Amino acids (HT only) (chemistry only) - AQA GCSE Chemistry Exam Questions

Practise 14 exam-style questions on Amino acids (HT only) (chemistry only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

14 questions
1–5 marks each
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14 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Amino acids have two different functional groups in each molecule.

Name the two functional groups.

Model answer

An amine group, –NH₂, and a carboxylic acid group, –COOH.

Explanation

The name 'amino acid' tells you both groups: 'amino' for –NH₂ and 'acid' for –COOH. Having two different groups that can react with each other is what allows amino acids to join into long chains.

Mark scheme

1 mark
amine group (–NH₂)
allow amino group; allow NH₂
1 mark
carboxylic acid group (–COOH)
allow carboxyl group; allow COOH

Question 2

2 marks

Explain why glycine, H₂NCH₂COOH, can polymerise on its own but ethanoic acid, CH₃COOH, cannot.

Model answer

Glycine has two different functional groups, –NH₂ and –COOH, so the amine group of one molecule can react with the acid group of another, and each molecule can react at both ends to form a chain. Ethanoic acid has only one functional group, the –COOH group, so its molecules cannot link together into a chain.

Explanation

A monomer for condensation polymerisation needs two functional groups. Glycine has two that react with each other, so one type of monomer is enough to make a polymer.

Mark scheme

1 mark
glycine has two functional groups (–NH₂ and –COOH) that can react with each other
1 mark
ethanoic acid has only one functional group, so it cannot form a chain
allow 'it has no group for the –COOH to react with'

Question 3

3 marks

Glycine, H₂NCH₂COOH, has a relative formula mass (Mr) of 75. When glycine polymerises, one molecule of water is lost for every glycine molecule that joins the chain.

Calculate the Mr of the repeating unit of the polypeptide. Calculate the approximate Mr of a polypeptide made from 150 glycine molecules.
Relative formula mass (Mr): H₂O = 18

Model answer

Mr of repeating unit = 75 − 18 = 57
Mr of polypeptide ≈ 57 × 150 = 8550

Explanation

In condensation polymerisation the repeating unit has fewer atoms than the monomer, because water is lost when each link forms. The repeating unit of poly(glycine) is –NH–CH₂–CO–, which has an Mr of 57. Multiplying by the number of units gives the approximate Mr of the chain.

Mark scheme

1 mark
Mr of repeating unit = 75 − 18
correct answer with no working scores full marks
1 mark
= 57
1 mark
Mr of polypeptide = 57 × 150 = 8550
allow ecf; allow 8568 (including one extra water for the chain ends)

Question 4

4 marks

Glycine, H₂NCH₂COOH, forms a polypeptide. Ethanediol, HOCH₂CH₂OH, and hexanedioic acid, HOOC(CH₂)₄COOH, form a polyester.

Compare the formation of these two polymers.

Model answer

Both polymers are formed by condensation polymerisation, so a molecule of water is lost each time two monomers join. The polypeptide is made from just one type of monomer, glycine, which has two different functional groups (–NH₂ and –COOH) that react with each other. The polyester needs two different monomers, each with two of the same functional group: ethanediol has two –OH groups and hexanedioic acid has two –COOH groups. In the polypeptide the links form between –NH₂ and –COOH groups (amide links), whereas in the polyester they form between –OH and –COOH groups (ester links).

Explanation

The principle is the same for both: monomers with two functional groups react at both ends and lose water. The differences are in how many types of monomer are needed and which functional groups react.

Mark scheme

1 mark
both are formed by condensation polymerisation / water is lost in both
max 4
1 mark
the polypeptide is made from one type of monomer, whereas the polyester is made from two different monomers
1 mark
glycine has two different functional groups (–NH₂ and –COOH), whereas each polyester monomer has two of the same functional group (–OH or –COOH)
1 mark
in the polypeptide the –NH₂ reacts with –COOH (forming an amide / peptide link), whereas in the polyester –OH reacts with –COOH forming an ester link
1 mark
n water molecules are lost for n glycine molecules, whereas 2n are lost for n of each polyester monomer

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