Energy is measured in Joules (J). It take 4.2 joules to heat up 1g of water by 1 degree. This amount of energy us called 1 calorie (C)
Information about the energy provided by food products is given in kilocaloires (kcal). Why any chemical change takes place it is accompanied by an energy change. The realtive amounts of energy produced by food or fuel can be measured using calorimetry.
Energy Released can be calculated using the formula: Q=m x c x (Delta)T
m= mass of water heated
c = specific heat capacity of water
Delta T= rise in temperature.
Making and Breaking Bonds
In a chemical reaction, new substances are produces. The bonds in the reactants must be broken and new bonds formed in order for this to happen.
Breaking a chemical bond requires a lot of energy - this is an endothermic process.
When a new chemical bond is formed, energy is given out - this is an exothermic process.
If more energy is required to break old bonds than is released when new bonds are formed, the reaction must be endothermic.
If more energy is released when the new bonds are formed than is needed to break the old bonds, the reaction must be exothermic.
Measuring the Energy of Chemical Reactions
The amount of energy produced in a chemical reaction in solution can be measured by mixing reactants in an insulated container. This method would be suitable for neutralisation reactions and reactions involving solids.
methane + oxygen -> carbon dioxide + water
CH4 + 2O2 -> CO2 + 2H20
Bond energies are C-H is 412kL/mol 0=0 is 496kL/mol C=0 is 805kJ/mol, H-0 is 463kJ/mol
Energy used to break = 4xC-H + 2x 0=0 = 2640kJ
Energy given ut when made= 2x C=0 + 4 x H-0 = 3462kJ
Energy change = energy used to break bonds -energy given out when making bonds = 2640kJ - 3462kJ =-822kJ - and exothermic reaction
Energy Level Diagrams
The energy changes in a chemical reaction can be illustrated using an energy level diagram:
- In an exothermic reaction, energy is released. This means it's being lost, so the products have less energy than the reactants.
- In an endothermic reaction, energy is being taken in. This means that energy is being gained, so the products have more energy than the reactants.
- The activation energy is the energy needed to start a reaction
- Catalysts reduce the activation energy needed for a reaction- this makes the reaction go faster.
Hydrogen as a fuel
Hydrogen can be used
- as a fuel in combustion engines -
- Hydorgen + oxygen -> water +heat
- in fuel cells, which produce electricity that can be used to power cars
Energy Level Diagrams