Energy changes

• What is the 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
• What is an exothermic reaction?

  An exothermic reaction is a chemical reaction that releases heat to its surroundings.

  -> e.g combustion, oxidation, neutralisation
• What is an endothermic reaction?

  An endothermic reaction is a chemical reaction that absorbs heat from its surroundings, resulting in a decrease in temperature

  -> e.g thermal decomposition, reaction of citric acid
• What is a reaction profile?

  A graph which shows the relative energies of reactants and product as well as the activation energy
• What occurs in a exothermic reaction in terms of bond energies?

  Energy released from forming bonds is greater than that needed to break the bonds
• What occurs in an endothermic reaction in terms of bond energies?

  Energy needed to break bonds is greater than energy released making them
• What is a cell?

  Composed of two electrodes dipped in an electrolyte solution

  It produces electricity from a chemical reaction
• What determines the voltage obtained from a cell?

  Identities of metals used as electrodes and the identity and concentration of an electrolyte
• Describe rechargeable and non-rechargeable cells

  Rechargeable: chemical reactions are reversed when an external current is supplied

  Non-rechargeable: reactants are used up
• What is a fuel cell?

  Supplied by fuel and oxygen to oxidise the fuel to generate electricity
• 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
• How do rechargeable cells work? 


  > An electric current is passed backwards through the cell

  > This works by reversing the chemical reactions so the cell can be used again
• How is voltage affected by the reactivity of the two metals used?

  The bigger the difference in reactivity between the two metals, the bigger the voltage
• How do chemical cells work?

  converts chemical energy into electrical energy

  two types: rechargeable and non rechargeable cells