C3.3 CALCULATING AND EXPLAINING ENERGY CHANGE

a) The relative amounts of energy released when substances burn can be measured by simple calorimetry, eg by heating water in a glass or metal container. This method can be used to compare the amount of energy released by fuels and foods. 

b) Energy is normally measured in joules (J).

c) The amount of energy released or absorbed by a chemical reaction can be calculated from the measured temperature change of the solution whene the reagents are mixed in an insulated container. This method can be used for reactions of solids with water or for neutralisation reactions. 

d) Simple energy level diagrams can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction. 

e) During a chemical reaction: 

• energy must be supplied to break bonds
• energy is released when bonds are formed.

f) In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. 

g) In an endothermic reactions, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

h) Catalysts provide a different pathway for a chemical reaction that has a lower activation energy.

i) Hydrogen can be burned as a fuel in combustion engines. 

hydrogen+oxygen---->water

It can also be used in fuel cells that produce electricity to power vehicles.