Energy Changes

• Chemical reaction


  1. Bonds are made

  2. Bonds are broken
• Exothermic process


  Heat is given out, the surroundings get hotter
• Endothermic process


  Heat and energy is taken in, the surroundings get colder
• Exothermic reaction


  • Combustion reaction
• Endothermic reaction


  • Sports Injury packs
  • Self-cooling drinks
• Measuring temperature change


  1. Get reactants

  2. Place in polystyrene cup

  3. Take initial temperature reading

  4. Calculate change in temperature
• Factors to reduce heat transfer

  • Use polystyrene cup
  • Put polystyrene cup in glass beaker
  • Wrap in insulated material
  • Put lid on
• Temperature increases


  Reaction is exothermic
• Temperature decreases


  Reaction is endothermic
• Variables in temperature change practical

  • Independent variable: reactants
  • Dependent variable: temperature change
  • Control variables: amount of reactants, amount of stirring
• Digital thermometer makes readings more accurate
• Repeat readings to make results reliable
• Reaction profile


  Shows the progress of the reaction and how the energy is changing
• Activation energy


  Minimum energy required for the reaction to start
• Endothermic reaction


  Reactants have less energy than products
• Exothermic reaction


  Reactants have more energy than products
• Using bond enthalpies to determine if a reaction is exothermic or endothermic


  1. Draw displayed formulas of reactants and products

  2. Identify bond breaking (endothermic) and bond making (exothermic)

  3. Calculate energy changes for bond breaking and bond making

  4. Determine if overall reaction is exothermic or endothermic
• Exothermic


  Giving out heat
• Endothermic


  Taking in heat energy from the surroundings
• Deciding if a reaction is exothermic or endothermic from bond enthalpies


  1. Draw displayed formulas of reactants

  2. Identify bond breaking (endothermic)

  3. Identify bond making (exothermic)

  4. Calculate overall bond enthalpy
• If overall bond enthalpy is negative, the reaction is exothermic
• If overall bond enthalpy is positive, the reaction is endothermic
• Fuel cell


  Two metals of different reactivity as electrodes, with an electrolyte solution
• How a fuel cell works

  1. More reactive metal (e.g. zinc) oxidises, giving up electrons

  2. Electrons flow to less reactive metal (e.g. copper)

  3. Difference in reactivity creates voltage
• Fuel cells


  • Can transport electricity around, but eventually run out as electrodes are used up
  • Rechargeable batteries can reverse the reaction to restore the electrodes
• Hydrogen fuel cell


  • Hydrogen and oxygen pumped in, with an electrolyte that allows ions but not molecules to pass through
  • Hydrogen oxidises, releasing electrons that flow through a circuit
  • Hydrogen ions react with oxygen to form water
• Advantages of hydrogen fuel cells

  • Only produce water, no harmful emissions
  • Can run continuously as long as fuel is supplied
  • High energy density
  • Abundant supply of hydrogen
• Disadvantages of hydrogen fuel cells

  • Dangerous to store hydrogen, which is flammable
  • Electrolysis of water to produce hydrogen is expensive and emits CO2
  • Less efficient in cold temperatures