Which group in an amino acid is acidic?

The carboxylic acid group, –COOH, which donates H⁺.

Which group in an amino acid is basic?

The amino group, –NH₂, which accepts H⁺ using its nitrogen lone pair.

What is a zwitterion?

A species with a positive and a negative charge but no overall charge.

What species does an amino acid form in dilute acid?

⁺H₃N–CHR–COOH (positive ion).

What species does an amino acid form in aqueous alkali?

H₂N–CHR–COO⁻ (negative ion).

Why are amino acids solids with high melting points?

Zwitterions are held by strong ionic attractions, which need much energy to overcome.

What does amphoteric mean?

Able to react as both an acid and a base.

Equation for alanine with NaOH?

CH₃CH(NH₂)COOH + NaOH → CH₃CH(NH₂)COO⁻Na⁺ + H₂O

Amino acidsAQA A-Level Chemistry: Flashcards

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What is the general formula of a 2-amino acid?

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What is the general formula of a 2-amino acid?: H₂N–CHR–COOH
Which group in an amino acid is acidic?: The carboxylic acid group, –COOH, which donates H⁺.
Which group in an amino acid is basic?: The amino group, –NH₂, which accepts H⁺ using its nitrogen lone pair.
What is a zwitterion?: A species with a positive and a negative charge but no overall charge.
Draw glycine as a zwitterion.: ⁺H₃N–CH₂–COO⁻
What species does an amino acid form in dilute acid?: ⁺H₃N–CHR–COOH (positive ion).
What species does an amino acid form in aqueous alkali?: H₂N–CHR–COO⁻ (negative ion).
Why are amino acids solids with high melting points?: Zwitterions are held by strong ionic attractions, which need much energy to overcome.
What does amphoteric mean?: Able to react as both an acid and a base.
Equation for alanine with NaOH?: CH₃CH(NH₂)COOH + NaOH → CH₃CH(NH₂)COO⁻Na⁺ + H₂O
What is the side chain R in glycine and alanine?: Glycine: H. Alanine: CH₃.
What is the overall charge of the species in neutral solution?: Zero.

Exam questions on Amino acids

Glycine, H₂N–CH₂–COOH, is the simplest amino acid. In the solid state and in neutral aqueous solution it exists mainly as a zwitterion.
Explain how glycine forms a zwitterion.2 marks
A student dissolves alanine, CH₃CH(NH₂)COOH, in solutions of different pH: dilute hydrochloric acid, water and aqueous sodium hydroxide.
Write an equation for the reaction of alanine with aqueous sodium hydroxide, showing the structure of the organic product.2 marks
Glycine, H₂NCH₂COOH, is a white crystalline solid that melts above 200 °C. Propanoic acid, CH₃CH₂COOH, has almost the same relative molecular mass but is a liquid at room temperature.
Explain why glycine, Mr = 75.0, is a solid at room temperature but propanoic acid, Mr = 74.0, is a liquid.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).