Energy changes

• What is an exothermic energy transfer?

  Transfer to the surroundings
• What is an endothermic energy transfer?

  Transfer from the surroundings
• What is a reaction profile?


  Diagram showing how the energy changes in a reaction
• What is the activation energy?


  Minimum amount of energy required before a collision will result in a reaction
• What is bond energy?


  Energy required to break a bond or the energy released when a bond is formed
• In terms of bond breaking and making, what is an exothermic reaction?


  Less energy is required to break the bonds than is released when making the bonds
• In terms of bond breaking and making, what is an endothermic reaction?


  More energy is required to break the bonds than is released when making the bonds
• How are chemical cells made?

  Connect two different metals (electrodes) in a solution (electrolyte)
• What is a battery?


  Two or more chemical cells connected in series
• How does the potential difference of a cell depend on the metals that the electrodes are made of?


  The bigger the difference in reactivity, the greater the potential difference
• How can some cells be recharged?


  By applying an external current
• Why can some cells not be recharged?

  The reaction cannot be reversed
• What is a fuel cell?


  Cell that uses a fuel and oxygen to generate electricity
• In the hydrogen fuel cell, what is the overall reaction?

  2H2 (g) + O2 (g) → 2H2O (l)
• In the alkaline hydrogen fuel cells, what are the half equations?

  2H2 (g) + 4OH- (aq) ---> 4H2O (l) + 4e-

  O2 (g) + 2H2O (l) + 4e- ---> 4OH- (aq)
• Give an advantage of the hydrogen fuel cell.


  Only product is water, do not need to be electrically recharged
• Give a disadvantage of the hydrogen fuel cell.


  Hydrogen is flammable, difficult to store and is often produced from non-renewable sources