But only those collisions in which the colliding species are associated with certain minimum amount of energy and collide in proper orientation result in the product formation , such collisions are called fruitful collisions or effective collision .
Here , rate = – ( dv / dt ) = collision frequency x fraction of effective collision = ZAB x f = ZAB x e -E a / RT
where , ZAB represents the collision frequency of reactants , A and B e -E a / RT represents the fraction of molecules with energies equal to or greater than
Ea .
So , to account for effective collisions , another factor , P called the probability or steric factor is introduced .
So , rate = PZABe -E a / RT
The Activated Complex Theory or Transition State Theory Reactants ⇔ Activated complex → Products
This theory is based on the fact that bond cleavage and bond formation , involved in a chemical reaction , must occur simultaneously . Hence , the reactants are not converted directly into the products . There is an energy barrier or activated complex [ intermediate product with partially formed bond ] between the reactants and products . The reactants must cross this energy barrier before converting into products . The height of the barrier determines the threshold energy .
Photochemical Reactions
Chemical reactions , that occur on exposure to visible radiation are called photochemical reactions .
1 . The rate of a photochemical reactions is affected by the the intensity of light .
2 . Temperature has little effect on photochemical reactions . Quantum yield or quantum efficiency of a photochemical reaction ,
φ = ( number of reactant molecules reacting in a given time / number of photons ( quanta ) of light absorbed ill the same time .