C2.4 Rates of reaction
Being able to speed up or slow down chemical reactions is important in everyday life and in industry.
Changes in temperature, concentration of solution, gas pressure, surface area of solids and the presence of catalysts all affect the rates of reactions. Catalysts can help to reduce the cost of some industrial processes.
Candidates should use their skills, knowledge and understanding to:
■ interpret graphs showing the amount of product formed (or reactant used up) with time, in terms of the rate of the reaction
■ explain and evaluate the development, advantages and disadvantages of using catalysts in industrial processes.
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. Chemical reactions can only occur when reacting particles collide with each other in the correct orientation with sufficient energy.The minimum amount of energy particles must have to react is called the activation energy.
Effect of temperature, pressure, concentration, surface area and catalysts on rates of reaction
1. Temperature: increasing the temperature increases the proportion of particles that achieve, or exceed, the activation energy for the reaction, hence increasing the proportion of successful collisions between reactants.
2. Pressure: increasing the pressure of reacting gases increases the frequency of collisions and so increases the proportion of successful collisions.
3. Concentration: increasing the concentration of reactants in solutions increases the frequency of collisions and so increases the proportion of successful collisions.
4. Surface area: increasing the surface area of solid reactants increases the number of surface active sites and so increases the proportion of successful collisions.
Catalysts change the rate of chemical reactions but are not used up during the reaction.
Catalysts provide an alternative route for the reaction to occur between reactants which is at a lower activation energy.
Hence, catalysts will increase the proportion of particles that achieve, or exceed, the activation energy, thereby increasing the proportion of successful collisions between reacting particles and therefore increase the rate of reaction.
Different reactions need different catalysts. Enzymes are examples of biological catalysts.
Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce costs.
For example, catalysts enable reactions to take place at a lower temperature, so saving energy resources when performing the reaction at a massive industrial level.
Catalysts do not alter the amount of energy released (for exothermic) or absorbed (for endothermic) by chemical reactions.
An action that increases the rate of reaction will do so by:
1) increasing the proportion of reacting particles that achieve, or exceed, the activation energy
2)hence, increase the proportion of successful collisions between reacting particles.
Assessment and practical opportunities
■ 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.
There are opportunities here for measurements using sensors (eg carbon dioxide, oxygen, light, pH, gas pressure and temperature) to investigate reaction rates.
Footprints simulation (click here)
Cool iodine clock reaction video
An excellent interactive educational resource produced by BP challenging you to speed up a real industrial reaction is listed below [click here]