2.4 Chemistry- Chemical reactions and energy

• Energy transfer during chemical reactions

  Energy is transferred to or from the surroundings - it is conserved. The amount of energy at the beginning is the same as at the end.
• Exothermic reactions


  • Transfer energy to the surroundings, so the temperature of the surroundings increases
  • Examples include combustion, many oxidation reactions and neutralisation
  • Everyday examples include self-heating cans and hand warmers
• Endothermic reactions


  • Take in energy from the surroundings, so the temperature of the surroundings decreases
  • Examples include thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate
  • Some sports injury packs are based on endothermic reactions
• Activation energy


  The minimum amount of energy that particles must have to react
• 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
• Endothermic reaction energy profile

  Shows the relative energies of reactants and products, the activation energy and the overall energy change
• Exothermic reaction energy profile

  Shows the relative energies of reactants and products, the activation energy and the overall energy change
• Bond energies


  The energy change of reactions is determined by the energy needed to break bonds in the reactants and the energy released when bonds are formed in the products
• Using bond energy data

  1. Sum of energy to break bonds - sum of energy released when bonds form = overall energy change

  2. If overall energy change is negative, the reaction is exothermic

  3. If overall energy change is positive, the reaction is endothermic
• Bond energies


  • N≡N: 946 kJ/mol
  • H-H: 436 kJ/mol
  • N-H: 389 kJ/mol
• Overall equation: N2 + 3H2 → 2NH3
• Energy in the reactants/bonds broken: 946 + 1,308 = 2,254 kJ/mol
• Energy in the products/bonds formed: 2,334 kJ/mol
• Overall energy change = ∑(bonds broken) - ∑(bonds formed) = -80 kJ/mol
• The overall energy change is negative, therefore the reaction is exothermic