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.