change.

  • Reaction is exothermic as products have less energy than reactants (shown by a lower line) and therefore energy was given to the surroundings, causing the temperature of the surroundings to increase.