Energy changes

• Chemical reaction


  1. Bonds are made

  2. Bonds are broken
• Exothermic process

  Heat is given out, the surroundings get hotter
• Exothermic reactions


  • Combustion reactions
• Endothermic process


  Heat and energy is taken in, the surroundings get colder
• Endothermic reactions


  • Sports injury packs
  • Self-cooling drinks
• Exothermic reactions

  • Hand warmers (reusable and disposable)
  • Self-heating cans
• Measuring temperature change in a reaction

  1. Get reactants and place in polystyrene cup

  2. Take initial temperature reading

  3. Calculate change in temperature (final - initial)
• Reducing heat transfer in temperature change experiment

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


  Reaction is exothermic
• Temperature decreases

  Reaction is endothermic
• Variables in temperature change experiment

  • Independent variable: reactants
  • Dependent variable: temperature change
  • Control variables: amount of reactants, amount of stirring
• Digital thermometer makes readings more accurate
• Repeating readings makes results more 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 enthalpy change from bond enthalpies
• 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) and bond making (exothermic)

  3. 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, 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, hydrogen ions travel through electrolyte to react with oxygen, producing water and electrons that power a circuit
• Advantages of hydrogen fuel cells

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

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