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Collision theory

CollisionTheory Definition

Collisiontheory of chemical reactionsexplains the completion process of chemical reaction and its rate.According to this theory, the reactant molecules are assumed to behard sphere and reaction is postulated to occur when molecule collidewith each other. The number of collision takes place at per secondand unit volume of the reaction mixture is known as collisionfrequency (Z).

Collisiontheory of reaction ratesis based on kinetic theory of gases and has some postulates which areas below;

(1)Areaction occurs on collision of two molecules only if they possess acertain minimum amount of energy in excess of the normal energy ofthe molecules and are in proper orientation.

(2)The minimum energy which molecules must possess before collisionshould be equal to or greater than the activation energy. The minimumamount of energy which is required for the colliding molecules togive the product is called threshold energy.

(3)Thereactant molecules must pass this energy barrier for converting intothe products.

(4)The average energy is possessed by the normal reactant moleculesunder standard conditions. So activation energy Ea= Threshold energy (ET) - Average energy (ER) 

(5)Some molecules are activated molecules which have energy equal to orgreater than the threshold energy during collisions and representedby asterisk (*).

(6)The collisions between activated molecules are known as activatedcollisions or effective collisions by which the products are formed.

A+ A -> A*+ A (normal collision)

A*+ A* -> products (effective collision) 

Where A,A* are normal and activated molecule. Thus activated molecules are asmall part of total molecules so the effective collision is also asmall part of total collisions. This is the reason that all thechemical reactions do not complete in a fraction of a second. 

(7)The number of collisions per unit time Z for a reaction

A ->Bis equal to:

Z="piσ2AB.(8kt / pi µ)1/2.nA.nB

Where σABandμ is Collision diameter and reduced mass, nA& nBare the number of molecules of reactants.

Theeffective collisions fraction increases with increasing the totalcollisions. So the rate of a reaction is directly proportional to thefrequency of collision.

R="ZAB.e -Ea/RT

(8)The Arrhenius equation shows the relation between k and Z; k="A." e-Ea/RT= p.ZAB.e -Ea/RT

Where k= rate constant 

A= pZ = Frequency factor, p = probability factor, Ea= Activation energy. 

Thetemperature and the activation energy are the two factors on whichthe rate of reaction depends.