Chemical Kinetics Definition

Understanding Chemical Kinetics and Rate of Reaction

colorful balls colliding
Chemical kinetics helps explain why increasing collisions between molecules increases chemical reaction rate. Don Farrall/Getty Images

Chemical kinetics is the study of chemical processes and rates of reactions. This includes analysis of conditions that affect speed of a chemical reaction, understanding reaction mechanisms and transition states, and forming mathematical models to predict and describe a chemical reaction.

Also Known As

Chemical kinetics may also be called reaction kinetics or simply "kinetics". The rate of a chemical reaction usually has units of sec-1

Chemical Kinetics History

The field of chemical kinetics developed from the law of mass action, formulated in 1864 by Peter Waage and Cato Guldberg. The law of mass action states the speed of a chemical reaction is proportional to the amount of reactants.

Rate Laws and Rate Constants

Experimental data is used to find reaction rates, from which rate laws and chemical kinetics rate constants are derived by applying the law of mass action. Rate laws allow for simple calculations for zero order reactions, first order reactions, and second order reactions.

  • The rate of a zero-order reaction is constant and independent of the concentration of reactants.
    rate = k
  • The rate of a first-order reaction is proportional to the concentration of one reactants:
    rate = k[A]
  • The rate of a second order reaction has a rate proportional to the square of the concentration of a single reactant or else the product of the concentration of two reactants.
    rate = k[A]2 or k[A][B]

Rate laws for individual steps must be combined to derive laws for more complex chemical reactions. For these reactions:

  • There is a rate-determining step that limits the kinetics.
  • The Arrhenius equation and Eyring equations may be used to experimentally determine activation energy.
  • Steady-state approximations may be applied to simplify the rate law.

Factors That Affect Chemical Reaction Rate

Chemical kinetics predicts the rate of a chemical reaction will be increased by factors that increase the kinetic energy of the reactants (up to a point), leading to increased likelihood the reactants will interact with each other. Similarly, factors that decrease the chance of reactants colliding with each other may be expected to lower the reaction rate. The main factors that affect reaction rate are:

  • concentration of reactants (increasing concentration increases reaction rate)
  • temperature (increasing temperature increases reaction rate, up to a point)
  • presence of catalysts (catalysts offer a reaction a mechanism that requires a lower activation energy, so the presence of a catalyst increases the rate of a reaction)
  • physical state of reactants (reactants in the same phase may come into contact via thermal action, but surface area and agitation affect reactions between reactants in different phases)
  • pressure (for reactions involving gases, raising pressure increases the collisions between reactants, increasing reaction rate)

Note that while chemical kinetics can predict the rate of a chemical reaction, it does not determine the extent to which the reaction occurs. Thermodynamics is used to predict equilibrium.