Rate of a Chemical Reaction - Chemical Kinetics | Chemistry Net

Rate of a Chemical Reaction - Chemical Kinetics

Rate of a Chemical Reaction


Rate of a Chemical Reaction - Chemical Kinetics

Chemical kinetics is the area of chemistry in which reaction rates are studied. Kinetics is largely an experimental science.

Reaction rate is called the speed at which a chemical reaction occurs.

Some chemical reactions are complete within a fraction of a second (explosions) while others take years (corrosion of metals) or even centuries (formation of mineral’s in Earth).

The basic concepts covered in this post are the following:

  • Factors that affect the rate of chemical reactions
  • Definition of the rate of reaction


What are the factors that affect the rate of chemical reactions?

Four factors allows us to change the rate at which a reaction occurs:


Physical state and nature of the reactants

Reactants in the same physical state tend to react faster since there is a greater chance for collision. Most of the reactions we consider are homogeneous involving either all gases or  all liquids

Reactants in different physical states tend to react slower since there is a smaller chance for collision

Gases and liquids react faster than solids because of the increase in surface area.

Large and complex molecules tend to react slower than small molecules since the reaction site may be hindered and therefore statistically there is smaller chance for collision at it.


Reaction temperature

Reaction rates generally increase with temperature (Fig. I.1). Increasing temperature increases the kinetic energies of molecules. The higher the temperature, the higher the kinetic energies of the molecules and the greater the number of collisions.

Fig I.1: Change of the rate of hydrolysis of ethyl acetate with temperature. As temperature increases by 10 K the reaction rate is almost doubled.


 Reactant Concentrations

The higher the concentration of reactants, the greater the chance of collision and the greater the reaction rate. For gaseous reactants, the pressure is directly related to the concentration. The greater the pressure the greater the reaction rate.



Catalysts increase reaction rates and are (theoretically) recoverable at the end of the reaction. Catalysts accomplish this by reducing the energy required for the reaction (activation energy)

Speed of a chemical reaction— reaction rate— is the change in the concentration of reactants or products per unit of time. The units for reaction rate are usually expressed as Molarity per second (M/s) —that is, the change in concentration measured in Molarity divided by a time interval measured in seconds.


Let us consider the general reaction:


A  +   2B   →   C  + 3D

The average rate of reaction can be expressed as:

Average rate of disappearance of A: - Δ[Α] / Δt

Average rate of disappearance of B: -(1/2) * Δ[B] / Δt

Average rate of appearance of C: + Δ[C] / Δt

Average rate of appearance of D: + (1/3) * Δ[D] / Δt

The first two expressions for the reactants are negative, because their concentrations will decrease with time

Where Δ[Α] Δ[B] Δ[C] and Δ[D] is the concentration change of A,B,C and D (Molarity) with time (s).

Relevant Posts - Relevant Videos



  1. David W. Oxtoby, H.P. Gillis, Alan Campion, “Principles of Modern Chemistry”, Sixth Edition, Thomson Brooks/Cole, 2008
  2. Steven S. Zumdahl, “Chemical Principles”  6th Edition, Houghton Mifflin Company, 2009
  3. Ralph H. Petrucci, “General Chemistry”, 3rd Edition, Macmillan Publishing Co., 1982

Key Terms

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