A2 · Practice questions · Condensation polymerisation

Lose a little, gain a chain.

Six original Cambridge-style questions on condensation polymerisation.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two reactive ends, one small molecule per link.

Diol + diacid (or diacyl dichloride): polyester, –COO–.
Diamine + diacid (or diacyl dichloride): polyamide, –CONH–.
Lost: H₂O from COOH, HCl from COCl.
Repeat unit M_r = monomers − small molecules lost.

Recall

[3 marks]

PET is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. (a) Name the type of polymerisation. (b) Name the linkage formed. (c) Draw the repeat unit.

(a) Condensation polymerisation. ✓
(b) Ester linkage. ✓
(c) –O–CH₂CH₂–O–CO–C₆H₄–CO–, with open bonds at both ends. ✓

Recall

[2 marks]

Write the structure of the repeat unit of nylon-6,6 and state the small molecule lost when it is made from hexanedioyl dichloride.

–NH(CH₂)₆NHCO(CH₂)₄CO–. ✓
Hydrogen chloride, HCl. ✓

Calculation

[2 marks]

Calculate the M_r of the repeat unit of Kevlar, C₁₄H₁₀N₂O₂, and the number of repeat units in a chain of M_r 47 600.

M_r = 14(12.0) + 10(1.0) + 2(14.0) + 2(16.0) = 238. ✓
47 600 ÷ 238 = 200 repeat units. ✓

Calculation

[2 marks]

900 g of lactic acid (M_r 90.0) is fully polymerised to poly(lactic acid). Calculate the mass of polymer and the mass of water formed.

10.0 mol of lactic acid gives 10.0 mol of repeat units, C₃H₄O₂, M_r 72.0: 720 g of polymer. ✓
Water = 900 − 720 = 180 g (10.0 mol, one per link). ✓

Explain

[3 marks]

Explain why proteins are described as polyamides.

Proteins are made from amino acids, each containing an NH₂ group and a COOH group. ✓
The NH₂ of one amino acid reacts with the COOH of the next, losing water. ✓
This forms an amide (peptide) link, –CONH–, repeated along the chain, so a protein is a condensation polyamide. ✓

Explain

[2 marks]

Explain why ethanol and ethanoic acid cannot form a polymer, but ethane-1,2-diol and hexanedioic acid can.

Ethanol and ethanoic acid each have only one reactive group, so once the ester forms there is no free group left to react. ✓
The diol and diacid each have two reactive groups, so ester links form at both ends and the chain keeps growing. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.