Rates of Reaction

The rate of a reaction is the speed at which a reaction occurs. It can be quantitatively defined as the decrease in concentration of reactants or the increase in concentration of products per unit time.
The rate of reaction can be determined by measuring the change in concentration of substrates or products over time. This can either be expressed as an average rate (change in concentration over a given time period) or an instantaneous rate (rate of reaction at a specific time).
Collision theory states that for a reaction to occur, reactant particles need to collide with sufficient energy and in the correct orientation. A successful collision that leads to a reaction is termed an effective collision.
The minimum energy required for an effective collision is called the activation energy. The greater the activation energy, the slower the reaction because fewer particles will have sufficient energy to react.
Catalysts speed up reactions by lowering the activation energy required for a reaction to occur. They work by providing an alternative reaction pathway with a lower activation energy.
Temperature influences the rate of a reaction. As temperature increases, the frequency of effective collisions between particles also increases due to an increase in kinetic energy, hence increasing the rate of the reaction.
The concentration of reactants also impacts the rate of reaction. Higher concentrations mean a greater number of particles in a given volume, leading to an increase in the frequency of effective collisions, and hence an increase in the reaction rate.
The pressure in case of gaseous reactants, impacts the reaction rate in a similar way to concentration. Higher pressure results in a higher concentration of particles, thus increasing the rate of reaction.
Surface area affects the rate of reactions involving solids. Increasing the surface area exposes more particles to react, hence increasing the rate of the reaction.
Rate equations help to establish the relationship between the rate of a reaction and the concentrations of reactants. The symbol ‘k’ in rate equations symbolises the rate constant of a reaction.
Order of reaction refers to the power to which the concentration of each reactant is raised in the rate equation. The overall order of the reaction is the sum of orders of all the reactants.
The Arrhenius equation relates the rate constant of a reaction with the temperature and activation energy. It illustrates that increasing temperatures or decreasing activation energy will increase the rate of a reaction.
The rate-determining step in reaction mechanisms is the slowest step. It determines the overall speed of the reaction. All preceding steps in reaction mechanisms are assumed to be in equilibrium.
Half-life is the time taken for half of the reactants to be consumed. For first order reactions, half-life is independent of the initial concentration.