Lesson 10 of 11 · 7 min
Collision theory
NCERT §3.5
Inside the club's flask, H₂O₂ and I⁻ ions collide billions of times every second. If every collision reacted, the flask would be finished in a flash. So why does it take 10 minutes?
The lesson in notes
In short
Collision theory (Max Trautz and William Lewis, 1916-18) treats molecules as hard spheres and says a reaction happens only when molecules collide.
Collision frequency Z counts how many collisions happen each second in a unit volume of the reacting mixture.
Only collisions carrying at least a threshold energy can succeed. Threshold energy = activation energy + energy the molecules already have.
For A + B → products, rate = Z_AB e^(−Ea/RT), where e^(−Ea/RT) is the fraction of collisions energetic enough to react. This mirrors the Arrhenius equation, with Z_AB in the role of A.
Proper orientation is also needed: for the formation of methanol from bromomethane and OH⁻, the OH⁻ must approach the carbon on the side away from bromine, or the collision fails even with enough energy.
Energy and orientation together make an effective collision. Adding a probability (steric) factor P gives rate = P Z_AB e^(−Ea/RT).
The theory's weakness is its hard-sphere picture of atoms and molecules; it ignores their structure, which is why P is needed at all.