Energy changes

• energy transfer during exothermic and endothermic reactions


  • when chemical reactions occur, energy is conserved.
  • the amount of energy in the universe at the beginning is the same as at the end.
  • this means if a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants.
• exothermic reactions


  • an exothermic reaction is one 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.
  • examples of exothermic reactions are: combustion, oxidation, neutralisation, self-heating cans, and hand warmers
• endothermic reactions


  • an endothermic reaction is one that takes in energy from the surroundings, so the temperature of the surroundings decreases.
  • product molecules must have more energy than reactants.
  • examples of endothermic reactions are: thermal decomposition, sports injury packs.
• reaction profiles


  • chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
• activation energy 


  minimum amount of energy that particles must have to react.
• energy change of reactions


  during a chemical reaction:

  • energy must be SUPPLIED to BREAK bonds in the reactants.
  • energy is RELEASED when bonds in the products are FORMED.

  the energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies.

  • sum of energy taken in to break bnds - sum of energy released to form bonds = overall energy change.

  energy taken in to break > energy released when formed = endothermic

  energy taken in to break < energy released when formed = exothermic
• What do cells contain that reacts to produce electricity?

  Chemicals
• What factors affect the voltage produced by a cell?

  The type of electrode and electrolyte
• How can a simple cell be constructed?

  By connecting two different metals in contact with an electrolyte
• What is a battery defined as?

  Two or more cells connected together in series
• What happens to non-rechargeable cells and batteries when one of the reactants is used up?

  Chemical reactions stop
• Are alkaline batteries rechargeable or non-rechargeable?

  Non-rechargeable
• Why can rechargeable cells and batteries be recharged?

  Because the chemical reactions are reversed when an external electrical current is supplied
• What are the differences between non-rechargeable and rechargeable cells and batteries?

  • Non-rechargeable:
  • Chemical reactions stop when reactants are used up
  • Example: Alkaline batteries
  • Rechargeable:
  • Chemical reactions can be reversed with external current
• fuel cells 


  • supplied by an external source of fuel and oxygen or air. the fuel is oxidised electrochemically within the fuel cell to produce a potential difference.
  • overall reaction a hydrogen fuel cell involves the oxidation of hydrogen to produce water.
  • hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.
• hydrogen fuel cells


  • fuel cells can be used constantly provided fuel keeps being put in.
  • hydrogen is a gas so needs to be stored at high pressure, so its harder to transport.
  • only produces water when burnt
• rechargeable cells and batteries


  • can be recharged by reversing reaction, so fuel foesnt need to keep being supplied.
  • hard to dispose of - non biodegradable.
  • will eventually stop working