energy transfer - topic 5

• When chemical reactions occur, energy is conserved
• The amount of energy in the universe at the beginning is the same as at the end - if a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred
• Exothermic reaction


  A reaction that transfers energy to the surroundings so the temperature of the surroundings increases
• Product molecules must have less energy than the reactants, by the amount transferred
• Exothermic reactions


  • combustion
  • oxidisation reactions
  • neutralisation
• Everyday examples of exothermic reactions

  • self-heating cans
  • hand warmers
• Endothermic reaction


  A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Product molecules must have more energy than reactants
• Endothermic reactions


  • thermal decomposition
  • citric acid
  • sodium hydrogen carbonate
• Everyday examples of endothermic reactions


  • Sports injury packs
• Measuring energy transfer: Practical

  1. Put 25mm of hydrochloric acid and sodium hydroxide in separate beakers

  2. Place beakers in a water bath and set to 25 degrees until they are both at the same temperatures

  3. Add the HCl acid and SH to a polystyrene cup with a lid

  4. Take the temp every 30 seconds and record the highest temperature
• Bond breaking is endothermic
• Bond making is exothermic