GAS PHASE REACTION

The Collision Theory and Activation Energy

Collision Theory Of Bimolecular Reaction

An effective (successful) collision is a collision that occurs with enough energy

and with particles aligned correctly

In an ineffective (unsuccessful) collision, the colliding particles remain unchanged

• For the reaction A + B C, A and B must collide Assumptions• Molecules are hard spheres• For reactions to occur, molecules must collide• Reactions occur only if molecules are energetic• Collisions should transfer certain minimum

energy

Collision Theory

Reaction Rate depends on:

1) The number of collision per unit time between the reacting species.

2) The fraction of these collisions that are successful in producing a new molecule.

Factors affecting rate of reaction

1. Concentration

2. Temperature

3. Particle size

4. Nature of reactants

5. Presence of a catalyst

Effect of Concentration

• Concentration reactants increases ↑• Frequency of collision increases ↑• Frequency of effective collision increases

Collision Theory

Particle size affects rate

• The smaller the particle size the faster the rate.

The Collision Theory: why higher temperatures helps

-Reactions result when atoms/molecules collide with sufficient energy to break bonds

- Molecules at a higher temperature move faster— have a greater energy (energy distribution increases)

The Collision Theory: why higher temperatures help

Nature of reactants And catalyst

We shall consider a bimolecular reaction A + B → P

ZAB =NA NB AB2 ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 ….(1)

ZAB = collision frequency

molecular diameter

AB = 𝐴 +𝐵

AB = mean molecular diameter

MA and MB are molecular weights

Rate of reaction = no of molecules colliding per unit time × fraction of effective collision

= NA NB AB2ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT

… .(2)

but rate =kሾ𝐴ሿሾ𝐵ሿ =k NA NB …(3)

from 2 and 3

k NA NB = NA NBAB2ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT

k= AB2ට8𝜋𝑅𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT (dm3/s)

k= AB2ට8𝜋𝐾𝑁𝑇𝑀𝐴+𝑀𝐵𝑀𝐴 𝑀𝐵 e-Ea/RT (dm3mol-1/s-1)

….(K=R/N)

• During a collision, the sum of PE + KE remains constant; that is, as particles collide they slow down and KE decreases but PE increases

• Activation Energy: the minimum energy with which reactant particles must collide to produce an effective collision

The sparks generated by striking steel against a flint provide the activation energy to initiate combustion in this Bunsen burner. The blue flame sustains itself after the sparks stop because the continued combustion of the flame is now energetically favorable.

Activation Energy

Activation Energy: the minimum energy with which reactant particles must collide to produce an effective collision

Activated complex: an unstable chemical species containing partially broken and partially formed bonds

•The activation energy can be thought of as a barrier to energy transfer (from the kinetic energy to the potential energy) between reacting molecules that must be overcome.

Steric factor or orientation

• If the values of the predicted rate constants are compared with the values of known rate constants

• collision theory fails to estimate the constants correctly and the more complex the molecules are, the more it fails.

• particles have been supposed to be spherical and able to react in all directions; that is not true, as the orientation of the collisions is not always the right one.

H2O2 I-

Bad orientation: no productive reaction occurs

I-O-HOH-

If collision orientation is favorable, a reaction occurs