Theories of Rate of Reaction Chemistry Notes

Theories of Rate of Reaction Chemistry Notes

Theories of Reaction Rate :

There are two theories of rate of reaction :

Collision Theory of Reaction Rate :

→ It is necessary to collide reactant molecules for completion of a chemical reaction. As a result of these collisions, the bonds present in molecules are broken and new bonds are formed. This theory can be explained with the help of kinetic molecular theory. According to this theory, when reactant molecules collide with each other then reaction takes place so the rate of reaction depends on the number of collisions of reactant molecules.

→ Hence, Rate of reaction ∝ Number of collisions between reactant molecules But, reaction does not take place as a result of all collisions between reactant molecules i.e., products are not formed, the collisions by which products are formed, are called effective collisions.

→ The number of effective collisions is very less as compared to number of total collisions. Its reason is that first molecules come in activated state and only those molecules, whose energy is equal to or more than activation energy, form products by taking part in reaction.

→ Following are the three main features of this theory:

• Effective collisions between reactant molecules.
• Presence of molecules in activated state.
• Proper orientation during collision.

According to collision theory,
Rate of reaction = Z_AB e^-Ea/RT
Here,
Z_AB = Collision frequency between reactant A and B
E_a = Activation energy
R = Gas constant
T = Temperature

→ As we know that all collisions can not be converted into products. The collisions, in which molecules have enough kinetic energy (threshold energy) and proper orientation due to which it is easy to break the bonds between reactant species and to form the products by formation of new bonds in products, are called effective collisions.” Example the formation of methanol from bromomethane depends on orientation of reactants.

We used a factor P, which is called probability factor for the explanation of effective collisions. So,
Rate of reaction PZ_AB e^-Ea/RT

Transition State Theory or Theory of Absolute Reaction Rate :

→ According to this theory, when reactant molecules react with each other then these form activated complex before forming products. The energy of this complex is more than both reactants and products and these remain in transition state. Here, the rate of reaction depends on following two factors :

Concentration of activated complex.
Rate of decomposition of activated complex.

Here, reaction takes place in following steps:

Step 1. Reactants react with each other and form an intermediate complex.

Step 2. Now intermediate complex breaks in products, Since its energy is high so its stability is also less.

Chemistry Notes