Long-chain alkanes from crude oil can be broken down in the laboratory. Fig. 6.1 shows apparatus a student uses to do this. Fig. 6.1 (a) Name the process sh...
Question 1 Report
Long-chain alkanes from crude oil can be broken down in the laboratory. Fig. 6.1 shows apparatus a student uses to do this.
Fig. 6.1
(a) Name the process shown in Fig. 6.1 and state what happens to the large hydrocarbon molecules during it. [2]
(b) State two conditions needed for this process to take place. [2]
(c) State the role of the broken pottery in the boiling tube. [1]
(d) One large alkane breaks down as shown. Complete the equation by giving the formula of the missing product:
C10H22 → C8H18 + ......... [2]
(e) Explain why this process is carried out on a large scale in the petrochemical industry. [2]
(f) The gas collected includes an alkene. Describe a test, with its result, that would show an alkene is present. [2]
Answer Details
What this tests: cracking long-chain alkanes, the conditions used, and testing for an alkene.
(a) [2] The process is cracking [1]. During it the large, long-chain hydrocarbon molecules are broken down into smaller molecules [1].
(b) Two conditions [2] Any two of: a high temperature (strong heat); a catalyst (broken pottery, aluminium oxide or silica); the vapour passed over the hot catalyst [1 + 1].
(c) Role of the broken pottery [1] It acts as a catalyst, speeding up the breakdown so it happens at a lower temperature; it is not used up.
(d) Missing product [2] Balance each element in \(C_{10}H_{22} \rightarrow C_8H_{18} + ?\): carbon \(10 - 8 = 2\), hydrogen \(22 - 18 = 4\), so the missing product is C2H4 (ethene).
\[ C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4 \]1 mark for the carbon balance (C2), 1 mark for the hydrogen balance (H4).
(e) Why cracking matters [2] It converts less useful long-chain alkanes into smaller molecules in high demand as fuels such as petrol [1]; and it makes alkenes such as ethene that are needed to manufacture plastics and other chemicals [1].
(f) Test for an alkene [2] Add bromine water and shake [1]; with an alkene the orange bromine water is decolourised (turns colourless) [1]. An alkane would leave it orange.
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