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• Overall reaction in hydrogen fuel cells

  2H₂(g) + O₂(g) → 2H₂O(1)
• Hydrogen fuel cells use a fuel and oxygen from the air to generate a potential difference
• Half equations in alkaline fuel cells

  1. 2H₂(g) + 4OH-(aq) → 4H₂O(l) + 4e¯

  2. O₂(g) + 2H₂O(l) + 4e¯ →40H-(aq)
• Types of hydrogen fuel cells

  • Alkaline fuel cells
• Metal displacement reaction

  One metal is oxidised - it loses electrons. These electrons are transferred to another metal, which gains the electrons and so is reduced
• Energy transfer in endothermic reactions


  From the surroundings
• Hydrogen is oxidised to produce water in hydrogen fuel cells
• Greater difference in reactivity between metals in a cell leads to a greater potential difference produced
• Energy transfer in exothermic reactions


  To the surroundings
• Chemical cell


  Conducts metal displacement reaction generating a current
• Batteries are formed of two or more cells connected in series
• Some batteries are rechargeable by applying an external electric current to reverse the reaction
• Temperature change in endothermic reactions

  Temperature of the surroundings decreases
• Temperature change in exothermic reactions

  Temperature of the surroundings increases
• Some batteries, like alkaline batteries, are not rechargeable because the reaction is not reversible
• Once reactants are used up in batteries, the chemical reaction stops and no more potential differences are released
• Key terms

  • Battery
  • Activation energy
  • Bond energy
  • Chemical cell
  • Combustion
  • Endothermic
  • Exothermic
  • Fuel cell
  • Neutralisation
  • Oxidation
  • Reaction profile
  • Rechargeable
  • Thermal decomposition
• Energy changes occur during a chemical reaction
• Ways energy can be transferred

  • To the surroundings (exothermic)
  • From the surroundings (endothermic)
• Thermal decomposition

  1. Process item 1

  2. Process item 2

  3. etc...
• Bonds

  Atoms are held together by strong chemical bonds. In a reaction, those bonds are broken and new ones are made between different atoms
• Energy transfer during a chemical reaction can cause a temperature change
• Energy is always conserved in chemical reactions
• Activation energy is the minimum amount of energy that particles must have to react when they collide
• When chemists refer to "the surroundings," they mean everything that isn't the reaction
• Breaking a bond requires energy, so it is endothermic

  Making a bond releases energy, so it is exothermic
• Different bonds require different amounts of energy to be broken (their bond energies)
• Bond calculations

  1. Step 1: Calculate the energy required to break all the bonds in the reactants

  2. Step 2: Calculate the energy released when making all the bonds in the products

  3. Overall energy transferred = energy required to break bonds - energy required to make bonds
• A positive number indicates an endothermic reaction, while a negative number indicates an exothermic reaction
• If a little energy is released when making the bonds and a lot is required to break them, the overall energy is taken in, and the reaction is endothermic
• If a lot of energy is released when making the bonds and only a little energy is required to break them, the overall energy is released, and the reaction is exothermic
• Required Practical Skills

  • Practise answering questions on the required practicals using the example below
  • You need to be able to apply your skills and knowledge to other practicals too
• Substance that reacts with acids in neutralisation reactions
• General equations for reactions of acids
• Formulas of ions involved in neutralisation reactions
• They react with the oxygen being formed
• Worked Example

  1. Write a method to investigate how the volume of sodium hydroxide added to hydrochloric acid affects the temperature change of the reaction

  2. Place a polystyrene cup inside a beaker. Pour 30 cm³ of dilute hydrochloric acid into the cup, place the lid on the cup and insert a thermometer through a hole in the cup. Record the temperature of the acid. Pour 5 cm³ sodium hydroxide solution into the cup and stir the solution. Record the highest temperature the reaction reaches on the thermometer. Repeat the experiment, increasing the amount of sodium hydroxide each time by 5 cm³, up to 40 cm³. Repeat the entire experiment two times to get repeat measurements
• Temperature changes

  1. This practical tests your ability to accurately measure mass, temperature, and volume to investigate changes during chemical reactions

  2. You should be able to describe how to measure temperature change after mixing a strong acid with a strong alkali
• Substance that dissolves in water to form a solution above pH 7
• Solid ionic substances not be electrolysed