Rates of Reaction
Physical Chemistry · 3 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 8% of your exam marks.
Factors affecting rate and collision theory are high-frequency multi-mark questions.
What a catalyst is
- A catalyst is a substance that speeds up a chemical reaction without being used up in it
- The mass of the catalyst is the same at the end of the reaction as at the start
- Only small amounts of catalyst are usually needed because each particle of catalyst can take part in many reactant turnovers
- A catalyst is neither a reactant nor a product, so its formula is written next to the arrow itself rather than inside the balanced equation
Industrial examples of catalysts
| Process | Catalyst | What it does |
|---|---|---|
| Haber process: N2 + 3 H2 ⇌ 2 NH3 | Iron | Speeds up the synthesis of ammonia |
| Contact process: 2 SO2 + O2 ⇌ 2 SO3 | Vanadium(V) oxide, V2O5 | Step in the manufacture of sulfuric acid |
| Catalytic decomposition of H2O2 | Manganese(IV) oxide, MnO2 | School demonstration of catalysis |
| Catalytic cracking of long-chain alkanes | Zeolites (aluminosilicates) | Breaks heavy fractions of crude oil into petrol-range products |
| Hardening of vegetable oils | Nickel | Adds hydrogen across C=C bonds to make margarine |
- Many catalysts are transition metals or their oxides — their variable oxidation states allow them to accept and donate electrons easily, which is what makes them catalytic
- Enzymes are biological catalysts: protein molecules that accelerate specific reactions inside cells
How a catalyst works
- A catalyst provides an alternative reaction pathway that has a lower than the uncatalysed route
- More reactant particles now have at least the (lower) activation energy at any given temperature, so a larger proportion of every collision is successful
- The catalyst is not consumed because it is regenerated at the end of the alternative pathway
Explain how a catalyst works (2-mark question)
What comes up: "Explain how a catalyst increases the rate of reaction."
Write (two marks): (1) The catalyst provides an alternative pathway (or route) for the reaction. (2) That pathway has a lower activation energy.
Watch out: general statements such as "the catalyst speeds up the reaction" or "the catalyst is not used up" — the mark scheme ignores these as they restate the question rather than explaining the mechanism. You must name the alternative pathway and the lower activation energy to score both marks.