C3.3 Calculating and explaining energy changes

When fuels and food react with oxygen, energy is released in an exothermic reaction.

Energy is normally measured in joules (j).

A simple calorimeter can be used to compare the amount of energy released by foods and fuels.

Energy released by different fuels can be compared by calculating how much energy is released per gram;

Energy released per gram = energy released / grams of fuel used

The energy released by different fuels can be calculated by the equation;

Q = mcT

Where;

Q = energy released by the fuel (joules)

m= mass of water heated(g)

c = specific heat capacity of water (amount neded to raise 1g of water by 1 degrees)

T = the rise in the temp (°C)

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

An exothermic reaction will release energy when taking place therefore the reactants will have a higher amount of energy than the products. The energy change can be worked out by the different in energy btween the reactants and the products. The activation energy however will be the height between the reactants and the top of the diagram.

An endothermal reaction is a positive reaction as energy is taken in when the reaction occurs. This means that the energy in the products made is higher than the energy in the reactants.

Overall energy = energy required to break the bonds + energy released forming new bonds

Overall energy can be positive or negative as energy can be taken in or given out.

Breaking the bonds in the reactant is endothermic as energy is taken in so the bonds can be broken.

Making new bond in a product is exothermic as when bonds form energy is released.

Catalysts have many uses;

• Speed up the reaction
• Not used up by the end of the reaction
• Nickel => hydrogenation
• Iron => Haber processes (production of NH3)

Catalysts can increase the rate of a reaction. They provide an altenate pathway for a chemical reaction which has a lower activation energy. More reactant particles will have enough energy to react.

Hydrogen can be burned as a fuel in combustion engines:

hydrogen + oxygen => water                            2H2 + O2 => 2H2O

Hydrogen can also be used in fuel cells that produce electricity to power vehicles.            Advantages;

• Less CO2 emissions
• Very abundant
• High efficiency

Disadvantages;

• hard to store - many safety issues with storing in tanks (high pressures)
• hard to supply -not many hydrogen fuel stations available - expensive-hard to match the price of ordinary fuel
• lifetime of M.E.A (membrane which is used) is not very long
• platinum catalyst - scarce
• highly flammable
• Rarely found in H2 form so electrolysis needs to take place