A student is making bread at home. Yeast is added to the dough mixture to help it rise. The student notices that the dough rises much faster when left in a warm kitchen compared to a cold room. Explain why the dough rises faster at higher temperatures, using ideas about particles and activation energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Yeast contains enzymes that act as biological catalysts. They break down sugars and release carbon dioxide gas, which causes the dough to rise.
Model answer (5 marks)
Higher temperature gives the particles more kinetic energy.
This means the particles move faster and collide more often, so the collision frequency rises.
At a higher temperature a greater proportion of the particles have energy equal to or greater than the activation energy of the reaction.
Consequently there are more successful collisions per unit time, so the rate of the reaction increases.
The enzymes in the yeast are biological catalysts that provide an alternative reaction pathway with a lower activation energy, which further increases the rate of the reaction.
This means the particles move faster and collide more often, so the collision frequency rises.
At a higher temperature a greater proportion of the particles have energy equal to or greater than the activation energy of the reaction.
Consequently there are more successful collisions per unit time, so the rate of the reaction increases.
The enzymes in the yeast are biological catalysts that provide an alternative reaction pathway with a lower activation energy, which further increases the rate of the reaction.
Examiner tips
- Use the word "activation energy" and explain how temperature affects it.
- Show the chain: temperature → kinetic energy → collision frequency → proportion of particles > Ea → reaction rate.
- Mention the enzymes as catalysts lowering Ea.
- Keep the answer concise and use exam‑style phrasing.
Common mistakes
- Failing to mention the activation energy or the role of enzymes.
- Using vague terms like "faster" without linking to kinetic energy or collision frequency.
- Not showing the logical sequence from temperature to reaction rate.
Mark scheme (5 marks)
- Higher temperature means particles have more kinetic energy
- Particles move faster so they collide more often / collision frequency increases
- A greater proportion of particles have energy equal to or greater than the activation energy
- This means there are more successful collisions per unit time / rate of reaction increases
- The enzymes in yeast are biological catalysts that provide a different / alternative reaction pathway with a lower activation energy, increasing the rate of reaction
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Energy questions
- Cold packs are used by athletes to treat injuries. When a cold pack is activated…
- A student uses a reaction profile to compare two reactions. Reaction X has a sma…
- A student is investigating two different reactions. Reaction A is exothermic and…
- Hand warmers contain chemicals that undergo an exothermic reaction when activate…