Fermentation is a process used to produce ethanol from glucose. During fermentation, yeast breaks down glucose in the absence of oxygen. A student notices that the reaction vessel feels slightly warm to the touch during fermentation. The student also learns that when baking bread, the same fermentation process occurs inside the dough. Explain what the temperature change tells us about the energy change during fermentation, and describe what happens to bonds during the reaction that accounts for this overall energy change.

OCR A-Level Chemistry A (H432) — 3.2 Physical chemistry · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Fermentation can be represented by the following equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The reaction vessel feels warm to the touch during fermentation.

Model answer (5 marks)

The vessel feeling warm shows that energy is being transferred to the surroundings, so the reaction is exothermic.

During fermentation glucose is broken down. Breaking C–C and C–O bonds in glucose requires energy (endothermic). New bonds are then formed in ethanol and carbon dioxide. The formation of these bonds releases more energy than was required to break the original bonds. The excess energy is released to the surroundings, giving a net negative ΔH for the overall reaction.

Examiner tips

  • Use the word "exothermic" to link the warm vessel to energy release.
  • Show the energy flow: bond breaking (endothermic) → bond making (exothermic) → net release.
  • Mention that the net ΔH is negative to summarise the overall energy change.

Common mistakes

  • Saying the reaction is endothermic because bonds are broken, ignoring bond making.
  • Forgetting to state that the energy released in bond making exceeds that required for bond breaking.
  • Using vague terms like "heat up" without linking to exothermicity.

Mark scheme (5 marks)

  1. The reaction vessel feeling warm indicates energy is transferred to the surroundings
  2. Fermentation is an exothermic reaction
  3. Bond breaking occurs in the reactants (glucose) and requires energy / is endothermic
  4. Bond making occurs when the products form and releases energy / is exothermic
  5. The energy released in bond making is greater than the energy required for bond breaking, giving a net release of energy / overall energy change is negative

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