Topic 1 - Chemical Reactions and Energy

• Two stages that occur in any chemical reaction

  Bond breaking - needs energy

  Bond making - releases energy
• Exothermic reaction

  Releases energy to surroundings which causes the temperature of surroundings to increase
• Reactions that are exothermic

  Combustion

  Neutralisation

  Oxidation
• Endothermic reaction

  Absorbs energy from surroundings which causes temperature of surroundings to decrease
• Reactions that are endothermic

  Thermal decomposition

  Electrolysis
• Energy profile diagram explanation - exothermic reaction

  More energy is released when new bonds are made than is needed to break existing bonds

  Overall energy change is negative as energy is given out to surroundings
• Uses of exothermic reactions

  Self-heating food cans

  Outdoor hand warmers
• Energy profile diagram explanation - endothermic reaction

  More energy is absorbed when the existing bonds are broken than is released in making new bonds

  Overall energy change is positive as energy is taken in from surroundings
• A use of endothermic reactions

  Instant ice packs to treat injuries
• Practical with polystyrene cup to show endo/exothermic reactions

  Sources of error and improvements

  Source -

  No lid on the cup so there will be heat loss

  Improvement -

  Add a lid to improve reliability

  Source -

  Incomplete reactions which could affect data

  Improvements -

  Stir reactants rapidly so all reacting particles collide

  Add a magnetic stirring bar
• Energy change equation 


  Reactants - Products