The diagram shows the cracking unit at a plastics-recycling research centre. Vapour from a long-chain hydrocarbon is passed over a hot aluminium oxide catal...
Question 1 Report
The diagram shows the cracking unit at a plastics-recycling research centre. Vapour from a long-chain hydrocarbon is passed over a hot aluminium oxide catalyst. The products are cooled and collected. One product is ethene, which can be used to make poly(ethene). Cracking changes large alkane molecules into smaller compounds, so the products are more useful as fuels or chemical feedstocks. A student records that the starting hydrocarbon has the formula C10H22 and that one product molecule is C2H4.
(a) Name the type of reaction occurring in the heated catalyst chamber. [1]
(b) State the purpose of the aluminium oxide in this reaction. [1]
(c) Complete the equation: C10H22 → C2H4 + C__H__. [2]
(d) Give one test for ethene and state the positive observation. [2]
(e) Explain why ethene is described as unsaturated. [2]
Answer Details
(a) Breaking long-chain alkane molecules into smaller molecules using heat and a catalyst is cracking. [1]
(b) Aluminium oxide is a catalyst: it increases the rate of reaction without being used up. [1]
(c) Atoms must be conserved. Removing \(\mathrm{C_2H_4}\) from \(\mathrm{C_{10}H_{22}}\) leaves eight carbon atoms and eighteen hydrogen atoms:
\[\mathrm{C_{10}H_{22}\rightarrow C_2H_4+C_8H_{18}}\]\(\mathrm{C_8}\) [1] and \(\mathrm{H_{18}}\) [1]
(d) Add or shake the product with bromine water. Ethene makes the orange-brown bromine water decolourise to colourless. [2]
(e) Ethene is unsaturated because it contains a carbon-carbon double bond. Atoms or groups can add across this bond, so it does not have the maximum possible number of hydrogen atoms. [2]
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