Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Most reactions do not happen in one collision. They happen as a reaction mechanism: a series of elementary steps, each one a single molecular event.
- The coefficients of an elementary step tell you how many particles react (its molecularity: uni-, bi- or, rarely, termolecular). The coefficients of the overall equation do not.
- When you add the steps and cancel species on both sides, you must get the overall balanced equation.
- An intermediate is present only while the reaction is running: it starts at zero, builds up a little and returns to zero.
- A mechanism is a model. Detecting an intermediate that only one proposed mechanism predicts is evidence for that mechanism.
Key relationships
| Species | Pattern in the steps | In the overall equation? |
|---|---|---|
| Reactant | Used, not made back | Yes, left side |
| Product | Made, not used again | Yes, right side |
| Intermediate | Made in an early step, used in a later step | No |
| Catalyst | Used in an early step, made back in a later step | No |
Assumptions behind the model
- Each step is a genuine elementary event, so its coefficients show the particles that collide.
- Steps involving three or more particles colliding at once are treated as unlikely.
- The mechanism describes the main route; it is a proposal supported by evidence, not a certainty.
Mistakes to avoid
- Treating the overall equation as one step. Its coefficients summarise; they do not describe a collision.
- Mixing up intermediates and catalysts. Check the order: made-then-used, or used-then-given-back.
- Forgetting a multiplier. If an intermediate is made twice but used once, the later step happens twice.
- Cancelling unequal amounts. Only cancel what is equal on both sides.
- Calling a transition state an intermediate. A transition state cannot be isolated.
- Thinking “adds up” means “proven”. Several mechanisms can add up to the same equation.
Quick self-check
- Steps: A₂ + C → A₂C, then A₂C + C → A₂ + C₂. What is the overall equation? (2C → C₂)
- In question 1, which species is the intermediate and which is the catalyst? (A₂C is the intermediate; A₂ is the catalyst)
- Mechanism X predicts intermediate M; mechanism Y does not. M is detected. What can you conclude? (The evidence supports X over Y, but does not prove X.)
Next: practice questions.