Collision Theory — Why Heating Speeds Up Reactions

A collision leads to a reaction only if it clears three gates: enough collisions happen (frequency Z), the colliding molecules have energy ≥ Ea (the e^−Ea/RT fraction), and they approach in the correct orientation (steric factor P). If any one fails, the molecules simply bounce apart. This is captured by k = P·Z·e^(−Ea/RT).

Z is the collision frequency — how often molecules hit. e^(−Ea/RT) is the fraction of those collisions energetic enough to cross the activation energy barrier. P is the steric (orientation) factor, always much less than 1, accounting for collisions wasted on a wrong approach angle. Their product gives the rate constant k.

Even when two molecules collide with enough energy, they react only if they meet at the correct orientation. For most molecules the reactive 'window' of approach angles is narrow, so the majority of energetic collisions are still wasted on a wrong angle. That fraction of correctly oriented collisions is P, which is therefore much less than 1.

No. Ea is a fixed property of the reaction pathway — the height of the energy barrier between reactants and products. Heating does not lower Ea; it raises the fraction of molecules that already have enough energy to clear it. This is exactly the trap NEET tests: rate rises because the energetic fraction grows, not because Ea drops.