Question

Glycine hydrochloride $\mathbf{\left(}{\mathbf{\text{Cl}}}^{\mathbf{\text{-}}}{\mathbf{\text{H}}}_{\mathbf{\text{3}}}{\mathbf{\text{N}}}^{\mathbf{\text{+}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH}}\mathbf{\right)}$is a diprotic acid that contains a carboxylic acid group and an ammonium group and is therefore called an amino acid. It is often used in biochemical buffers.

(a) Which proton would you expect to dissociate at a lower pH, the proton of the carboxylic acid group or the ammonium group?

(b) Write the chemical equations describing the dissociation of the first and second protons of ${\mathbf{\text{Cl}}}^{\mathbf{\text{-}}}{\mathbf{\text{H}}}_{\mathbf{\text{3}}}{\mathbf{\text{N}}}^{\mathbf{\text{+}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH}}$.

(c) A solution containing${\mathbf{\text{0.01MCl}}}^{\mathbf{\text{-}}}{\mathbf{\text{H}}}_{\mathbf{\text{3}}}{\mathbf{\text{N}}}^{\mathbf{\text{+}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH}}$ and$\mathbf{\text{0.02M}}$ of the monodissociated species has$\mathbf{\text{pH=2.65}}$ . What is the of this dissociation?

(d) In analogy with Figure $\mathbf{\text{2-18}}$, sketch the titration curve of this diprotic acid.

Short answer

(a) Carboxylic group (COOH) lose protons more readily at lower pH.

(b) The chemical equation is :

${\text{NH}}^{\text{3}}{\text{CH}}_{\text{2}}\text{COOH}\rightleftarrows {\text{NH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{COO}}^{\text{-}}{\text{+H}}^{\text{+}}\rightleftarrows {\text{NH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{COO}}^{\text{-}}{\text{+H}}^{\text{+}}$

(c) The value of pK is $\text{2.35}$.

(d) The titration curve of this diprotic acid is –

Step-by-step solution

Definition of Concept

Diprotic acid refers to any acid with two hydrogen atoms in its molecule that can be released or ionised in water, such as sulphuric acid and carbonic acid.

Step 2:Find the proton would we expect to dissociate at a lower pH

(a)Carboxylic groups (COOH) lose protons more readily at lower pH because they are stronger acids than ammonium groups (NH).

Therefore, the carboxylic group (COOH) lose protons more readily at lower pH .

Write the chemical equations describing the dissociation of the first and second protons

(b)The chemical equations describing the dissociation of the first and second protons of ${\text{Cl}}^{\text{-}}{\text{H}}_{\text{3}}{\text{N}}^{\text{+}}{\text{CH}}_{\text{2}}\text{COOH}$is :

${\text{NH}}^{\text{3}}{\text{CH}}_{\text{2}}\text{COOH}\rightleftarrows {\text{NH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{COO}}^{\text{-}}{\text{+H}}^{\text{+}}\rightleftarrows {\text{NH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{COO}}^{\text{-}}{\text{+H}}^{\text{+}}$

Find the value of  PK

(c)Consider the given information:

$\text{pH=2.65}$

Use the Henderson-Hesselbach formula to calculate pK:

$\begin{array}{c}\text{pH=pK+log}\frac{\text{A}}{\text{HA}}\\ \text{2.65=pK+log}\frac{\text{0.02M}}{\text{0.01M}}\\ \text{pK=2.65-0.3}\\ \text{pK=2.35}\end{array}$

Therefore, the required value of pK is $\text{2.35}$.

The titration curve

The titration curve is drawn as follows –

Therefore, the curve is obtained