3) The energy change of reactions

• In a chemical reaction, new substances are produced:

  • The bonds in the reactants are broken.
  • New bands 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 bands are formed than is needed to break bonds, the reaction must be exothermic.
• The amount of energy produced in a chemical reaction in solution can be measured by mixing the reactants in an insulated container.
• This enables the temperature change to be measured before heat is lost to the surroundings.
• This method would be suitable for neutralisation reactions and reactions involving solids, e.g. metal and acid reactions.