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.

Rate/Velocity of chemical reaction

The rate of change of concentration with time of different chemical species taking part in a chemical reaction is known as rate of reaction of that species.

Rate is always defined in such a manner so that it is always a positive quantity.

Types of Rates of chemical reaction

For a reaction R à P

Average rate = Total change in concentration / Total time Taken

Instantaneous Rate: Rate of reaction at a particular instant.

Instantaneous rate can be determined by drawing a tangent at time t on curve drawn for concentration versus time.

Initial Rate: Instantaneous rate ‘t = 0’ is called initial rate [slope of tangent at t = 0]

Relation between reaction rates of different species involved is a reaction

These rates are not equal. Therefore, by convention the rate of a reaction is defined as

Note: Rate of reaction value is dependent on the stoichiometric coefficients used in the reaction while rate of any species will be fixed value under given conditions.