What is a Reaction Mechanism?
A reaction mechanism is the detailed sequence of elementary steps that describes how a chemical reaction actually proceeds at the molecular level. It explains the pathway from reactants to products, including any short-lived intermediate species.
A reaction mechanism is a series of elementary steps that combine to give the overall chemical reaction. Each step involves a specific collision between molecules, and intermediates are species formed in one step and consumed in another.
- 1↓Step 1 (Elementary)A + B → AB (bimolecular collision, rate-determining)
- 2↓Intermediate FormedAB is produced and consumed; not in overall equation
- 3↓Step 2 (Elementary)AB + C → ABC (unimolecular, fast)
- 4Overall ReactionA + B + C → ABC (sum of steps; AB cancels)
Step-by-step worked examples
The decomposition of N₂O₅ has two elementary steps. Step 1: N₂O₅ → N₂O₃ + O₂ (slow). Step 2: N₂O₃ → N₂O + O₂ (fast). What is the intermediate?
An intermediate is formed in one step and consumed in another. N₂O₃ is formed in Step 1 and consumed in Step 2. Intermediate = N₂O₃ Overall: N₂O₅ → N₂O + 2O₂ (N₂O₃ cancels)
For a two-step mechanism: (1) Cl + H₂ → HCl + H (slow) and (2) H + Cl₂ → HCl + Cl (fast), identify the rate-determining step.
The rate-determining step (RDS) is the slowest step in the mechanism. It controls the overall reaction rate. RDS = Step 1 (Cl + H₂ → HCl + H) The predicted rate law: rate = k[Cl][H₂]
If the overall reaction is 2NO + O₂ → 2NO₂ and the mechanism is (1) 2NO ⇌ N₂O₂ (fast equilibrium) and (2) N₂O₂ + O₂ → 2NO₂ (slow), write the rate law.
Step 1 is fast equilibrium: K₁ = [N₂O₂]/[NO]² Step 2 is rate-determining: rate = k₂[N₂O₂][O₂] Substitute [N₂O₂] = K₁[NO]²: rate = k₂K₁[NO]²[O₂] = kₒbₛ[NO]²[O₂]
Flashcards
Quick quiz
Q1.Which species is an intermediate in the following steps? Step 1: A + B → C. Step 2: C + D → E.
Q2.In a reaction mechanism, what does the rate-determining step determine?
Q3.For the mechanism: (1) NO + O₃ → NO₂ + O₂ (fast) and (2) NO + O₃ → NO₂ + O₂ (slow), what is the rate-determining step?
Q4.Why are reaction mechanisms important?
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Common mistakes
Confusing intermediates with reactants or products. — Correct: Intermediates are formed and consumed within the mechanism; they do not appear in the overall equation.
Thinking the slowest step always comes first. — Correct: The rate-determining step can occur at any position in the mechanism.
Assuming the mechanism must have only two steps. — Correct: Mechanisms can have any number of elementary steps.
Ignoring fast pre-equilibrium steps when deriving rate laws. — Correct: Fast equilibrium steps must be used to express intermediates in terms of reactants.
FAQ
What is the difference between a reaction equation and a mechanism?
A reaction equation shows only reactants and products. A mechanism shows the detailed sequence of elementary steps, including intermediates.
How do we determine a reaction mechanism experimentally?
By measuring the overall rate law and comparing it to rate laws predicted by candidate mechanisms. The mechanism that matches is likely correct.
Can a reaction mechanism change if we change temperature or pressure?
Generally no—the mechanism describes the molecular pathway. However, different mechanisms may dominate under extreme conditions.
What is molecularity and how does it relate to mechanisms?
Molecularity is the number of molecules involved in an elementary step. A bimolecular step involves two molecules.