Topic 5 - energy changes

• Energy must always be supplied to break bonds

  • during a chemical reaction old bonds are broken and new ones are formed
  • energy must be supplied to break existing bonds - bond breaking is an endothermic process
  • energy is released when new bonds are formed - bond formation is an exothermic process
• In Exothermic reactions energy released by forming bonds is greater than the energy used to break them
• In Endothermic reactions the energy used to break bonds is greater than the energy released by forming them
• Bond energy calculations

  • every chemical bond has a particular bond energy associated with it
  • you can use these known bond energies to calculate the overall energy change for a reaction
• fuel cells use fuel and oxygen to produce electrical energy
• fuel cells :


  • an electrical cell thats supplied with a fuel and oxygen and uses energy for the reaction between them to produce electrical energy
  • when the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell
  • there are multiple types of fuel cells using different fuels and electrolytes
• Electrochemical cells :


  • a basic system made up of 2 electrodes in contact with electrolyte
  • the 2 electrodes must be able to conect electricity so are usually metals
  • the electrolyte is a liquid that contains ions which react with electrodes
  • the chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes
  • if the elecrodes are connected by a wire thr charge is able to flow and electricity is produced, a voltmetre can also be connected to the circuit to mesure the voltage of the cell
• different metals will react differently with the same electrolyte - this is what causes the charge difference (or voltage) of the cells- so the type of electrodes will affect the voltage of the cell
• non rechargeable batteries :


  • in some cells the chemical reactions that happen at electrodes are irriverable
  • over time the reacting particles- ions in the electrolyte and the metal ions on the electrodes - get used up and turned into the products of the reaction
  • once any one of the reactants is used up, the reaction cant happen so no electricity is produced
  • the products cant be turned back into the reactants so the cell cant be recharged
• what is an exothermic reaction?

  a reaction which transfers energy to the surroundings, this is shown by a rise in temperature
• examples of exothermic reactions:


  • combustion
  • oxidation
  • neutralisation
• everyday uses of exothermic reactions:


  • hand warmers
  • self heating cans
• what is an endothermic reaction 


  a reaction which takes in energy from the surroundings, this is shown by a fall in temperature
• examples of endothermic reactions


  • reaction between citric acid and sodium hydrogencarbonate
  • thermal decomposition
• everyday uses of endothermic reactions


  • sports injury packs
• in exothermic reactions reactants have more energy than products
• in endothermic reactions products have a higher energy than the reactants
• what is activation energy ?


  the minimum amount of energy needed for the reactants to collide with each other and react
• what is a fuel cell?


  an electrical cell thats supplied with a fuel and oxygen and uses energy from the reaction between them to produce energy efficiently