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.
· 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.