GCSE / Chemistry
The Rate and Extent of Chemical Change
Learn how to calculate and interpret rates of reaction, explain the effects of concentration, pressure, surface area, temperature and catalysts using collision theory, and understand reversible reactions and dynamic equilibrium. This GCSE Chemistry topic also covers activation energy, reaction graphs and how concentration, temperature and pressure affect equilibrium.
The rate of a chemical reaction describes how quickly reactants are used up or products are formed. Mean rate can be calculated by dividing the quantity of reactant used or product formed by the time taken.
Reaction rate can be measured using changes in mass, volume of gas or amount of substance. Common units include g/s, cm³/s and, for Higher Tier calculations, mol/s.
A reaction rate graph shows how the amount of reactant or product changes with time. A steeper gradient means the reaction is happening faster, while a flatter gradient means the reaction is slowing down.
The rate of reaction at a particular point on a curved graph is found by drawing a tangent at that point and calculating the gradient of the tangent using change in y divided by change in x.
The main factors that affect reaction rate are concentration of reactants in solution, pressure of reacting gases, surface area of solid reactants, temperature and the presence of a catalyst.
Increasing the concentration of a reactant increases the number of reacting particles in a given volume. This causes more frequent collisions and therefore increases the rate of reaction.
Increasing the pressure of reacting gases forces the gas particles closer together. This increases the frequency of collisions between reacting particles and causes a faster reaction.
Increasing the surface area of a solid reactant exposes more particles to other reactants. Small pieces or powders therefore usually react faster than larger pieces of the same substance.
Increasing temperature gives reacting particles more kinetic energy. Particles move faster, collide more often and a greater proportion of collisions have enough energy to result in a reaction.
Collision theory states that chemical reactions occur only when reacting particles collide with each other and the collision has sufficient energy to overcome the activation energy.
Activation energy is the minimum amount of energy that reacting particles must have when they collide for a successful chemical reaction to occur.
A catalyst increases the rate of a reaction without being used up. It provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful.
A reversible reaction is one in which the products can react to form the original reactants again. The direction of a reversible reaction can be changed by changing the reaction conditions.
Dynamic equilibrium is reached in a closed system when the forward and reverse reactions continue at equal rates. The amounts of reactants and products then remain constant even though both reactions continue.
The position of equilibrium can be changed by altering concentration, temperature or pressure. Increasing pressure favours the side with fewer gas molecules, while temperature changes favour the endothermic or exothermic direction depending on the change applied.