A student has two gas jars. One contains ethane and the other contains ethene, but the labels have come off. Fig. 4.1 shows apparatus that can be used to bu...
Question 1 Report
A student has two gas jars. One contains ethane and the other contains ethene, but the labels have come off. Fig. 4.1 shows apparatus that can be used to bubble a gas through a portion of bromine water.
Fig. 4.1
(a) Plan how the student could use bromine water to find out which gas jar contains the alkene. State the apparatus used and the observations expected. [3]
(b) State the colour change seen with the alkene. [1]
(c) Name the alkene used in this experiment. [1]
(d) Suggest why the same volume of bromine water should be used for each gas. [1]
Answer Details
This question tests how to design a fair test to identify an alkene using bromine water, and how to describe the result.
(a) A suitable plan [3]:
- Bubble the gas from each jar separately through its own portion of bromine water using the delivery tube shown [1].
- The jar whose gas turns the bromine water from orange to colourless contains the alkene [1].
- The other jar's gas (the alkane) leaves the bromine water orange, giving no change [1].
(b) With the alkene, the bromine water changes from orange to colourless (decolourised) [1].
(c) The alkene used here is ethene [1] (the alkane in the pair is ethane).
(d) The same volume of bromine water should be used for each gas so that the comparison is fair and any colour change is due only to the gas, not to different amounts of bromine water [1].
Exam tip: "decolourises bromine water" is the single observation that identifies an alkene; describe it as orange to colourless, not "clear".
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