A reaction mechanism is the step-by-step molecular story of how reactants become products. The overall chemical equation shows only the starting and ending substances, but the mechanism shows which bonds break, which bonds form, and in what order. This matters because reactions with the same overall equation can happen by different pathways and have different speeds.
Mechanisms help chemists explain observations and design better reactions in the lab and industry.
Each elementary step in a mechanism represents a single molecular event with its own transition state and activation energy. Unstable species called intermediates may form in one step and be consumed in a later step, so they do not appear in the overall equation. The slowest elementary step is often the rate-determining step because it limits how fast the whole reaction can proceed.
For many reactions, the mechanism also explains the rate law, which connects reaction speed to the concentrations of specific reactants.
Key Facts
- A reaction mechanism is a sequence of elementary steps that add up to the overall balanced equation.
- An elementary step describes one molecular event, so its rate law follows directly from its molecularity.
- For an elementary step A + B -> products, rate = k[A][B].
- For an elementary step 2A -> products, rate = k[A]^2.
- An intermediate is formed in one step and consumed in a later step, so it cancels when steps are added.
- The rate-determining step is the slowest step and usually has the largest activation energy, Ea.
Vocabulary
- Reaction mechanism
- A reaction mechanism is the detailed sequence of elementary steps that explains how reactants are converted into products.
- Elementary step
- An elementary step is a single molecular event in a mechanism, such as one collision or one bond rearrangement.
- Intermediate
- An intermediate is a short-lived species that is produced during a reaction mechanism and then consumed before the final products form.
- Transition state
- A transition state is a high-energy arrangement of atoms at the top of an energy barrier during an elementary step.
- Rate-determining step
- The rate-determining step is the slowest elementary step in a mechanism and has a major effect on the overall reaction rate.
Common Mistakes to Avoid
- Treating the overall balanced equation as the mechanism is wrong because the equation only shows reactants and products, not the actual sequence of molecular events.
- Including intermediates in the final overall equation is wrong because intermediates are made in one step and consumed in another, so they cancel out.
- Writing a rate law directly from the overall equation is wrong for most multi-step reactions because the rate law must come from experiment or from the rate-determining elementary step.
- Confusing intermediates with transition states is wrong because intermediates can exist briefly as species, while transition states are unstable energy maxima that cannot be isolated.
Practice Questions
- 1 A proposed mechanism is Step 1: NO2 + NO2 -> NO3 + NO, slow. Step 2: NO3 + CO -> NO2 + CO2, fast. Write the overall reaction and the predicted rate law.
- 2 For the elementary step 2A + B -> products, write the rate law. If [A] is doubled and [B] is tripled, by what factor does the rate change?
- 3 A reaction has two possible mechanisms. Mechanism 1 has one large activation energy barrier, while Mechanism 2 has three smaller barriers with a stable intermediate between the first and second steps. Explain how the energy diagrams would differ and how you would identify the rate-determining step.