Comparing energy produced by fuels.
Heat is produced when fuels burn. It is an exothermic reaction.
A bomb calorimeter can measure the amount of energy produced by fuels. Energy is measured in joules (J).
We can measure and compare the energy produced by foods as well.
1 Calorie = 4.2 J.
A calorimeter is used to measure the amount of heat produced when a substance is burned.
A simple calorimeter is a beaker of water with a thermometer in it which measures the temperature of the water as the substance is set alight and held underneath the beaker.
4.2 J of heat energy increases 1g of water by 1 degree C.
Energy changes in reactions.
Endothermic reactions - energy is taken from surroundings - supplied to break bonds.
Exothermic reactions - energy is released - so new bonds are formed.
The energy needed to break bonds is the activation energy for a reaction.
Energy level diagrams the difference in enery between products and reactants.
The energy change is represented by delta H (ΔH).
ΔH is negative on the diagram when it is an exothermic reaction. (because the products are at a lower energy level than the reactants and the temperature of the surroundings increases.)
ΔH is positive on the diagram when it is an endothermic reaction. (because the products are at a higher energy level than the reactants and the temperature of the surroundings decreases.)
Calculations using bond energies.
The energy needed to break bonds in an atom is known as bond energy.
To calculate the energy change for a chemical reaction:
1) calculate how much energy is needed to break all bonds in the reactants
2) calculate how much energy is released when the new bonds are formed in the products
3) take the difference between the 2 totals