A cable-recycling plant receives offcuts of copper electrical cable covered with poly(chloroethene). The plastic coating protects the metal wire from water ...
Question 1 Report
A cable-recycling plant receives offcuts of copper electrical cable covered with poly(chloroethene). The plastic coating protects the metal wire from water and reduces the risk of an electric shock. Before recycling, the manager checks that the coating has not been mixed with another polymer. Fig. 1 shows part of one polymer chain. The section inside the brackets is repeated many thousands of times.
Fig. 1
The monomer chloroethene can be made by heating 1,2-dichloroethane. The reaction is:
C2H4Cl2 → C2H3Cl + HCl
Hydrogen chloride dissolves readily in water to make hydrochloric acid. The plant collects this acidic gas rather than releasing it. Burning waste polymer is also controlled carefully because harmful acidic gases can form.
(a) Give the molecular formula of chloroethene. [1]
(b) Describe the change in bonding when many chloroethene molecules form poly(chloroethene). [2]
(c) Use Fig. 1 to give the formula of one repeating unit. [1]
(d) Use C = 12, H = 1 and Cl = 35.5 to calculate the percentage by mass of chlorine in one repeating unit. [3]
(e) Describe why hydrogen chloride from the monomer process should be collected before it reaches the air. [2]
(f) Give two properties of poly(chloroethene) that make it suitable as insulation around copper cable. [2]
(g) Describe one environmental problem caused when waste poly(chloroethene) is burned without gas-cleaning equipment. [1]
Answer Details
(a) The molecular formula of chloroethene is \(\mathrm{C_2H_3Cl}\). [1]
(b) In addition polymerisation, the \(\mathrm{C=C}\) double bond opens and becomes a \(\mathrm{C-C}\) single bond. Many chloroethene monomers then join to form a long-chain polymer. [2]
(c) One repeating unit contains two carbon atoms, three hydrogen atoms and one chlorine atom, so its formula is \(\mathrm{C_2H_3Cl}\). [1]
(d) The relative formula mass of one repeating unit is:
\[M_r=(2\times12)+(3\times1)+35.5=62.5\]
\[\text{percentage chlorine}=\frac{35.5}{62.5}\times100=56.8\%\]
So the percentage by mass of chlorine is \(56.8\%\), or \(57\%\). [3]
(e) Hydrogen chloride is acidic and dissolves in water to form hydrochloric acid. Releasing it could cause acid rain and damage plants, buildings or aquatic life, so it should be collected. [2]
(f) Poly(chloroethene) is suitable for cable insulation because it is an electrical insulator and waterproof. It is also flexible and tough or durable. Any two of these properties gain credit. [2]
(g) Burning waste poly(chloroethene) without gas cleaning can release hydrogen chloride gas, causing acidic pollution or acid rain. [1]
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