Energy Changes

• Conservation of energy principle


  Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place
• Exothermic reaction


  A reaction where energy is transferred to the surroundings so that the surroundings temperature increases
• Exothermic reactions


  • Combustion
  • Oxidation reactions
  • Neutralisation (acid + alkali) reactions
• Endothermic reaction


  A reaction where energy is taken in from the surroundings so the surroundings temperature decreases
• Endothermic reactions


  • Thermal decomposition
  • Reaction of citric acid and sodium hydrogencarbonate
• Activation energy


  Minimum amount of energy that particles need to react
• Reaction profile


  A graph which shows the relative energies of reactants and product, as well as activation energy of the reaction
• Chemical reactions in terms of bond energies

  • Energy is supplied to break bonds and energy is released when bonds are made
  • Exothermic - energy released from forming bonds is greater than that needed to break the bonds
  • Endothermic - energy needed to break bonds is greater than energy released making them
• Energy of reaction = sum of bonds broken – sum of bonds made
• Cell


  Composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction
• Battery


  Consists of two or more cells connected in series
• Voltage obtained from a cell


  Determined by the identities of metals used as electrodes and the identity and concentration of an electrolyte
• Advantages and disadvantages of using cells and batteries

  • More or less cheap
  • Some are rechargeable
  • A convenient source of electrical energy
  • Harmful chemicals
• Rechargeable cells


  Chemical reactions are reversed when an external current is supplied
• Non-rechargeable cells


  Reactants are used up, cannot be recharged
• Fuel cell


  Supplied by fuel and oxygen to oxidise the fuel to generate electricity
• Overall reaction in a hydrogen fuel cell

  1. Cathode: 2 H2 → 4 H+ + 4 e−

  2. Anode: O2 + 4 H+ + 4 e−→ 2 H2O

  3. Overall: 2 H2 + O2 → 2 H2O
• Advantages and disadvantages of hydrogen fuel cells

  • Advantages: no pollutants, no recharging
  • Disadvantages: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis