Rate expression and reaction mechanism

Rate expression and reaction mechanism

Rate Expression

  • The rate of reaction measures how quickly reactants are turned into products.
  • Rate expressions describe the relationship between the rate of a reaction and the concentrations of the reactants.
  • In a rate expression, each concentration is raised to a power equal to the number of molecules of that reactant involved in the reaction.
  • The rate constant (k) in the rate expression is specific to a particular reaction and is affected by the temperature and the presence of catalysts.

Order of Reaction

  • The order of a reaction is the sum of the powers to which the concentrations of the reactants are raised in the rate expression.
  • The order of a reaction with respect to a single reactant indicates how a change in that reactant’s concentration will affect the rate of reaction.
  • A reaction is zero-order if changing the concentration of the reactant does not affect the rate of reaction. In other words, the rate is independent of that particular reactant’s concentration.

Reaction Mechanism

  • A reaction mechanism describes the sequence of steps, or elementary reactions, that make up the overall chemical reaction.
  • Each elementary reaction occurs in a single step with a single transition state and has its own rate equation.
  • The rate-determining step is the slowest step in the reaction mechanism. This step determines the overall rate of the reaction.
  • The order of reaction and the rate constant (k) can provide insight into the reaction mechanism.

Rate Determining Step

  • The rate determining step is the slowest step in a reaction mechanism and by its nature, determines the rate of the entire reaction.
  • The rate equation for the overall reaction matches the rate equation for the rate determining step, and the reactants in the rate determining step are included in the rate equation.
  • The step with the highest energy transition state is typically the rate determining step.
  • The rate determining step also heavily influences the way in which changes to conditions, such as concentration, temperature, or catalysts, will affect the rate of reaction.