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The fermentation equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, as yeast converts sugar into ethanol and carbon dioxide. Free notes and 11 questions answered.
Fermentation and Alcohol Production is part of Reactions of alkenes and alcohols (chemistry only) in GCSE AQA Higher Chemistry.
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The fermentation equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, as yeast converts sugar into ethanol and carbon dioxide. Free notes and 11 questions answered.
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Use this lesson to stay inside Reactions of alkenes and alcohols (chemistry only) while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Fermentation and Alcohol Production before testing yourself from memory.
Picture a bottle of grape juice left open for weeks. Eventually, it starts to smell different and might even bubble slightly. What's happening? Tiny organisms called yeast are eating the sugar and producing alcohol - a process we call fermentation.
This isn't just chemistry in a lab - it's how humans have made alcoholic drinks for thousands of years. Wine, beer, and spirits all start with fermentation. The key player is yeast, a living organism that acts like a biological factory.
The fermentation process follows a specific chemical equation. When yeast encounters sugar (glucose), it breaks it down in the absence of oxygen. Here's what happens at the molecular level:
Notice that one glucose molecule produces two ethanol molecules. This is why fermentation can produce significant amounts of alcohol from sugar-rich starting materials like grapes or grain.
Yeast is a living organism, so it needs the right conditions to work effectively. Get these wrong, and your fermentation will fail or produce unwanted products. Here are the critical factors:
Temperature: Yeast works best at 25-30°C. Too cold and the enzymes work slowly. Too hot (above 40°C) and the enzymes denature, killing the yeast. This is why wine cellars are kept cool but not cold.
pH: Yeast prefers slightly acidic conditions (pH 4-6). This also prevents harmful bacteria from growing and spoiling the fermentation.
Oxygen absence: The mixture must be anaerobic. Even small amounts of oxygen will switch yeast to aerobic respiration, producing carbon dioxide and water instead of alcohol.
Sugar concentration: Too little sugar means low alcohol yield. Too much sugar can actually inhibit yeast growth through osmotic stress.
In industrial fermentation, knowing how much alcohol you can produce from a given amount of sugar is crucial for economics. Let's work through a typical calculation that appears in GCSE exams.
Worked example: Calculating Ethanol Yield from Glucose
A brewery starts with 180g of glucose. Calculate the maximum mass of ethanol that could be produced. [4 marks]
Answer: 92 g of ethanol
In reality, fermentation never reaches 100% efficiency. Industrial processes typically achieve 85-95% of the theoretical yield due to side reactions and incomplete conversion.
Fermentation isn't just about making alcoholic drinks. It's a major industrial process with both advantages and limitations that you need to understand for exam questions.
Advantages: Fermentation uses renewable resources (crops), works at relatively low temperatures, and produces a valuable product. It's also carbon-neutral since the CO₂ released was recently absorbed by the plants.
Limitations: The process is slow (days to weeks), produces dilute ethanol solutions (typically 10-15%), and competes with food production for agricultural land. The ethanol also needs expensive distillation to concentrate it.
Modern industrial fermentation uses specially selected yeast strains that can tolerate higher alcohol concentrations and work more efficiently. Some can even ferment at higher temperatures, speeding up the process.
Fermentation questions appear regularly in GCSE chemistry papers. Here's how to maximise your marks and avoid the traps that catch many students.
Common exam mistakes: Writing 'alcohol' instead of 'ethanol' (be specific), forgetting that fermentation needs the absence of oxygen, and mixing up the conditions for fermentation with those for other biological processes.
For evaluation questions, remember to discuss both advantages (renewable, low temperature, valuable product) and disadvantages (slow, dilute product, land use competition) of fermentation compared to synthetic methods of ethanol production.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The equation for fermentation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Yeast converts glucose into ethanol and carbon dioxide, so one glucose molecule produces two ethanol molecules.
Fermentation needs yeast, a sugar solution, a warm temperature of 25-30°C, slightly acidic conditions (pH 4-6) and anaerobic conditions with no oxygen. Even small amounts of oxygen make yeast switch to aerobic respiration, producing carbon dioxide and water instead of alcohol.
Ethanol is made by fermentation when yeast converts sugar into ethanol and carbon dioxide in the absence of oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. One glucose molecule produces two ethanol molecules.
Fermentation uses renewable crops, works at relatively low temperatures and produces a valuable product. However, it is slow, taking days to weeks, produces dilute ethanol of about 10-15% that needs expensive distillation, and competes with food production for land.
Fermentation is described as carbon-neutral because the CO₂ released was recently absorbed by the plants (crops) used for the sugar. It also uses renewable resources (crops) and works at relatively low temperatures.
180g of glucose can produce a maximum of 92 g of ethanol. Moles of glucose = 180 ÷ 180 = 1.0 mol; the equation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ gives 2.0 mol of ethanol, and 2.0 × 46 = 92 g.
Fermentation stops at high temperatures because the enzymes in yeast denature when it is too hot (over 40°C), killing the yeast. If it is too cold, the enzymes work slowly, so yeast works best at 25-30°C.
Yeast prefers slightly acidic conditions, pH 4-6, for fermentation. These conditions also prevent harmful bacteria from growing and spoiling the fermentation, alongside a temperature of 25-30°C and no oxygen.
Fermentation produces dilute ethanol solutions, typically 10-15%. The ethanol needs expensive distillation to concentrate it, which is one of the limitations of fermentation, along with being slow and competing for land.
Industrial fermentation typically achieves 85-95% of the theoretical yield, because of side reactions and incomplete conversion. In theory, 180g of glucose gives 92 g of ethanol, but fermentation never reaches 100% efficiency.
Too much sugar can inhibit yeast growth through osmotic stress, while too little sugar means a low alcohol yield. Sugar concentration is one of the conditions, along with temperature, pH and no oxygen, that must be right for fermentation.