Q.Explain briefly the collision theory of bimolecular reactions.

Step 1. Collision theory (Trautz 1916, Lewis 1918) explains bimolecular reactions like $A_2(g)+B_2(g)\rightarrow2AB(g)$ as occurring purely through molecular collisions -- so the rate is proportional to the number of collisions occurring per second, $\text{Collision rate}=Z[A_2][B_2]$, with Z the collision-frequency constant from the kinetic theory of gases.

Step 2. Not every collision reacts. At 298 K, 1 atm, a molecule undergoes about $10^9$ collisions per second, which if all effective would finish any reaction in about a nanosecond -- clearly most collisions fail. The requirement is a minimum energy, the activation energy $E_a$; only the fraction $f=e^{-E_a/RT}$ of collisions carry enough energy.

Step 3. Even energetic collisions can fail if the molecules are not correctly ORIENTED to form the transition state; the fraction with the right geometry is the steric factor p. …

Build up collision theory in its three logical layers -- raw collision frequency, then the energetic fraction, then the orientation fr …

• Stating that ALL collisions between reactant molecules lead to reaction -- the entire point of the theory is that only a tiny, specific …