Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A catalyst increases the rate of a reaction and is not used up overall.
- It works by increasing the number of effective collisions: it opens a new path with a lower activation energy, and/or it helps reactants meet with a better orientation.
- The catalysed reaction has a different mechanism: new elementary steps and new intermediates.
- In the mechanism the catalyst is used up in one step and made again later. It is often used up in the rate-determining step, so it can appear in the rate law.
- Three types: binding catalysts such as enzymes (catalyst–reactant complex), acid–base catalysis (a proton is gained or lost), surface catalysis (reactants bond to a solid surface).
Key relationships
| Idea | What it means | Use it to |
|---|---|---|
| Catalyst vs intermediate | Catalyst: reactant first, product later. Intermediate: product first, reactant later | label species in a mechanism |
| Rate law from the slow step | If the catalyst is in the slow (first) step, rate ∝ [catalyst] | predict the effect of adding more catalyst |
| Highest point of the path | Overall barrier = highest transition state − starting energy | compare catalysed and uncatalysed barriers, both directions |
| ΔH = E(products) − E(reactants) | Same for both paths | show the catalyst does not change ΔH |
Assumptions behind the reasoning
- The temperature is the same with and without the catalyst, so the spread of collision energies is the same.
- The proposed mechanism is consistent with the experimental rate law; a mechanism can be supported by data but never proved.
- For a surface catalyst, only atoms on the surface can take part.
Mistakes to avoid
- “Not used in the reaction.” It is used, then regenerated.
- “Gives particles more energy.” It lowers the threshold; the energy spread is unchanged.
- “Changes ΔH.” Reactants and products keep the same energies.
- “Forward only.” The reverse barrier drops by the same amount.
- Swapping catalyst and intermediate. Check which appears first as a reactant.
- Drawing one lower hump. A catalysed path usually has extra humps and a dip for the new intermediate.
Quick self-check
- In the steps Q + R → QR, then QR + T → V + R, which species is the catalyst? (R: used in step 1, made again in step 2. QR is the intermediate.)
- A reaction has Ea = 90 kJ mol⁻¹ and ΔH = −25 kJ mol⁻¹. A catalyst lowers the highest barrier to 55 kJ mol⁻¹. What is the reverse barrier with the catalyst? (55 + 25 = 80 kJ mol⁻¹, down from 115 kJ mol⁻¹.)
- A catalyst is consumed in the slow first step. What happens to the rate if you triple its concentration? (The rate triples, if the step is first order in the catalyst.)
- Why is powdered nickel a better hydrogenation catalyst than a nickel bar of the same mass? (More surface atoms, so more sites where H₂ and the alkene can bind.)
Next: practice questions.