4.5 CHEMISTRY

• Exothermic reaction


  A reaction that transfers energy to the surroundings so the temperature of the surroundings increases
• Endothermic reaction


  A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Exothermic reactions


  • Combustion
  • Many oxidisation reactions
  • Neutralisation
• Endothermic reactions


  • Thermal decomposition
  • Reaction of citric acid and sodium hydrogencarbonate
• Everyday examples of exothermic reactions


  • Self-heating cans (e.g. for coffee)
  • Hand warmers
• Everyday examples of endothermic reactions


  • Sports injury packs
• Activation energy


  Minimum amount of energy that particles must have to react
• Reaction profile


  1. Shows the relative energies of reactants and products

  2. Shows the activation energy

  3. Shows the overall energy change of a reaction
• Exothermic reaction


  Products have less energy than reactants
• Endothermic reaction


  Products have more energy than reactants
• Energy changes during a chemical reaction


  1. Energy must be supplied to break bonds in the reactants

  2. Energy is released when bonds in the products are formed
• The sum of energy taken in to break bonds - the sum of energy released to form bonds = overall energy change
• Endothermic reaction


  Energy taken in to break bonds > energy released when bonds are formed
• Exothermic reaction


  Energy taken in to break bonds < energy released when bonds are formed
• Cells


  Contain chemicals which react to produce electricity
• Voltage produced by a cell


  • Depends on type of electrode & electrolyte
• Making a simple cell


  Connect two different metals in contact with an electrolyte
• Batteries


  Two or more cells connected together in series to provide a greater voltage
• Non-rechargeable cells & batteries


  • Chemical reactions stop when one of the reactants has been used up
  • Alkaline batteries are non-rechargeable
• Rechargeable cells & batteries

  • Can be recharged because the chemical reactions are reversed when an external electrical current is supplied
• Fuel cells


  Supplied by an external source of fuel (e.g. hydrogen) and oxygen or air, the fuel is oxidised electrochemically within the fuel cell to produce a potential difference
• Overall reaction in a hydrogen fuel cell


  Oxidation of hydrogen to produce water
• Hydrogen fuel cells


  Offer a potential alternative to rechargeable cells & batteries
• Advantages of hydrogen fuel cells over rechargeable cells & batteries


  • Fuel cells can be used constantly provided fuel keeps being put in
  • Rechargeable cells and batteries can be recharged by reversing reaction, so fuel doesn't need to keep being supplied
  • Hydrogen is a gas so needs to be stored at high pressure and so is harder to transport
  • Hard to dispose of - non-biodegradable
  • Only produces water when burnt
• Disadvantages of hydrogen fuel cells compared to rechargeable cells & batteries


  • Will eventually stop working
• Reactions in a hydrogen fuel cell


  1. At the anode (positive electrode): H2(g) -> 2e- + 2H+(aq)

  2. At the cathode (negative electrode): 4H+(aq) + O2(g) + 4e- -> 2H2O(g)