Reaction Rate Definition (Chemistry)

What the reaction rate is and the factors that affect it

Reaction rate is how fast a chemical reaction proceeds. Temperature is the key factor affecting reaction rate.
Reaction rate is how fast a chemical reaction proceeds. Temperature is the key factor affecting reaction rate. WLADIMIR BULGAR / Getty Images

The reaction rate is defined as the rate at which the reactants of a chemical reaction form the productsReaction rates are expressed as concentration per unit time.

Reaction Rate Equation

The rate of a chemical equation may be calculated using the rate equation. For a chemical reaction:

a A + b B → p P + q Q

The rate of the reaction is:

r = k(T)[A]n[B]n

k(T) is the rate constant or reaction rate coefficient.

However, this value is not technically a constant because it includes the factors that affect reaction rate (most notably temperature).

n and m are reaction orders. They equal the stoichiometric coefficient for single-step reactions, but are determined by a more complicated method for multi-step reactions.

Factors That Affect Reaction Rate

There are several factors that influence the rate of a chemical reaction:

  • Temperature: Usually this is a key factor. In more cases, raising the temperature increases the rate of a reaction because higher kinetic energy leads to more collisions between reactant particles. This, in turn, increases the chance some of the colliding particles will have sufficient activation energy to react with each other. The Arrhenius equation is used to quantify the effect of temperature on reaction rate. It is important to note some reaction rates are negatively impacted by temperature, while a few are independent of temperature.
  • The Chemical Reaction: The nature of the chemical reaction plays a large role in determining reaction rate. In particular, the complexity of the reaction and the state of matter of the reactants is important. For example, reacting a powder in a solution typically proceeds faster than reacting a large chunk of solid.
  • Concentration: Increasing the concentration of the reactants increases the rate of a chemical reaction.
  • Pressure: Increasing the pressure increases the rate of reaction.
  • Order: The reaction order determines the nature of the effect of pressure or concentration on rate.
  • Solvent: In some cases, a solvent does not participate in a reaction, but does affect its rate.
  • Light: Light or other electromagnetic radiation often speeds up reaction rate. In some cases, the energy causes more particle collisions. In others, light acts to form intermediate products that affect the reaction.
  • Catalyst: A catalyst lowers activation energy and increases reaction rate in both forward and reverse directions.

Sources

  • Connors, Kenneth (1990). Chemical Kinetics:The Study of Reaction Rates in Solution. VCH Publishers. p. 14. ISBN 978-0-471-72020-1.
  • Isaacs, Neil S. (1995). "Section 2.8.3". Physical Organic Chemistry (2nd ed.). Harlow: Addison Wesley Longman. ISBN 9780582218635.
  • IUPAC (1997) Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").
  • Laidler, K. J.; Meiser, J.H. (1982). Physical Chemistry. Benjamin/Cummings. ISBN 0-8053-5682-7.