Reaction Rates & Collision Theory
- Define reaction rate and express it in terms of a reactant or product
- Use collision theory and activation energy to explain why reactions occur
- Predict how concentration, temperature, surface area, and a catalyst change the rate
What "rate" actually measures
Kinetics asks a different question from thermodynamics: not whether a reaction happens, but how fast. The reaction rate is how quickly a reactant is consumed or a product is formed — a change in concentration per unit time, with units of M·s⁻¹. Because reactants vanish while products appear, we attach a minus sign to reactant terms so every rate comes out positive.
Collision theory: three things must go right
A reaction happens when particles collide, but not every collision reacts. Three conditions must be met: the particles must (1) collide, (2) collide with enough energy to break existing bonds — at least the activation energy, Eₐ — and (3) collide with the correct orientation. Most collisions fail at least one test, which is why reactions are far slower than the raw collision rate would suggest.
Temperature and the Maxwell–Boltzmann picture
At any temperature, molecules have a spread of kinetic energies described by the Maxwell–Boltzmann distribution. Only the molecules in the high-energy tail — those with KE ≥ Eₐ — can react. Raising the temperature shifts the whole distribution to higher energy and fattens that tail, so a much larger fraction of collisions clear the Eₐ barrier. That fraction effect, not the modest increase in collision frequency, is why a 10 °C rise can roughly double a rate.
For the reaction A → products, [A] falls from 0.100 M to 0.060 M in 20. s. Find the average rate of reaction over this interval.
- 1.Rate is defined so it comes out positive: rate = −Δ[A]/Δt.
- 2.Compute the change: Δ[A] = 0.060 − 0.100 = −0.040 M.
- 3.Divide by the time interval: rate = −(−0.040 M) / (20. s) = 0.040 / 20.
Raising the temperature of a reaction speeds it up primarily because:
Temperature does not lower Eₐ — it raises the fraction of collisions that already exceed Eₐ. Only a catalyst lowers Eₐ, by opening a new pathway. Keep those two mechanisms separate; the AP exam tests the distinction directly.
Grinding a solid reactant into a fine powder speeds up its reaction with a liquid because it:
When asked to justify a rate change, name the mechanism: more frequent collisions, a greater fraction of collisions exceeding Eₐ, or a lower Eₐ (catalyst only). Naming the factor without the mechanism rarely earns the point.
Answer the 2 checkpoints as you read.
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