PET is the polyester used to make transparent drinks bottles. It is made from ethane-1,2-diol (Fig. 12.1) and benzene-1,4-dicarboxylic acid (Fig. 12.2). Fig...
Question 1 Report
PET is the polyester used to make transparent drinks bottles. It is made from ethane-1,2-diol (Fig. 12.1) and benzene-1,4-dicarboxylic acid (Fig. 12.2).
Fig. 12.1:
Fig. 12.2:
(a) Name the class of compound to which each monomer belongs. [2]
(b) Draw the structure of the ester linkage formed when the -OH group of one monomer reacts with the -COOH group of the other. [3]
(c) Name the small molecule eliminated and state how many of these molecules are lost each time one ester linkage is formed. [2]
(d) Describe how a long PET molecule is built up from these two monomers. [3]
(e) When PET is hydrolysed, what are the products? [2]
(f) State two reasons why recycling PET bottles is better than sending them to landfill. [2]
Answer Details
PET is a polyester built by condensation of a diol (ethane-1,2-diol, HO-CH2-CH2-OH) and a dicarboxylic acid (benzene-1,4-dicarboxylic acid). The question tests the ester linkage, the loss of water, and how a chain grows from difunctional monomers.
(a) Class of each monomer [2]. Ethane-1,2-diol is an alcohol (specifically a diol, two -OH groups) [1]; benzene-1,4-dicarboxylic acid is a carboxylic acid (a dicarboxylic acid, two -COOH groups) [1].
(b) Ester linkage [3]. When an -OH reacts with a -COOH, the -OH of the acid and the -H of the alcohol leave as water, leaving the ester group -C(=O)-O-. The displayed linkage is:
Marks: carbon double-bonded to oxygen (C=O) [1]; that carbon single-bonded to a second oxygen (C-O) [1]; the linkage correctly joining the acid residue on one side to the alcohol residue on the other [1].
(c) Small molecule eliminated [2]. Water, H2O [1]; exactly one molecule of water is lost for every one ester linkage formed [1].
(d) Building a long PET molecule [3]. Each diol carries two -OH groups and each acid carries two -COOH groups [1]. An -OH reacts with a -COOH to form an ester linkage and release a molecule of water [1]. Because both ends of every monomer are reactive, the chain keeps growing alternately -acid-diol-acid-diol- into a long polymer [1].
(e) Hydrolysis products [2]. Hydrolysis reverses the polymerisation: water splits every ester linkage, regenerating the two starting monomers, ethane-1,2-diol [1] and benzene-1,4-dicarboxylic acid [1].
(f) Two reasons to recycle rather than landfill [2]. Any two of: it saves crude oil (the raw material for the plastic); it saves energy; it reduces the volume of non-biodegradable waste and litter in landfill; it reduces carbon dioxide emissions from making new plastic [1] [1].
Exam tip: the ester group is C=O and C-O on the SAME carbon; drawing the two oxygens on different carbons loses the linkage mark.
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