Chemical kinetics

Chemical Kinetics Overview

  • Chemical kinetics is the study of rate at which reactions proceed and the factors that influence these rates.
  • It is important in predicting the speed of reactions and determining reaction mechanisms.

Collision Theory

  • Reactions can only occur when particles collide.
  • Only a fraction of collisions lead to a reaction. These are referred to as successful collisions.
  • A successful collision is characterised by particles having sufficient kinetic energy (above the activation energy) and the correct spatial alignment.

Rate of Reaction

  • Measured as the rate of change of concentration of a reactant or product over time, usually in mol dm^-3 s^-1.
  • An increase in concentration of particles can increase the rate of reaction as it increases the likelihood of collisions.

Activation Energy

  • The minimum energy needed for a reaction to proceed.
  • The fraction of particles with sufficient energy can be increased by raising the temperature.
  • A catalyst can lower the activation energy, increasing the rate of reaction.


  • An increase in temperature increases the rate of reaction as particles possess more kinetic energy, leading to more successful collisions.
  • Temperature is the single most powerful way of increasing reaction rate.


  • For reactions involving gases, an increase in pressure increases the rate of reaction as it leads to a higher concentration of particles, and thus more frequent collisions.


  • Catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
  • They are not consumed in the reaction, and can therefore be used repeatedly.

Rate Equations

  • The rate equation shows how rate is affected by the concentration of each reactant.
  • Rates can be zero order (rate is independent of concentration), first order (rate is directly proportional to concentration) or second order (rate is proportional to the square of the concentration).

Determining Order of Reaction

  • The order of reaction can be determined from concentration-time graphs, initial rates method or the method of half lives.
  • The overall order of reaction is the sum of the individual orders for each reactant.

Reaction Mechanisms

  • A reaction mechanism describes the steps that take place during a reaction on a molecular level.
  • Each step is known as an elementary step or a reaction intermediate.
  • The rate-determining step (RDS) is the slowest step in the whole reaction and governs the rate. Determine the RDS from the experimental rate equation.

Using the Arrhenius Equation

  • The Arrhenius equation quantifies the effect of temperature on the rate of reaction.
  • It relates the energy of activation and the temperature to the rate constant, k.

Remember, it’s crucial to understand and apply these concepts when tackling problems in chemical kinetics. Repetitive practice and revision of these core ideas will ensure your overall understanding of chemical kinetics is solid and comprehensive. The more you practice applying these concepts to different problems, the better. Happy revising!