Class 12 Chemistry Chemical Kinetics Collision theory

Collision theory

• It states that:
• According to collision theory the molecules collides with great kinetic energy in order to bring about a chemical reaction.

The molecules of the reacting species collide through the space in a rectilinear motion.

Rate of a chemical reaction is proportional to the number of collisions between the molecules of the reacting species.

The molecules must be properly oriented.

• Rate of successful collisions ∝ Fraction of successful collisions X Overall collision frequency.
• The number of collisions per second per unit volume of the molecules in a chemical reaction is called collision frequency (Z).

Let A+B --> C + D

Rate = ZABe-Ea/RT

Here ZAB = collision frequency of A and B.

• In many reactions Rate = P ZABe-Ea/RT

Where p= steric factor which takes into account the proper orientation of the molecules participating in a chemical reaction.

PROBLEM.   The activation energy for the reaction 2HI(g) → H2 + I2(g) is 209.5 kJ mol-1 at 581 K. Calculate the fraction of molecules of reactants having energy equal to or greater than activation energy?

SOLUTION.   Ea = 209.5 kJ mol - 1 = 209500 J mol - 1

T = 581 K

R = 8.314 JK - 1 mol - 1

Fraction of molecules of reactants having energy equal to or greater than activation energy is as follows:

x = e-Ea/RT

ln x = -Ea/RT

log x = Ea/2.303RT

log x = 209500 J mol-1/2.303 X 8.314 J K-1 mol-1 X 581 = 18.8323

x = antilog (18.323)

= antilog 19.1977

= 1.471 X 10-19

.