Question

Choose the incorrect statement regarding collision theory:

• A. The minimum amount of energy the colliding molecules must possess to participate in a chemical reaction is the threshold energy.
• B. For a collision to be effective, orientation of the molecules does not matter.
• C. For a collision to be effective, kinetic energy of molecules does matter.
• D. Kinetic energy and orientation factor both play an important role for an effective collision to take place

Answer 1

The correct option is B For a collision to be effective, orientation of the molecules does not matter.

For a bimolecular elementary reaction
$A+B\to \text{Products}$
$\text{Rate}=Z_{AB}{e}^{-\frac{E_a}{RT}}...eqn\left(i\right)$
where,
$Z_{AB}$ is collision frequency of reactants, A and B
$e^{-\frac{E_a}{RT}}$ : Fraction of molecules with energies equal to or greater than activation energy.

From Arrhenius theory:
$k=A{e}^{-\frac{E_a}{RT}}...eqn\left(ii\right)$

Comparing both equations above:
Arrhenius factor (A) is related to collision frequency $Z_{AB}$
Rate expression in equation (i) predicts the value of rate constant fairly accurately for simple atoms/molecules.For complex molecules, significant deviations are observed.
Reason:
Not all collisions lead to product formation i.e. only few collisions are effective.
Effective collisions :
The collisions in which molecules collide, with sufficient kinetic energy and proper orientation, so as to facilitate breaking of bonds between reacting species and formation of new bonds to form products.
Thus an effective collision has:

1. Sufficient kinetic energy
2. Proper Orientation

Sufficient Kinetic Energy
The minimum amount of energy the colliding molecules must possess to participate in a chemical reaction is the threshold energy.
The molecule whose energy of colliding molecule is greater than or equal to the threshold energy, can cross the energy barrier .
Proper Orientation
Energy alone does not determine the effectiveness of a collision.The reacting molecules must
collide in a proper orientation to make the collision effective.
The proper orientation of reactant molecules leads to bond formation whereas improper orientation makes them simply bounce back and no products are formed
To account for proper orientation, a factor “P” is introduced which is called Probability or steric factor
Final expression of rate becomes

$\text{Rate}=P{Z}_{AB}{e}^{-\frac{E_a}{RT}}$.
Thus option (b) is a incorrect statement.