The method and the results are on the alcohols and carboxylic acids lab. Do the analysis yourself first, then check the answers below.
Quick answer: Alcohols contain –OH and carboxylic acids contain –COOH. These answers go with the Chemistry 9-10 lab on alcohols and carboxylic acids: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
These model answers use the sample data to show the graph shape, the rate calculations and the explanation of enzyme damage. Check that you have used the correct molar volume and that the mole ratio for glucose to carbon dioxide is 1 : 2.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C7
Temperature on the x axis, volume on the y axis. The volume rises to a maximum at 40 °C and then falls sharply at 50 and 60 °C, giving a peaked curve.
Question 2 Optimum temperature
The best temperature is 40 °C. Rate = 36 cm³ ÷ 10 min = 3.6 cm³/min.
Question 3 Compare 20 and 40
Rate at 20 °C = 11 ÷ 10 = 1.1 cm³/min. Ratio = 36 ÷ 11 = 3.3 times faster.
Question 4 Explain the fall
The enzymes in yeast are proteins. At high temperature their shape changes: they are denatured and can no longer speed up fermentation, so less carbon dioxide forms.
Question 5 Moles and mass at 40 °C
CO₂ = 36 ÷ 24 000 = 0.0015 mol. Glucose gives 2 CO₂, so glucose = 0.00075 mol and mass = 0.00075 × 180 = 0.135 g. Ethanol = 0.0015 mol × 46 = 0.069 g.
Question 6 Control variables
The mass of yeast, mass of sugar and volume of water (also the shaking and the time of 10 minutes) must be the same in every flask.
Question 7 Identify the gas
The gas is carbon dioxide. Bubble it through limewater: the limewater turns cloudy (milky).
Question 8 Evaluate the method
Some gas dissolves, so the measured volumes are slightly too low. Improve by repeating each temperature three times and taking a mean, and by using a thermostatic water bath to hold the temperature steady.
