Unit 5 · Topic 5.7 Beta

Introduction to Reaction Mechanisms

A reaction mechanism is a sequence of elementary steps that adds up to the overall balanced equation.

Practice 1: Models and RepresentationsPractice 4: Model Analysis

Question set for this topic

Part 1 · Hook

Why this matters

A recipe on a cereal box says "flour, eggs, sugar, milk → pancakes", but nobody makes pancakes by throwing everything into the pan at once. There is an order of small steps, and some half-finished mixtures along the way. Chemical reactions are the same: the balanced equation is the recipe, and the mechanism is the step-by-step method.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is the rate law for the elementary step NO + O₃ → NO₂ + O₂?

  1. rate = k[NO][O₃]
  2. rate = k[NO₂][O₂]
  3. rate = k[NO]²
  4. It must be measured
Show the answer

For an elementary step, the coefficients are the orders.

  • Correct: rate = k[NO][O₃]:
  • rate = k[NO₂][O₂]:
  • rate = k[NO]²:
  • It must be measured:

2. Which elementary step is least likely to happen?

  1. One with four particles colliding at once
  2. One with two particles colliding
  3. One particle breaking apart
  4. One with a small activation energy
Show the answer

Four particles meeting in the same instant essentially never happens.

  • Correct: One with four particles colliding at once:
  • One with two particles colliding:
  • One particle breaking apart:
  • One with a small activation energy:

Part 4 · See it

See it first

Two elementary steps stacked and added. Step 1: two nitrogen dioxide molecules form NO3 and NO. Step 2: NO3 and carbon monoxide form nitrogen dioxide and carbon dioxide. NO3 is formed in step 1 and used in step 2, so it cancels: it is an intermediate. One NO2 on each side also cancels. The sum is the overall equation, NO2 plus CO gives NO plus CO2.
The steps of a mechanism add up to the overall equation; species made and then used cancel. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Many-particle collisions are too improbablemost reactions happen in a series of one- and two-particle elementary steps
  2. Some species are made in one step and used in a later onethey cancel when the steps are added: they are intermediates
  3. The steps must add up to the overall equationthis sum test rules out some proposed mechanisms
  4. Several mechanisms can pass the sum testa mechanism is only consistent with the evidence, never proved by it

Part 6 · Key ideas

Key ideas

  • A reaction mechanism is a series of elementary steps that adds up to the overall equation.
  • An intermediate is made in one step and used in a later step; it does not appear in the overall equation.
  • Add the steps and cancel species that appear on both sides to check a mechanism.
  • A mechanism that passes the tests is consistent with the evidence, not proven.

Part 7 · Misconception

A common mistake

The wrong idea: An intermediate is the same thing as a transition state.

What actually happens: An intermediate is a real species made in one step and used in another; it sits in a valley between steps. A transition state is the peak of a single step and cannot be isolated.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model

A proposed mechanism for making water from NO and H₂

The overall reaction 2 NO(g) + 2 H₂(g) → N₂(g) + 2 H₂O(g) is proposed to happen in three elementary steps:

  1. 2 NO → N₂O₂
  2. N₂O₂ + H₂ → N₂O + H₂O
  3. N₂O + H₂ → N₂ + H₂O

1. Which species are intermediates in this mechanism? Select all that apply.

  1. N₂O₂
  2. N₂O
  3. H₂
  4. H₂O
Show the answer

N₂O₂ forms in step 1 and is used in step 2; N₂O forms in step 2 and is used in step 3. Neither appears in the overall equation. H₂ is a reactant and H₂O a product.

  • Correct: N₂O₂: Right: made in step 1, used in step 2.
  • Correct: N₂O: Right: made in step 2, used in step 3.
  • H₂: H₂ is used in steps 2 and 3 and never made: it is a reactant.
  • H₂O: H₂O is made in steps 2 and 3 and never used: it is a product.

2. Does the mechanism add up to the overall reaction?

  1. Yes: the intermediates cancel and the sum matches
  2. No: the steps make just one H₂O in total
  3. No: N₂O₂ is left over at the end
  4. Yes, provided step 1 is written twice
Show the answer

Add the steps: 2 NO + N₂O₂ + H₂ + N₂O + H₂ → N₂O₂ + N₂O + H₂O + N₂ + H₂O. Cancel N₂O₂ and N₂O from both sides: 2 NO + 2 H₂ → N₂ + 2 H₂O.

  • Correct: Yes: the intermediates cancel and the sum matches: Right: the sum matches the overall equation.
  • No: the steps make just one H₂O in total: Steps 2 and 3 each make one H₂O, two in total.
  • No: N₂O₂ is left over at the end: N₂O₂ is made in step 1 and used in step 2, so it cancels.
  • Yes, provided step 1 is written twice: Each step is used once; writing step 1 twice would give 4 NO.

3. What is the molecularity of step 2?

  1. Unimolecular
  2. Bimolecular
  3. Termolecular
  4. Tetramolecular
Show the answer

Step 2 is one N₂O₂ colliding with one H₂: two particles.

  • Unimolecular: One particle would be unimolecular; this step has two.
  • Correct: Bimolecular: Right: two reactant particles.
  • Termolecular: Three particles would be termolecular.
  • Tetramolecular: Four particles never collide at once in an elementary step.

Model

Two mechanisms proposed for one reaction

The overall reaction is 2 O₃(g) → 3 O₂(g). Two students propose mechanisms. In each, every step occurs once each time the overall reaction occurs.

Mechanism X

  1. O₃ → O₂ + O
  2. O + O₃ → 2 O₂

Mechanism Y

  1. O₃ → O₂ + O
  2. O + O → O₂

4. Which mechanism adds up to the overall reaction?

  1. Mechanism X
  2. Mechanism Y
  3. Both mechanisms
  4. Neither mechanism
Show the answer

X: O₃ + O + O₃ → O₂ + O + 2 O₂; cancel O: 2 O₃ → 3 O₂. ✓. Y: O₃ + O + O → O₂ + O + O₂; cancel one O: O₃ + O → 2 O₂, which is not the overall equation.

  • Correct: Mechanism X: Right: X sums correctly and Y does not.
  • Mechanism Y: Y sums to O₃ + O → 2 O₂, which is not the overall reaction.
  • Both mechanisms: Y fails the sum test.
  • Neither mechanism: X passes the sum test.

5. In mechanism X, what is the oxygen atom, O?

  1. An intermediate
  2. A reactant
  3. A product
  4. A transition state
Show the answer

O is made in step 1 and used in step 2, and it does not appear in the overall equation: an intermediate.

  • Correct: An intermediate: Right: formed, then consumed.
  • A reactant: A reactant is used and not made; O is made first.
  • A product: A product is made and not used; O is used in step 2.
  • A transition state: A transition state is the top of one step's energy barrier, not a species made in one step and used in the next.

6. The mechanism (1) A + B → C; (2) C + B → D is proposed. What is the overall reaction?

  1. A + 2 B → D
  2. A + B → D
  3. A + B + C → C + D
  4. C + B → D
Show the answer

Add the steps: A + B + C + B → C + D. Cancel C: A + 2 B → D.

  • Correct: A + 2 B → D: Right: C cancels; B appears twice.
  • A + B → D: This forgets that B is used in both steps.
  • A + B + C → C + D: C must be canceled, because it appears on both sides.
  • C + B → D: That is only step 2.

Part 9 · Summary

Summary

A reaction mechanism is a sequence of elementary steps that adds up to the overall balanced equation. Intermediates are made in one step and used in a later one, so they cancel from the sum. A mechanism must sum correctly and use reasonable steps, and it is a proposed explanation, consistent with the evidence rather than proven.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections