Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- An elementary reaction is one event: one particle reacts on its own, or two (rarely three) particles collide and react in that collision.
- An overall reaction is the sum of one or more elementary steps.
- A step’s rate depends on how often the right particles meet, which is proportional to the product of their concentrations.
- So, for an elementary step only, each reactant’s coefficient is its order in the rate law. Products do not appear.
- Molecularity is the number of reactant particles in the step.
- Termolecular steps are rare: three particles must meet at the same instant with the right energy and orientation.
Key relationships
| Elementary step | Molecularity | Rate law | Units of k |
|---|---|---|---|
| A → products | unimolecular | k[A] | s⁻¹ |
| A + B → products | bimolecular | k[A][B] | M⁻¹ s⁻¹ |
| 2A → products | bimolecular | k[A]² | M⁻¹ s⁻¹ |
| 2A + B → products | termolecular | k[A]²[B] | M⁻² s⁻¹ |
Testing an overall reaction:
| Experimental rate law vs rate law from coefficients | Conclusion |
|---|---|
| Different | Not a single elementary step |
| Same | One step is possible, but not proved |
| Four or more reactant particles | Cannot be one step |
Assumptions
- You have been told the step is elementary. You cannot tell from the equation alone.
- Temperature is constant, so k is constant.
- The rate law is for the forward step only.
Mistakes to avoid
- Writing a rate law from the coefficients of an overall equation.
- Calling 2A → products unimolecular. Count particles, not substances.
- Putting products in the rate law.
- Saying termolecular steps are impossible (they are rare).
- Treating a matching rate law as proof of a single step.
Quick self-check
- Write the rate law for the elementary step NO + NO₃ → 2 NO₂. (rate = k[NO][NO₃]; bimolecular)
- For the elementary step A + 2B → products, by what factor does the rate change if [B] doubles? (× 4)
- And if [A] and [B] both double? (× 8)
- What are the units of k for a termolecular step? (M⁻² s⁻¹)
- An overall reaction 2X + Y → Z has rate = k[X][Y]. Can it be one step? (No: one step would give k[X]²[Y].)
Next: practice questions.