Chemical reactions and energy

• 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
• Energy changes in chemical reactions

  Used to classify reactions as exothermic or endothermic
• Exothermic reaction


  • Energy is transferred to the surroundings
  • Temperature of the surroundings increases
  • Examples: combustion, neutralisation, oxidation
• Exothermic reactions


  • Used in self-heating food cans and outdoor hand warmers
• Endothermic reaction


  • Energy is taken in from the surroundings
  • Temperature of the surroundings decreases
  • Examples: thermal decomposition, electrolysis
• Endothermic 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
• Activation energy


  • Starts at the energy of the reactants
  • Equal to the difference in energy between the top of the 'hump' and the reactant
• Overall change in energy in a reaction


  Difference between the energy of the reactants and the energy of the products
• Breaking and making bonds in a chemical reaction


  1. Energy is needed to break the bonds in the reactants

  2. Energy is released when the bonds in the products are formed

  3. The difference between the energy needed to break bonds and the energy released when forming bonds determines the type of reaction
• Exothermic reactions - breaking and making bonds


  • 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 - breaking and making bonds

  • 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