Chemistry GCSE : Rates of reaction

Rates of reaction

a) The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed over time:

Rate of reaction = amount of reactant used over time

Rate of reaction = amount of product formed over time

b) Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy. The minimum amount of energy particles must have to react is called the activation energy.

c) Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of reaction.

d) Increasing the pressure of reacting gases increases the frequency of collisions and so increases the rate of reaction.

e) Increasing the concentration of reactants in solutions increases the frequency of collisions and so increases the rate of reaction.

f) Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

g) Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts.

h) Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce costs.

You need to recall how the steps of practical work when:

■ designing and carrying out investigations into factors such as:

– temperature, eg magnesium with acids at different temperatures

– surface area, eg different sizes of marble chips

– catalysts, eg the decomposition of hydrogen peroxide using manganese(IV) oxide, potato and/ or liver; the ignition of hydrogen using platinum; oxidation of ammonia using platinum; cracking liquid paraffin using broken pot

– concentration, eg sodium thiosulfate solution and dilute hydrochloric acid.

You need to know how to do measurements using sensors (eg carbon dioxide, oxygen, light, pH, gas pressure and temperature) to investigate reaction rates.