Factors Influencing the Rate of Reaction

  • The rate of reaction is governed by number of factors. Those having appreciable effect are described below:

a) Nature of reactants

  • Reactions involving polar and ionic substances including the proton-transfer reactions are usually very fast.
  • On the other hand, the reactions in which bonds are rearranged, or electrons transferred, are slow.
  • Oxidation- reduction reactions, which involve transfer of electrons, are also slow as compared to the ionic reactions.
  • Substitution reactions are relatively much slower.
  • For example, the reaction between H3O+ and OH is extremely fast (k = 1011 L mol-1 s-1).
  • The fastness of the reaction may be seen from the fact that if 0.1M solutions of a strong acid and strong base are mixed, 99% of the reaction will be over within 10-6s.
  • The rate constant for the reaction

          CH3COOCH3 + H2O      ⇌ (H+)      CH3COOH + CH3OH

Which involves breaking and reformation of bonds is of the order 10-2 s-1.

b) Temperature

  • The reaction rate generally increases with a rise in temperature.
  • The effect of temperature is commonly described in terms of temperature coefficient of the reaction.
  • The temperature coefficient is defined as the ratio of the rate constants at two temperatures differing by 100C, usually from 25 to 350C.
  • The temperature coefficients for most of the reactions lie between 2 and 3, i.e., the reaction rates increase by a factor of 2 to 3, for every 10 degree rise in temperature.

c) Concentration of the reactants

  • The rate of reaction increases with an increase in the concentration of the reactants.
  • This is due to an increased probability of the molecular collisions at higher concentrations.
  • The law of mass action explains the effect of concentration on the rate of reaction.
  • According to this law, if the reaction proceeds in a single step as indicated by the balanced chemical equation, then the rate of a chemical reaction is proportional to the product of the concentrations of the reacting substances, each raised to the power equal to the stoichiometric coefficient occurring in the balanced chemical equation.

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d) Catalyst

  • Catalyst is a substance that changes the reaction rate without undergoing any permanent change in its chemical behavior.
  • It was given by Ostwald in 1895.
  • Broadly, there are two types of catalyst; positive and negative catalysts.
  • The positive catalysts increase the rate of reaction, while negative catalysts decrease the reaction rate.
  • A positive catalyst is believed to provide the reaction with an alternative pathway involving lower activation energy.
  • The lowering of activation energy by the catalyst permits more and more molecules to take part in the chemical reaction, leading to an increase in the rate of reaction.

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e) Physical state of the reactants

  • The reaction rate also depends upon the ease with which the reactants are brought in contact with each other.
  • Thus, ionic reactions are faster in the high dielectric constant solvents than in the solid state.
  • A powdered substance tends to react faster than the material in the lump form.
  • For example, coal (lump form) burns at a normal rate when ignited, but if the powdered coal is ignited, it burns in an explosive way.
  • This happens due to larger surface area of the coal in the powdered form as compared to that in the lump form.

f) Pressure

  • Pressure also has a significant effect on the rate of certain reactions.
  • Increase in pressure e.g., by hammering can cause many reactions going in an explosive way.


i) https://www.atascientific.com.au/detecting-rapid-kinetics/

ii) https://sciencing.com/what-factors-influence-the-rate-of-a-chemical-reaction-13710452.html

iii) https://alevelchemistry.co.uk/notes/rate-of-a-reaction/

iv) https://classnotes.org.in/class12/chemistry12/chemical-kinetics/factors-influencing-rates-chemical-reactions/

Factors Influencing the Rate of Reaction