Measuring Energy Changes

• Energy

  Transferred in chemical reactions
• Exothermic reactions

  • Heat energy is transferred to the surroundings
  • Temperature of the surroundings increases
• Exothermic reactions

  • Combustion
  • Many oxidation reactions
  • Neutralisation
• Endothermic reactions

  • Heat energy is taken in from the surroundings
  • Temperature of the surroundings decreases
• Endothermic reactions

  • Thermal decomposition of metal carbonates
• Measuring energy changes

  1. Mix reactants in an insulated container

  2. Measure temperature change before heat is lost to surroundings
• Suitable method

  For neutralisation reactions and reactions involving solids, e.g. metal and acid reactions
• Energy Change of Reactions

  . In a chemical reaction, new substances are produced:

  - The bonds in the reactants are broken

  - New bonds are made to form the products

  . Breaking a chemical bond requires energy - it is an endothermic process

  . When a new chemical bond is formed, energy is given out - it is an exothermic process

  . If more energy is required to break bonds than is released when bonds are formed, the reaction must be endothermic

  . If more energy is released when bonds are formed than is needed to break bonds, the reaction must be exothermic
• Key Point

  . Endothermic: energy required to break old bonds > energy released when new bonds are formed

  . Exothermic: energy required to break old bonds < energy released when new bonds are formed
• Key Point

  You will always be given any bond energies that you need for questions. You are not expected to be able to recall them