5. Energy changes

• Exothermic reaction


  A reaction which transfers energy to the surroundings, shown by a rise in temperature
• Endothermic reaction


  A reaction which takes in energy from the surroundings, shown by a fall in temperature
• The total amount of energy doesn't change in a reaction, as energy is conserved - it can only be moved around
• Exothermic reactions


  • Burning fuels (combustion)
  • Neutralisation reactions (acid + alkali)
  • Many oxidation reactions
• Endothermic reactions


  • Reaction between citric acid and sodium hydrogencarbonate
  • Thermal decomposition of calcium carbonate
• Endothermic reactions are much less common than exothermic reactions
• Everyday uses of endothermic reactions

  • Sports injury packs
• Measuring energy transfer in a reaction

  1. Take temperature of reagents before mixing

  2. Mix in polystyrene cup and measure final temperature

  3. Reduce energy lost to surroundings by insulation and lid
• This method works for neutralisation reactions, reactions between metals and acids, or carbonates and acids
• Investigating effect of acid concentration on energy released in neutralisation

  1. Use 0.25, 0.5 and 1 mol/dm³ hydrochloric acid

  2. Mix with sodium hydroxide in polystyrene cup

  3. Measure maximum temperature reached
• Reaction profile/energy level diagram


  Diagram showing relative energies of reactants and products, and activation energy
• Exothermic reaction profile


  • Products have lower energy than reactants
  • Difference in height represents overall energy change (energy given out)
• Endothermic reaction profile


  • Products have higher energy than reactants
  • Difference in height represents overall energy change (energy taken in)
• Activation energy


  Minimum energy needed for reactants to collide and react
• Greater activation energy means more energy needs to be supplied to start the reaction
• Bond breaking


  Endothermic process - energy must be supplied
• Bond forming


  Exothermic process - energy is released
• In exothermic reactions, the energy released by forming new bonds is greater than the energy used to break existing bonds
• In endothermic reactions, the energy used to break existing bonds is greater than the energy released by forming new bonds
• Calculating overall energy change of a reaction

  1. Find energy required to break original bonds

  2. Find energy released by forming new bonds

  3. Overall energy change = energy to break bonds - energy released by forming bonds
• You can't compare overall energy changes of reactions unless you know the numerical differences in the bond energies
• Cells and Batteries
• 431 kJ/mol


  Energy required to break the original bonds
• Find the energy required to break the original bonds

  1. + 242 kJ/mol

  2. = 678 kJ/mol

  3. (1x H-H) + (1 x CI-CI) = 436 kJ/mol
• Find the energy released by forming the new bonds

  1. 2xH-CI 2x 431 kJ/mol

  2. = 862 kJ/mol
• Find the overall energy change for the reaction

  1. Overall energy change = energy required to break bonds - energy released by forming bonds

  2. = 678 kJ/mol - 862 kJ/mol

  3. E = -184 kJ/mol
• Chlorine and bromine react with hydrogen in a similar way. B-bonds are weaker than Cl-Cl bonds and H-B bonds are weaker than H-Cl bonds. So less energy is needed to break the bonds in the reaction with bromine but less energy is released when the new bonds form. So unless you know the exact difference, you can't say which reaction releases more energy.
• You can't compare the overall energy changes of reactions unless you know the numerical differences in the bond energies.
• Electrochemical cell


  A basic system made up of two different electrodes in contact with an electrolyte
• Electrochemical cell


  • The two electrodes must be able to conduct electricity and are usually metals
  • The electrolyte is a liquid that contains ions which react with the electrodes
  • The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes
  • If the electrodes are then connected by a wire, the charge is able to flow and electricity is produced
• Voltmeter


  Can be connected to the circuit to measure the voltage of the cell
• Voltage of a cell


  • Different metals will react differently with the same electrolyte, this is what causes the charge difference or the voltage of the cell
  • The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell
  • The electrolyte used in a cell will also affect the size of the voltage since different ions in solution will react differently with the metal electrodes used
• Voltages of two simple cells with the same electrolyte and different electrodes

  • Iron/Iron = -0.30 V
  • Tin/Lead = -0.31 V
• Calculate the voltage of a cell where Electrode A is lead and Electrode B is tin

  The voltages of both cells are negative, so iron is more reactive than tin and lead. The voltage for the iron/iron cell is more negative than the iron/tin cell, so lead is less reactive than tin. The difference in voltage of the two cells is 0.01 V. So, for a cell where Electrode A is lead and Electrode B is tin, the voltage is +0.01 V
• Battery


  Formed by connecting two or more cells together in series. The voltages of the cells in the battery are combined so there is a bigger voltage overall.
• Non-rechargeable batteries


  • The chemical reactions that happen at the electrodes are irreversible
  • Over time the reacting particles - the ions in the electrolyte and the metal ions on the electrode - get used up and turned into the products of the reaction
  • Once any one of the reactants is used up, the reaction can't happen and so no electricity is produced
  • The products can't be turned back into the reactants, so the cell can't be recharged
• Fuel cell


  An electrical cell that's supplied with a fuel and oxygen to produce electrical energy
• How a hydrogen-oxygen fuel cell works

  1. Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment

  2. At the -ve electrode (the anode), hydrogen loses electrons to produce H+ ions. This is oxidation.

  3. H+ ions in the electrolyte move to the cathode (+ve electrode)

  4. At the +ve electrode (the cathode), oxygen gains electrons from the cathode and reacts with H+ ions (from the electrolyte) to make water. This is reduction.

  5. The electrons flow through an external circuit from the anode to the cathode - this is the electric current.

  6. The overall reaction is hydrogen plus oxygen, which gives water.
• Hydrogen-oxygen fuel cells


  • The electrolyte is often an acid, such as phosphoric acid
  • The electrodes are often porous carbon with a catalyst
• Fuel cell vehicles don't produce as many pollutants as other fuels - no greenhouse gases, nitrogen oxides, or carbon monoxide. The only by-products are water and heat.