The Steps a Reaction Really Takes
Introduced| English | Meaning |
|---|---|
| elementary step/ˌelɪˈmentəri step/ | A single molecular event. |
| reaction mechanism/rɪˈækʃn ˈmekənɪzəm/ | A proposed sequence of elementary steps explaining how reactants become products. |
| intermediate/ˌɪntəˈmiːdɪət/ | A species formed in one step of a reaction mechanism and consumed in a later step. |
The hidden choreography
- A balanced equation shows the start and the finish.
- But reactants rarely leap straight to products in one bound.
- They usually go through a hidden series of small moves.
- That secret sequence is the mechanism.
Elementary steps
- A reaction mechanism 反应机理 is the real sequence of small steps.
- Each elementary step 基元反应 is a single molecular event.
- Add up the steps and you get the overall reaction.
A two-step reaction: each step has its own barrier and the slow step sets the rate
The elementary steps of a mechanism add up to the overall reaction.
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Summing the steps (cancelling intermediates) gives the overall equation.
A single molecular event within a mechanism is an ____ step.
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Each single event is an elementary step.
Intermediates
- An intermediate 中间体 is made in one step and used up in a later one.
- It does not appear in the overall equation.
- Spot it: it shows up first as a product, then as a reactant.
An intermediate is a species that is...
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An intermediate forms then is consumed, so it cancels overall.
How many collide
- Molecularity counts how many particles meet in a single step.
- One particle is unimolecular; two is bimolecular.
- Three-particle collisions are rare, so most steps involve one or two.
Molecularity of a step
Classify each elementary step by how many particles must collide.
A single step in which two particles collide is called...
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Two colliding particles make a bimolecular step.
Steps requiring three particles to collide at once are common.
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Three-body collisions are very unlikely, so they are rare.
Steps: $A + B \to C$ (fast), then $C + B \to D$.
- $C$ is made and then consumed, so it is an intermediate.
- Overall: $A + 2B \to D$, since $C$ cancels out.
How does an intermediate differ from a catalyst?
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An intermediate appears as a product first; a catalyst appears as a reactant first.
An intermediate appears then disappears (a product of one step, a reactant of a later one) — do not confuse it with a catalyst, which appears first as a reactant then reappears. Elementary steps can use their coefficients as orders, unlike the overall reaction. And three-body collisions are so unlikely that real mechanisms avoid them.
A reaction mechanism is the sequence of elementary steps that add up to the overall reaction. An intermediate is made in one step and used up later, so it cancels from the overall equation. Molecularity counts the particles meeting in a step — almost always one or two.