- 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.
- 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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- 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.
- 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.
Factors Influencing the Rate of Reaction