Rates and Reaction Mechanisms

Rates and Reaction Mechanisms

Reaction Rates

  • Rate of reaction refers to how quickly or slowly the reactants are used up or the products are formed in a chemical reaction.
  • It can be determined by measuring the change in concentration of a reactant or product over time, typically in mol dm⁻³ s⁻¹.
  • Reaction rate can be affected by several factors such as concentration of reactants, pressure, temperature, and catalysts.
  • The rate equation shows the relationship between the rate of reaction and the concentrations of the reactants. It takes the form: rate = k[A]^m [B]^n, where k is the rate constant, m and n are orders of reaction.
  • Order of reaction can be zero (no effect on rate), first (rate proportional to concentration), or second (rate proportional to square of concentration), and are determined experimentally.
  • The value of k, the rate constant, changes with temperature.
  • The half-life of a reaction (t½) is the time taken for the concentration of a reactant to decrease by half.

Collision Theory

  • Collision theory states that chemical reactions can only occur when particles collide with each other with sufficient energy and correct orientation.
  • Activation energy (Ea) is the minimum energy that colliding molecules need in order to react.
  • An Arrhenius plot can be used to find the activation energy of a reaction by plotting the natural log of the rate constant, k, against 1/Temperature.


  • Catalysts are substances that increase the rate of reaction but remain chemically unchanged at the end.
  • They provide an alternative reaction pathway with a lower activation energy by forming an intermediate.
  • Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase.

Reaction Mechanisms

  • A reaction mechanism is a series of steps that make up the overall reaction.
  • Most reactions proceed in multiple steps, each with its own rate.
  • The rate determining step is the slowest step in the reaction mechanism and determines the rate of the overall reaction.
  • In a multi-step reaction, the rate equation is determined by the rate determining step.
  • Intermediate species are formed and used up during the reaction mechanism and do not appear in the overall equation. They can often be identified by their appearance in the rate determining step.