GCSE · Chemistry · Edexcel · Spec 1CH0

Polymer definition

One unit of poly(ethene) has a relative molecular mass of just 28, lighter than a carbon dioxide molecule. So how does a whole chain reach millions?

Chemistry · Polymers

One small unit. Now join 100,000 of them.

the number of repeating units: 1/5. Repeating units in the chain 10. Each unit adds 28. Chain's relative molecular mass (about) 280

Repeating units in the chain10Each unit adds28Chain's relative molecular mass (about)280

Each repeating unit of poly(ethene), C₂H₄, has a relative molecular mass of just 28 (you'll check that yourself in a minute). The bar is drawn on a ten-times scale: every step to the right means ten times as many units in the chain. Real chains often go further still, to hundreds of thousands of units.

Chemistry · Relative molecular mass

Where does the 28 come from?

Relative atomic masses: H = 1, C = 12, O = 16. Find the relative molecular mass of a water molecule, H₂O. Then do the same for one poly(ethene) repeating unit, C₂H₄.

Relative molecular mass is the sum of the relative atomic masses of all the atoms in the molecule. So count each type of atom first.
H₂O has 2 hydrogen atoms and 1 oxygen atom.
missing step

Which line is step 3?

1 + 16 = 17(2 × 1) + (1 × 16) = 182 × (1 + 16) = 34

Chemistry · Polymers

From a long chain to one tidy formula

View: The chain. Showing 1 layer: The chain

View

Explore

Try both views, then pick each part.

Look along it: the same two-carbon section, with its four hydrogen atoms, turns up again and again.

Switch between the chain and its shorthand, then pick each part to see what it means.

Chemistry · Polymers

So what's really behind the huge mass?

One repeating unit of poly(ethene) has a relative molecular mass of 28. A whole chain can run into the millions.

Which of these is closest to what you think right now?

Polymers must be made of unusually heavy atoms.Lots of small units are joined into one chain, so their masses add up. Chains differ, so we quote an average.Every chain of the same polymer has exactly the same mass.n is a small number: it's how many units are drawn in the formula.

How sure are you?

I'm sureI think soI'm guessing

Polymers must be made of unusually heavy atoms.

This is a common misconception

Look at what poly(ethene) is made of: carbon (12) and hydrogen (1), among the lightest atoms there are. Nothing heavy anywhere. The size comes from how many units are joined, not from what they are made of.

Lots of small units are joined into one chain, so their masses add up. Chains differ, so we quote an average.

This matches the evidence

Spot on, and you've spotted why the definition says AVERAGE. The chains in a sample don't all have the same number of units, so they don't all have the same mass. A polymer is a substance of high average relative molecular mass made up of small repeating units.

Every chain of the same polymer has exactly the same mass.

This is a common misconception

Tempting, but no. The number of units, n, differs from chain to chain, so some chains are longer and heavier than others. That's exactly why a polymer's relative molecular mass is given as an average.

n is a small number: it's how many units are drawn in the formula.

This is a common misconception

The formula only ever draws ONE repeating unit. n is how many are really joined in the chain, and it's large: often hundreds of thousands.

High average relative molecular mass = a huge, varying number of small repeating units.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why a chain of small repeating units ends up with such a huge relative molecular mass

What you need to know

What a polymer is, word for word, and what a monomer is
Why joining very many small units gives a high average relative molecular mass
How to work out a relative molecular mass by adding relative atomic masses
How to read and draw a repeating unit with open bonds, brackets and n

The big picture

A polymer is a substance of high average relative molecular mass made up of small repeating units. It is made by joining together very many small molecules called monomers. A single chain often contains hundreds of thousands of them, and every unit adds its own mass, which is why the total is so high. Chains in a sample have different lengths, so the value is an average. On paper, a polymer is written as one repeating unit, with open bonds on each side, inside brackets with n outside.

Key points

1A polymer is a substance of high average relative molecular mass made up of small repeating units.

2Polymers are made by joining together many small molecules called monomers. A chain often contains hundreds of thousands of them.

3Relative molecular mass is the sum of the relative atomic masses of all the atoms in a molecule.

4The mass is high because of the sheer number of units, not because the atoms are heavy. It is an average because chains differ in length.

5The repeating unit is the section that repeats along the chain. It is drawn with open bonds on each side, inside brackets, with n outside for the large number of monomer units.

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Worked example

Problem

Part of a polymer chain looks like this: –CH₂–CH(CH₃)–CH₂–CH(CH₃)– and so on. Its repeating unit is C₃H₆. One chain contains 5,000 repeating units. Estimate the relative molecular mass of that chain. (Relative atomic masses: C = 12, H = 1)

⚠ Watch out

Thinking n is a small number, or the number of units you can see drawn. The formula only ever shows one repeating unit. n stands for the number of monomer units joined in the chain: a large number that varies from chain to chain.

🧠

Memory hook

Small unit × huge n = huge mass. The brackets hold the unit; n does the counting.

✓

Check yourself

Without looking back: write down the definition of a polymer, then name the one word in it that is there because different chains have different lengths.

Flashcards

(9)

What is a polymer?

A substance of high average relative molecular mass made up of small repeating units.

What is a monomer?

One of the small molecules that join together, in very large numbers, to make a polymer.

Why is a polymer's relative molecular mass so high?

Each chain is made of a huge number of small units (often hundreds of thousands), and every unit adds its mass.

What is relative molecular mass?

The sum of the relative atomic masses of all the atoms in a molecule.

Why is a polymer's relative molecular mass an AVERAGE?

The chains in a sample contain different numbers of units, so they have different masses.

What is the repeating unit of a polymer?

The section of the polymer that repeats along the chain.

Why is a repeating unit drawn with open bonds on each side?

To show it is bonded to further repeating units on both sides: it is part of a chain.

What does the n outside the brackets stand for?

The number of monomer units joined in the chain: a large number that varies between chains.

Are polymers heavy because their atoms are heavy?

No. Poly(ethene) is only carbon and hydrogen, both light atoms. The mass comes from the number of units.

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Where does the 28 come from?

So what's really behind the huge mass?

More Edexcel GCSE Chemistry topics