C5 - Energy Changes

• Chemicals store a certain amount of energy - and different chemicals store different amounts
• If the products of a reaction store more energy than the original reactants, then they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction.
• If the products of a reaction store less energy, then the excess energy was transferred to the surroundings during the reaction
• Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
• If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
• An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
• An exothermic reaction is shown by rise in temperature
• Example of exothermic reaction - burning fuels - Combustion
• Exothermic reactions:

  • Combustion
  • Neutralisation reactions
  • Oxidation reactions
• Self heating cans and hand warmers are everyday exothermic reactions
• An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
• Endothermic reactions are shown as a fall in temperature
• Endothermic reactions:

  • citric acid and sodium hydrogencarbonate
  • thermal decompositions
• Some sports injury packs are based on endothermic reactions.
• Energy transfer - Practical equipment:

  • polystyrene cup
  • thermometer
  • 250 cm3 glass beaker
  • measuring cylinder
  • top pan balance
• Energy transfer Practical Method - Metal and acid:

  1. Place the polystyrene cup inside the glass beaker to make it more stable.
  2. Measure an appropriate volume of the solution, eg 25 cm3.
  3. Measure an appropriate mass of the solid, or select a suitable sized piece of metal.
  4. Place the solution in a polystyrene cup.
  5. Record the temperature of the solution.
  6. Add the solid and record the highest or lowest temperature obtained.
  7. Change your independent variable and repeat the experiment.
• Energy transfer practical -

  Your independent variable could be the surface area of the solid, or the type of acid being used, or the type of metal being used.
• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
• Exothermic reaction profile - products are at a lower energy than the reactants
• Exothermic reaction profile

  A) products

  B) energy

  C) energy change

  D) reactants

  E) activation energy

  
• Activation energy - initial rise in energy = energy needed to start the reaction
• Activation energy - minimum amount of energy reactants need to collide with each other and react.
• The greater the reaction energy, the more energy needed to start the reaction - this has to be supplied (e.g heating)