Chemical reactions and factors affecting Rate

Chemical Reaction is a process that involves rearrangement of the molecular or ionic structure of a substance, as distinct from a change in physical form or a nuclear reaction.


Factors affecting rate are as follows:


1.      Concentration: We know from Law of mass action that Rate is proportional to concentration of reactions. So rate of reaction decreases with passage of time, since concentration of reactants decreases.


2.      Temperature: Effect of temperature on reaction rate was expressed in terms of temperature coefficient which was defined as the ratio of rate of reaction at two different temperature differing by 10% (usually these temperatures where taken as 25C and 30C)


3.      Nature of Reactants and Products

·         Physical state of reactants: Gaseous state > Liquid state > Solid state. Decreasing order of rate of reaction because collisions in homogeneous system are more effective than heterogeneous system.

·         Physical size of reactants: As we decrease the particle size rate of reaction increases since surface area increases.

·         Chemical nature of reactants: (a) If more bonds are to be broken, the rate of reaction will be slow. (b) Similarly bond strength is more, rate of reaction will be slow.


4.      Catalyst

·         Presence of positive catalyst lower down the activation energy hence increases the rate if reaction.

·         Presence of negative catalyst increases activation energy hence decreases the rate of reaction.


5.      PH of the solution

·         This reaction takes place with appreciable rate in acidic medium, but does not takes place in basic medium.


6.      Dielectric constant of the medium: More is the dielectric constant of the medium greater will be the rate of iconic reactions.


7.      Radiations/Light: Radiations are useful for photochemical reaction.


8.      Pressure: Pressure is important factor of gaseous reaction.


9.      Electrical and magnetic field: Electric and magnetic fields are rate determining factors if a reaction involves polar species.


Note: First four factors generally affect rate of almost all reactions while other factors are specific to some reactions only.

Post By : Mona Thakur 25 Feb, 2019 2144 views Chemistry