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Proton Transfer Reactions · Paper 2 — Free Chemistry SL Practice Questions

1ChallengeSAQ-LAcid-base reactions: Bronsted-Lowry theory9 marksPaper 2~14 min

Glycine (

\text{NH}_2\text{COOH}

) is the simplest amino acid. In aqueous solution, glycine exists a zwitterion:

^+\text{NH}_3\text{CH}_2\text{COO}^-

. A chemist titrates

25.0\,\text{cm}^3

of a

0.200\,\text{mol}\,\text{dm}^{-3}

solution of glycine hydrochloride (

^+\text{NH}_3\text{CH}_2\text{COOH}\cdot\text{Cl}^-

, the fully protonated form) with

0.200\,\text{mol}\,\text{dm}^{-3}

NaOH.

pK_{a1}\ (\text{carboxyl group, } {-}\text{COOH}) = 2.34

pK_{a2}\ (\text{ammonium group, } {-}^+\text{NH}_3) = 9.60

K_w = 1.0 \times 10^{-14}\ \text{at } 298\,\text{K}

(a)

Calculate the pH at the first equivalence point, where the zwitterion

^+\text{NH}_3\text{CH}_2\text{COO}^-

is the predominant species. [3 marks]

(b)

At a point during the titration, the pH equals

9.60

. Determine the ratio

\dfrac{[\text{NH}_2\text{COO}^-]}{[^+\text{NH}_3\text{CH}_2\text{COO}^-]}

and identify which glycine species is acting as the Brønsted–Lowry acid at this pH. [2 marks]

(c)

Calculate

K_{a2}

for the zwitterion acting as an acid and

K_b

for the zwitterion acting as a base. Hence evaluate whether the isoelectric point of glycine lies above, below, or at pH

7.00

, justifying your answer by comparing these two equilibrium constants. [4 marks]

Solutions

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