Energy from Reactions
It is possible to use calorimetry to measure the relative amounts of energy released when substances burn. This is commonly done by heating water in a glass or metal container. Calorimetry can be used to compare the amount of energy released by different fuels or foods.
The equation for calculating the amount of energy released by different fuels is:
Q = mc ΔT
Q = energy released (joules per kilogram, J/kg or joules per gram, J/g)
m = mass of substance heated (kilograms, kg or grams, g)
c = specific heat capacity (joules, J) [the specific heat capacity of water is 4.18J]
ΔT = temperature change (°C)
The specific heat capacity of water is 4.18J, meaning that it takes 4.18J of energy to heat 1g of water by 1°C.
When a chemical reaction is carried out and the reagents are mixed in an insulated container, it is possible to calculate the amount of energy released or absorbed by the chemical reaction. This method is effective for neutralisation reactions or reactions of solids with water.
We can use simple energy level diagrams to show the relative energies of products and reactants, the activation energy and the overall energy change of a reaction.
During a chemical reaction:
Breaking bonds requires energy
Forming bonds releases energy
In an exothermic reaction, there is more energy released from forming new bonds than there is taken in to break existing bonds. Thus, energy is given out to the surroundings in the form of heat.
In an endothermic reaction, there is more energy taken in to break existing bonds than there is released from forming new bonds. Thus, energy in the form of heat is taken in from the surroundings.
A catalyst works by providing an alternative pathway for a chemical reaction. This pathway has a lower activation energy, so the reaction happens more quickly.
Hydrogen can be burned as a fuel in a combustion engine.
hydrogen + oxygen → water
It can also be used in fuel cells that produce electricity to provide power for vehicles.
