Liquid paraffin was cracked using the apparatus in Fig. 5.1 and the gas produced was collected. The student then carried out two tests on the gas, shown in ...
Question 1 Report
Liquid paraffin was cracked using the apparatus in Fig. 5.1 and the gas produced was collected. The student then carried out two tests on the gas, shown in Table 5.1.
Fig. 5.1
| test | observation | conclusion |
|---|---|---|
| add the gas to bromine water and shake | the orange colour disappears | ? |
| hold a lighted splint to the gas | ? | the gas is flammable |
(a) Name a catalyst that can be used in this experiment. [1]
(b) Complete the two empty boxes in Table 5.1. [2]
(c) Use the results in the table to explain why the gas is an alkene and not an alkane. [2]
(d) Describe the precaution the student must take at the end of the experiment to stop water being sucked back into the hot tube. [2]
(e) State the general name given to breaking down large alkane molecules in this way. [1]
Answer Details
This question tests cracking and the two standard tests on the gas produced: the bromine water test (for unsaturation) and the lighted splint test (for flammability).
(a) A catalyst for cracking is aluminium oxide (broken pot, porcelain or pumice are accepted) [1].
(b) The completed table is:
| test | observation | conclusion |
|---|---|---|
| add the gas to bromine water and shake | the orange colour disappears | the gas is unsaturated / an alkene |
| hold a lighted splint to the gas | the gas burns (with a yellow, smoky flame) | the gas is flammable |
Bromine-water conclusion: the gas is unsaturated / an alkene [1]. Lighted-splint observation: the gas burns [1]. [2]
(c) The gas is an alkene because it decolourises bromine water, which shows it is unsaturated and contains a carbon-carbon double bond [1]. An alkane is saturated, has only single bonds, and would leave the bromine water orange, so the colour change rules out an alkane [1]. [2]
(d) To stop water being sucked back into the hot tube (which could crack the hot glass), remove the delivery tube from the water [1] before the flame is removed / before heating is stopped [1]. This "suck-back" happens because the gas inside cools and contracts, lowering the pressure. [2]
(e) Breaking large alkane molecules into smaller ones in this way is called cracking [1].
Exam tip: always state the suck-back precaution as remove-tube-then-stop-heating, in that order.
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