Chemguide: Support for CIE A level Chemistry


Learning outcomes 21.4(a), 21.4(b), 21.4(c) and 21.4(d)

These statements are about the degradability of polymers.

Before you go on, you should find and read the statements in your copy of the syllabus.


Statement 21.4(a)

Polyalkenes contain only carbon-carbon single bonds and carbon-hydrogen bonds, both of which are relatively unreactive. Most organisms don't have the biochemical pathways necessary to break them down either.

Polyalkenes are therefore difficult to biodegrade, and remain in the environment for a considerable time. There isn't much more you can say about that!


Statement 21.4(b)

Many common polymers can be degraded by the action of light - particularly UV light. It leads to discolouring, and breaking of the polymer chains, which causes cracking or crumbling of the material.

This is new for 2016, and it is difficult to see what CIE might expect apart from a knowledge that it can happen. The mechanisms involved are complicated, and vary from polymer to polymer.

As an example, low density polythene (LDPE), which is used in plastic bags amongst other things, becomes brittle on long exposure to UV light.

LDPE contains lots of branches off the polymer chains, and reactions involving UV light produce free radicals at these junctions. Further reactions involving oxygen can convert these into carbonyl groups (C=O). More interactions with UV can split the chains around the carbonyl groups.

This is all very complicated, and I am not the least bit confident about it! Nobody is going to expect you to learn it. Just be aware that some polymers can be degraded by light, and perhaps remember that LDPE is one example of this.


Statement 21.4(c)

I'm not certain what CIE want here. The statement talks about polyesters and polyamides being biodegradable, which implies that they can be broken down by organisms involving enzyme-controlled hydrolysis of the ester or amide links.

But the details of that are way beyond an A level chemistry course. To be on the safe side, I am giving you links to straightforward chemical reactions involved in hydrolysing these links.

You will find the hydrolysis of polyesters at the bottom of the page about polyesters. You might want to follow the link to the hydrolysis of esters to remind yourself about the basic chemistry.

You will find the hydrolysis of polyamides at the bottom of the page about polyamides. Again, you might want to follow the link to the hydrolysis of amides to remind yourself about the basic chemistry.


Statement 21.4(d)

You will find what you need on the page about protein hydrolysis.

You will find that I give a time of 24 hours for the hydrolysis; CIE's teacher support material gives 8 hours. I suggest you learn their value.

Take your time to be sure that you understand exactly what happens when the peptide bond is broken. Compare the original dipeptide with the hydrolysis products, and work out exactly what gets broken, and what gets added to each side of the broken bond.


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© Jim Clark 2011 (last modified July 2014)