Energy Transfer

• Chemical changes can do many things. But when they occur, what happens?
• Chemical reactions transfer energy to or from surroundings.
• Many reactions are accompanied by temperature changes.
• Exothermic reactions are accompanied by a rise in temperature.
• Exothermic reactions transfer thermal energy to the surroundings.
• Examples of exothermic reactions include combustion, rusting, neutralisation, displacement and respiration.
• Some real life examples of exothermic reactions are self heating cans and hand warmers.
• Endothermic reactions are accompanied by a fall in temperature.
• Endothermic reactions take in energy from the surroundings.
• Some examples of endothermic reactions are thermal decomposition and photosynthesis.
• Some real life examples of endothermic reactions are sports injury packs.
• If a reversible reaction is exothermic in one direction, it must be endothermic in the opposite direction.
• The Law of Conservation of Energy states that energy is neither created nor destroyed; it can only be transferred via pathways.
• The same amount of energy is transferred in each case.
• Activation energy is the minimum amount of energy required for a chemical reaction to start.
• Atoms that are rearranged as 'old bonds' are broken and 'new bonds' are formed.
• For old bonds to be broken, the reacting particle must collide with sufficient energy.
• Activation energy is the minimum amount of energy needed for a chemical reaction to occur.
• Energy is given out and the reactants have more energy content than the products in exothermic reactions.
• Catalysts reduce the activation energy needed for a reaction, making the reaction go faster.
• As energy is taken in, the products have more energy content than the reactants.
• Energy changes in a chemical reaction can be shown using an energy level diagram or a reaction profile.
• Activation energy is used to break the bonds in the reactant molecules.
• Exothermic reaction


  Energy is transferred from the chemical to the surroundings
• Endothermic reaction


  Chemicals gain energy from the surroundings
• Calculating energy changes


  1. Look at formed energy when breaking chemical bonds

  2. Get structures of molecules involved

  3. Get energy values for each bond

  4. Break bonds on left side

  5. Make bonds on right side

  6. Add up energy to break bonds

  7. Add up energy to make bonds

  8. Subtract energy to make bonds from energy to break bonds
• Bond energy


  Energy required to break that bond
• Breaking a bond is endothermic, so energy is gained
• Making a bond is exothermic, so energy is released
• Breaking bonds


  Requires 678 kJ
• Making bonds


  Releases 862 kJ
• The final energy change for the reaction is -184 kJ
• This reaction is exothermic