Fig. 2.1 shows the apparatus a student set up to make ethanol from a warm sugar solution. The flask was left in a warm place for four days. Fig. 2.1 (a) Nam...
Question 1 Report
Fig. 2.1 shows the apparatus a student set up to make ethanol from a warm sugar solution. The flask was left in a warm place for four days.
Fig. 2.1
(a) Name the substance that must be added to solution G so that the reaction takes place. [1]
(b) The reaction works best at about 30 °C. Explain why the reaction is very slow at 5 °C and why it stops completely at 80 °C. [2]
(c) A gas bubbles through liquid L. Name this gas and state the change you would see in L. [2]
(d) Explain why air must be kept out of the flask during the reaction. [1]
(e) Write a word equation for the reaction taking place inside the flask. [1]
Answer Details
This question is about making ethanol by fermentation, the effect of temperature on the enzymes in yeast, and testing the gas given off.
(a) The substance that must be added to sugar solution G is yeast [1]. Yeast contains the enzymes that ferment the sugar.
(b) Fermentation is catalysed by enzymes, which are temperature sensitive. At 5°C the particles/enzymes have too little energy, so the reaction is very slow [1]; at 80°C the enzymes in the yeast are denatured (their shape is destroyed), so the reaction stops completely [1]. This is why around 30°C is ideal.
(c) The gas that bubbles through liquid L is carbon dioxide [1]; the limewater L turns milky / cloudy / white [1], which is the confirmatory test for carbon dioxide.
(d) Air must be kept out to stop the ethanol being oxidised (to ethanoic acid / vinegar) by oxygen in the air [1].
(e) The word equation for fermentation is:
glucose → ethanol + carbon dioxide [1].
Exam tip: "denatured" is the correct word at 80°C, not "killed" or "melted"; heat changes the enzyme's shape so it can no longer work.
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