Rate Equations

Understanding Rate Equations

  • A rate equation shows how the rate of a reaction depends on the concentration of the reactants.
  • General form of a rate equation is rate = k[A]^m[B]^n where k is the rate constant, and m and n are the orders of reaction with respect to A and B.
  • The sum of m and n gives the overall order of the reaction.

Defining Order of Reaction

  • Zero order: The rate of reaction is unaffected by the concentration of the reactant. The graph of concentration against time is a straight line with a negative slope.
  • First order: The rate of reaction is proportional to the concentration of the reactant. The graph of concentration against time follows an exponential decay curve.
  • Second order: The rate of reaction is proportional to the square of the concentration of the reactant.

Determining Rate Equations

  • Experimental data for variation of rate of reaction with changing concentrations is used to determine the rate equation.
  • Various methods like initial rates method, graphical method etc., are used to identify the order of reaction and rate constant.
  • For reactions involving more than one reactant, each reactant’s order is determined individually by changing its concentration and keeping others constant.

Rate Constants and Temperature

  • The rate constant k depends on the temperature of the reaction system.
  • As the temperature increases, the rate constant usually increases, demonstrating the specificity of reactions at different temperatures.
  • The Arrhenius equation relates the rate constant to the temperature and activation energy of a reaction.

Applications of Rate Equations

  • Rate equations can be used to predict the behaviour of a reaction over time.
  • Chemical engineers and chemists use rate equations to design and optimise chemical processes.
  • By controlling variables linked to the rate equation (such as temperature and reactant concentrations), production rates can be maximised, and energy and waste minimised.

Safety Considerations for Rate Equations

  • Exothermic reactions, which release heat, may cause temperature increases that can significantly speed up reactions; understanding the rate equation can help control these reactions.
  • If an industrial process is happening too quickly due to high reactant concentrations, it can be slowed down by diluting the reactants or lowering the temperature.
  • Knowing a process’s rate equation and the factors affecting the rate allows for forecasts on potential hazards, helping to establish safe operating parameters.