Energy Changes

• Exothermic reaction


  Reaction that transfers energy to the surroundings so the temperature of the surroundings increases
• Exothermic reactions


  • Combustion
  • Many oxidation reactions
  • Neutralisation
• Endothermic reaction


  Reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Endothermic reactions


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


  Minimum amount of energy that particles must have to react
• Chemical reactions


  1. Reacting particles collide with each other

  2. Reacting particles have sufficient energy
• Reaction profile

  Shows the relative energies of reactants and products, the activation energy and the overall energy change of a reaction

  
• 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
• Bond energy


  Energy needed to break bonds and energy released when bonds are formed
• In an exothermic reaction

  The energy released from forming new bonds is greater than the energy needed to break existing bonds
• In an endothermic reaction


  The energy needed to break existing bonds is greater than the energy released from forming new bonds
• Chemical cell


  Chemicals react to produce electricity
• Voltage produced by a cell


  • Dependent on type of electrode and electrolyte
• Simple cell


  Two different metals in contact with an electrolyte
• Battery


  Two or more cells connected together in series to provide a greater voltage
• In a chemical cell

  1. Chemical reaction between electrodes and electrolyte sets up a charge difference between the electrodes

  2. If electrodes are connected by a wire, the charge is able to flow and electricity is produced

  3. Different metals will react differently with the same electrolyte
• Non-rechargeable cells and batteries

  Chemical reactions stop when one of the reactants has been used up
• Rechargeable cells and batteries

  Chemical reactions are reversed when an external electrical current is supplied
• Fuel cell


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

  
• Hydrogen fuel cell


  1. Hydrogen gas loses electrons and forms hydrogen ions at the anode

  2. Hydrogen ions move to the other electrode through the electrolyte

  3. Electrons move to the other electrode through a wire

  4. Hydrogen ions and electrons react with oxygen gas at the cathode to make water