energy changes

• When chemical reactions occur, energy is conserved
• The amount of energy in the universe at the beginning is the same as at the end
• If a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred
• Exothermic reaction


  A reaction that transfers energy to the surroundings so the temperature of the surroundings increases
• Product molecules in an exothermic reaction must have less energy than the reactants, by the amount transferred
• Examples of exothermic reactions

  • Combustion
  • Many oxidisation reactions
  • Neutralisation
• Everyday examples of exothermic reactions

  • Self-heating cans (e.g for coffee)
  • Hand warmers
• Endothermic reaction


  A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
• Product molecules in an endothermic reaction must have more energy than reactants
• Examples of endothermic reactions

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


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


  1. Reacting particles collide with each other and with sufficient energy

  2. Activation energy is reached

  3. Reaction occurs
• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• You can tell which reaction is exothermic or endothermic by looking at whether the reactants or products have more energy (higher up energy scale=higher energy)
• Exothermic: heat released to the surroundings, so products have less energy than reactants
• Endothermic: heat taken in from surroundings, so products have more energy than reactants
• Energy change of reactions

  • Energy must be supplied to break bonds in the reactants
  • Energy is released when bonds in the products are formed
• The energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies
• Sum of energy taken in to break bonds - sum of energy released to form bonds = overall energy change
• Energy taken in to break > energy released when formed = ENDOTHERMIC (because overall energy has been taken in)
• Energy taken in to break < energy released when formed = EXOTHERMIC
• Cells and batteries


  • Cells contain chemicals which react to produce electricity
  • The voltage produced by a cell is dependent upon a number of factors (e.g. type of electrode & electrolyte)
• Simple cell


  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
• The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water
• Hydrogen fuel cells


  • Offer a potential alternative to rechargeable cells & batteries
  • Can be used constantly provided fuel keeps being put in
  • 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
  • Will eventually stop working
• Equations for each half cell 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)