Topic 5 - energy changes

• Exothermic reactions


  Reactions that transfer heat energy to the chemical surroundings and result in an increase in temperature
• Endothermic reactions


  Reactions that take in energy from the surroundings and result in a decrease in temperature
• Examples of exothermic and endothermic reactions


  • Exothermic examples - self heating cans and hand warmer • endothermic - sports injury packs
• Energy transfers in chemical reactions


  • When chemical reactions occur, energy is transferred from the chemicals to or from the surroundings, resulting in a temperature change
• Exothermic reactions


  Accompanied by a temperature rise, transfer heat energy from the chemicals to the surroundings, give out heat energy
• Examples of exothermic reactions


  • Self-heating cans (for coffee) and hand warmers
• Endothermic reactions


  Accompanied by a fall in temperature, heat energy is transferred from the surroundings to the chemicals, they take in heat energy
• Examples of endothermic reactions


  • Some sports injury packs use endothermic reactions
• If a reversible reaction is exothermic in one direction, then it is endothermic in the opposite direction
• The same amount of energy is transferred in each case (exothermic and endothermic)
• Activation energy


  The minimum amount of energy the particles must have for a reaction to take place
• Energy level diagrams


  • Show the energy changes in a chemical reaction
  • In an exothermic reaction, energy is given out to the surroundings and the products have less energy than the reactants
  • In an endothermic reaction, energy is being taken in from the surroundings and the products have more energy than the reactants
• Catalysts reduce the activation energy needed for a reaction, increasing the rate of reaction
• Required practical: Investigate the variables that affect temperature changes in reacting solutions


  1. Set up equipment

  2. Take temperature of acid

  3. Add metal powder and stir

  4. Record highest temperature reached

  5. Calculate temperature change

  6. Repeat with different metals
• Variables in the required practical

  • Independent variable: type of metal
  • Dependent variable: temperature change
  • Control variables: type, concentration and volume of acid
• There should be a correlation between the reactivity of the metal and the temperature change, i.e. the more reactive the metal, the greater the temperature change
• Measurements are accurate if they are close to the true value, and precise if the results are all close together
• There is a low risk of a corrosive acid damaging the experimenter's eye, so eye protection must be used
• Energy changes in chemical reactions

  Energy is released when new bonds are made, and energy is required to break old bonds
• If more energy is required to break bonds than is released when bonds are formed, the reaction must be endothermic
• If more energy is released when bonds are formed than is needed to break bonds, the reaction must be exothermic
• Measuring energy changes


  1. Mix reactants in an insulated container to measure temperature change before heat is lost to surroundings

  2. Suitable for neutralisation reactions and reactions involving solids, e.g. metal and acid reactions
• Simple cells


  Made by placing two different metals into a beaker containing an electrolyte
• Batteries


  Made when two or more cells are connected together
• Voltage produced by a cell


  Dependent on the type of electrode and electrolyte used
• Cells joined in series can produce a higher voltage
• Non-rechargeable batteries


  Stop producing electricity when one of the reactants has been used up
• Fuel cells


  A very efficient way of producing electrical energy, most use hydrogen but other fuels can be used
• How a hydrogen fuel cell works

  1. Hydrogen and oxygen are supplied

  2. Hydrogen is oxidised at the anode to produce hydrogen ions and electrons

  3. Hydrogen ions travel through the electrolyte to the cathode

  4. At the cathode, hydrogen ions react with oxygen to form water
• The overall equation for the reaction in a hydrogen fuel cell is 2H2 + O2 → 2H2O
• Advantages of fuel cells

  • Produce water, which is non-polluting
  • Lightweight and small
  • No moving parts so unlikely to break down
  • More efficient and less polluting than traditional electricity production methods