Chemistry 5.4

• Exothermic reaction


  A chemical reaction that transfers energy to the surroundings, usually as heat energy, causing the reaction mixture and surroundings to become hotter
• Endothermic reaction


  A chemical reaction that takes in energy from the surroundings, usually as heat energy, causing the reaction mixture and surroundings to become colder
• What happens during a chemical reaction


  1. Bonds in the reactants are broken

  2. New bonds are made in the products
• Breaking bonds


  Requires energy to be absorbed
• Forming new bonds


  Releases energy
• Exothermic reaction


  The energy released when new bonds form is greater than the energy absorbed to break bonds, so the overall energy change is negative and energy is released to the surroundings
• Endothermic reaction


  The energy absorbed to break bonds is greater than the energy released when new bonds form, so the overall energy change is positive and energy is taken in from the surroundings
• Bond energy calculations


  • Add bond energies for reactants (energy in)
  • Add bond energies for products (energy out)
  • Calculate energy change = energy in - energy out
• Exothermic reactions


  • Combustion
  • Neutralisation between acids and alkalis
  • Oxidation between water and calcium oxide
• Endothermic reactions


  • Electrolysis
  • Reaction between sodium carbonate and citric acid
  • Thermal decomposition
• Energy profile diagram


  Shows the key energy levels of the reactants and products, indicating whether the reaction is exothermic (energy released) or endothermic (energy absorbed)
• Activation energy


  The maximum amount of energy required for a reaction to take place, as reactants must collide with enough kinetic energy and in the correct orientation