Remember 9.1 Practical Skills for Chemistry: Practical Skills
This experiment is fairly simple and safe. It cannot be done in one period but if possible the experiment should be conducted so that measurements can be taken at hourly intervals.
- Place 100 mL of water in a 200-250 mL conical flask and place a teaspoon of yeast and 0.1 mol of glucose (to make calculations easier).
- Place the conical flask over a Bunsen flame so it reaches slightly above lukewarm temperatures.
- Attach a balloon so that it seals the mouth of the flask, wool and stoppers can also work and allows mass variance to be calculated over time.
- Weigh and record results.
- Place the flask in a warm place such as a window sill or near (but not close) to a heater.
- Weigh and record results at hourly intervals, until constant mass (or eight hours for the balloon).
Testing for presence of carbon dioxide:
Run a rubber stopper connected with tubing to a flask containing limewater, the water will turn cloudy in presence of carbon dioxide. Remembering that Ca(OH)2(aq) + H2CO3(aq) → 2H2O(l) + CaCO3(s)
The loss of mass should be equal to the carbon dioxide produced. The solution will be a mixture of water, very dilute ethanol and sugar (also a slightly acidic broth of H2CO3 near gas bubbles). Monitoring the mass changes at regular intervals will show the conversion and conversion rates of the sugar to ethanol. It can be noted that less and less CO2(g) is produced over time.
Slight changes in the mass that do not conform to the experimental values as compared to theoretical can be due to inaccuracy of the measurement equipment, human error, gas escaping, hydration of the solution, etc. To reduce error an accurate electronic scale should be used as well as accurate measurement of the glucose and water. The rate of conversion slows down as yeast die when the product (ethanol) exceeds yeast tolerance levels. In order to continue producing more ethanol the products (ethanol and CO2) must be removed and sugar added.
C6H12O6(aq) → 2CH3CH2OH(aq) + 2CO2(g)