chemical reactions

• Chemical reaction

  Almost every chemical reaction is accompanied by an energy change
• Energy change in chemical reactions

  Energy is transferred to or from the surroundings
• Energy changes in chemical reactions

  Can be explained by examining the changes in chemical bonding during a reaction
• Exothermic reaction

  Energy is transferred to the surroundings
• Exothermic reactions

  • During an exothermic reaction, the temperature of the surroundings increases
  • Examples: combustion, neutralisation and oxidation reactions
  • Used in self-heating food cans and outdoor hand warmers
• Endothermic reaction

  Energy is taken in from the surroundings
• Endothermic reactions

  • During an endothermic reaction, the temperature of the surroundings decreases
  • Examples: thermal decomposition and electrolysis reactions
  • Used in instant ice packs to treat injuries
• Energy level diagram

  Shows whether a reaction is exothermic or endothermic
• Exothermic reactions in energy level diagram

  • Products are at a lower energy level than reactants
  • Energy is given out to the surroundings
  • Downwards arrow shows energy is given out
• Endothermic reactions in energy level diagram

  • Products are at a higher energy level than reactants
  • Energy is taken in from the surroundings
  • Upwards arrow shows energy is taken in
• Reaction profile

  Shows how the energy of the chemicals changes during a reaction
• Reaction profile

  • Includes the activation energy - the minimum energy needed to start a reaction
  • Activation energy is shown as a 'hump' in the line
• The activation energy starts at the energy of the reactants and is equal to the difference in energy between the top of the 'hump' and the reactants
• The overall change in energy in a reaction is the difference between the energy of the reactants and the energy of the products
• Breaking and making bonds in chemical reactions

  • Energy is needed to break the bonds in the reactants
  • Energy is released when the bonds in the products are formed
  • The difference between the energy needed to break bonds and the energy released when forming bonds determines the type of reaction
• Exothermic reactions

  • More energy is released when new bonds are made than is needed to break existing bonds
  • The overall energy change is negative - energy is given out to the surroundings
• Endothermic reactions


  • More energy is taken in when the existing bonds are broken than is released in making new bonds
  • The overall energy change is positive - energy is taken in from the surroundings
• Calculating bond energies

  1. Add together 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. Overall energy change = energy in - energy out