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What is Collision Theory?

Collision theory states that reactions occur when molecules collide with sufficient kinetic energy and proper orientation. Only a fraction of collisions (those with E ≥ Ea) lead to reaction; the collision frequency and fraction with proper energy together determine the reaction rate.

Short answer

Collision theory: molecules must collide with kinetic energy ≥ activation energy (Ea) and correct orientation to react. Rate = (collision frequency) × (orientation factor) × (fraction with E ≥ Ea).

Energy distribution of molecules and activation energy

x: Kinetic energy · y: Number of moleculesAt low TAt high T (Ea threshold shown)

Try it: interactive calculator

Reaction rate constant k

17,198.17s^-1

= 10,000,000,000,000 * exp(-50 / (8.314 * 298 / 1000))

Pre-exponential factor A10,000,000,000,000 s^-1Activation energy50 kJ/molTemperature298 KGas constant (fixed)8.314 J/(mol·K)

Step-by-step worked examples

A reaction has Ea = 50 kJ/mol and A = 1e13 s^-1. At 298 K, what is k? (Assume Arrhenius equation)

k = A × e^(-Ea/RT)
k = 1e13 × e^(-50000/(8.314×298))
k = 1e13 × e^(-20.16)
k = 1e13 × 1.97e-9 ≈ 20000 s^-1

At what temperature will k double if Ea = 60 kJ/mol and A stays constant?

ln(k2/k1) = Ea/R × (1/T1 - 1/T2)
ln(2) ≈ 60000/8.314 × (1/298 - 1/T2)
0.693 = 7,217 × (0.00336 - 1/T2)
Solving: T2 ≈ 310 K (rough ~37°C rise)

A collision has frequency Z = 1e34 collisions/s and only 0.01% have E ≥ Ea. How many react per second?

Reacting collisions = Z × fraction with E ≥ Ea
= 1e34 × 0.0001
= 1e30 collisions/s → reaction rate per mole ≈ 1e30/6.022e23 ≈ 1,660 mol/L·s

Flashcards

Quick quiz

Q1.A collision has high kinetic energy but wrong orientation. Will it react?

Correct answer: B. Both energy AND orientation are needed; collision theory requires both.

Q2.Activation energy Ea represents…

Correct answer: B. Ea is the energy threshold; only collisions with E ≥ Ea can react.

Q3.If Ea is very large, the reaction is…

Correct answer: B. Large Ea means few molecules have enough energy; few collisions succeed.

Q4.A 10°C rise typically increases reaction rate by…

Correct answer: A. Rule of thumb: ~2–3× rate increase per 10 K (depends on Ea and T).

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Common mistakes

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All molecular collisions lead to reaction. — Correct: Only collisions with E ≥ Ea and proper orientation react.

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Activation energy is the reaction's total energy change. — Correct: Ea is the barrier height, independent of ΔE.

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Orientation doesn't matter if energy is high enough. — Correct: Collision theory requires both sufficient energy AND correct orientation.

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Collision frequency depends only on concentration. — Correct: Frequency depends on concentration, temperature, and molecular size.

FAQ

How does collision theory explain reaction rates?

Rate is the product of collision frequency, orientation factor, and the fraction of collisions with E ≥ Ea.

Why is activation energy important?

It determines how sensitive a reaction's rate is to temperature changes; high Ea means rate is very temperature-dependent.

How do catalysts fit into collision theory?

Catalysts lower Ea by providing an alternative pathway; more collisions exceed the lower Ea, speeding up reaction without changing ΔE.

What is the steric factor?

The fraction of collisions with correct orientation (for bonds to break and form); most collisions fail this test.