Rates of Reaction Factors
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
What is Rate of Reaction?
The rate of reaction is how quickly reactants are converted into products.
rate of reaction = amount of reactant used (or product formed) ÷ time taken
A fast reaction (e.g. an explosion) is over in a fraction of a second; a slow reaction (e.g. rusting) can take years.
Five Factors That Change the Rate
The rate of a reaction can be increased by:
- increasing the concentration of a reactant in solution
- increasing the pressure of a reacting gas
- increasing the surface area of a solid reactant (smaller pieces)
- increasing the temperature
- adding a catalyst (including enzymes)
Why each factor speeds up a reaction is explained fully, using collision theory, in the next two topics.
Worked Example: Slowing a Reaction Down
- Question: an experiment measuring the rate of reaction between 1 g of solid X and 100 cm³ of solution Y happens too quickly to take measurements. Which change would slow the reaction down: adding a catalyst, decreasing the concentration of Y, decreasing the particle size of X, or increasing the temperature?
- Answer: decrease the concentration of solution Y - this is the only option that reduces the rate. Adding a catalyst, decreasing particle size (increasing surface area), and increasing temperature would all make the reaction faster, not slower.
Catalysts
A catalyst is a substance that increases the rate of a reaction and is chemically unchanged at the end of the reaction.
- Not used up - can be used again and again
- Only a small amount is needed
- Does not change the products formed or the overall yield
- Different reactions generally need different catalysts
- Enzymes are biological catalysts (e.g. in digestion, respiration, fermentation)
Worked Example: Defining "Catalyst"
- Question: describe what is meant by the term catalyst. [2]
- Answer: a substance that speeds up the rate of a reaction [1], without being used up (chemically unchanged at the end of the reaction) [1]
| Catalyst | Reaction | Use |
|---|---|---|
| Iron | N₂ + 3H₂ → 2NH₃ | Haber process (ammonia) |
| Vanadium(V) oxide | 2SO₂ + O₂ → 2SO₃ | Contact process (sulfuric acid) |
| Manganese(IV) oxide | 2H₂O₂ → 2H₂O + O₂ | decomposing hydrogen peroxide |
| Platinum/palladium | breaking down exhaust pollutants | catalytic converters |
Common Mistakes
- ❌ Saying a catalyst is "used up slowly"
- ✓ a catalyst is not used up at all - the same mass is present at the end as at the start
- ❌ Saying a catalyst increases the amount of product made
- ✓ a catalyst only changes how fast the reaction reaches completion, not the final yield
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