C7 - Energy Changes

• Exothermic reaction


  Energy is transferred to the surroundings
• Endothermic reaction

  Energy is transferred from the surroundings
• Increase in temperature = ?


  Exothermic
• Decrease in temperature = ?

  Endothermic
• Reaction profile


  A graph that shows whether a reaction is exothermic or endothermic
• Activation energy


  The minimum energy that particles must have to react when they collide
• If a reaction profile goes from high to low...

  The reaction is exothermic
• If a reaction profile goes from low to high...


  The reaction is endothermic
• • Bond breaking requires energy
  • A reaction where more bonds are broken than made is endothermic
• • Making bonds releases energy
  • A reaction where more bonds are made than broken is exothermic
• How to work out the overall energy change of a reaction


  • work out how much energy is required to break all the bonds in the reactants
  • work out how much energy is released when making all the bonds in the products
  • overall energy transferred = energy required to break bonds - energy required to make bonds
  • a positive number means an endothermic reaction
  • a negative number means an exothermic reaction
• Chemical cells


  • In a metal displacement reaction, one metal is oxidised and loses electrons
  • The electrons are transferred to another metal, which gains electrons and is reduced
  • A chemical cell conducts this reaction, and the electrons' movement generates a current
  • The greater the difference in reactivity, the greater the potential difference produced
• Batteries


  • Formed of two or more cells connected in series
  • Rechargeable batteries can be recharged by applying an external current that reverses the reaction
  • Non-rechargable batteries cannot be recharged because the reaction is not reversible
  • Once the reactants are used up, the chemical reaction stops and no more potential differences are released
• Hydrogen fuel cells


  • Use fuel and oxygen from the air to produce a potential difference
  • Hydrogen is oxidised to produce water
• Advantages of hydrogen fuel cells


  • the only waste is water
  • do not need to be electrically charged
• Disadvantages of hydrogen fuel cells


  • hydrogen is highly flammable and difficult to store
  • hydrogen is often produced from non-renewable sources