Textbook Problem

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The cyanide ion, CN⁻, accepts a proton from water to form HCN. Is CN⁻ a Bronsted acid or base or is it amphiprotic?

1. (a) acid
2. (b) base
3. (c) amphiprotic

Interpretation Introduction

Interpretation:

The cyanide ( $C{N}^{-}$) ion parotic nature should be identified, using the three different statements.

Concept introduction:

An acid was defined as a substance whose turns blue litmus to red, water solution tastes sour and neutralizes the bases.

A base was defined as a substance whose turns red litmus to blue, aqueous solution tasted bitter and neutralizes acids.

They are three important concepts of acids and bases.

Arrhenius Acids and Bases

Acid release hydrogen ion in water

Base release hydroxide ions in water.

$\begin{array}{l}\text{HCl(aq) }\to {\text{H}}^{\text{+}}{\text{(aq) + Cl}}^{\text{-}}\text{(aq)}\mathrm{.........}\text{Acid}\\ \text{NaOH(aq) }\to {\text{Na}}^{\text{+}}{\text{(aq)+ OH}}^{\text{-}}\text{(aq)}\mathrm{...........}\text{Base}\end{array}$

An acid is a substance that produces hydronium ions, ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ when dissolved in water.

Bronsted –Lowry definitions:

A Bronsted –Lowry acid is a proton donor, it donates a hydrogen ion, ${\text{H}}^{\text{+}}$

A Bronsted-Lowry base is a proton acceptor, it accepts a hydrogen ion ${\text{H}}^{\text{+}}$

Lewis definition:

A Lewis acid is a substance that can accept and share an electron pair.

A Lewis base is a substance that can donate and share an electron pair.

Bronsted –Lowry conjugate acid-Base pairs:

When an acid is dissolved in water, the acid $\text{(HA)}$ donates a proton to water to form a new acid (conjugate acid) and a new base (conjugate base).

$\begin{array}{l}\downarrow Conjugateacid-basepair\downarrow \\ C{H}_{3}COOH+H_{2}O\underset{}{\overset{}{\rightleftharpoons }}{H}_3{O}^{+}+C{H}_{3}CO{O}^{-}\\ WeekacidweakbaseStrongacidStrongbase\\ \uparrow Conjucageacid-basepair\uparrow \\ \end{array}$

Explanation of Solution

Reason for correct answer:

The cyanide (CN^-) ions is weak base, it can accept a hydrogen ion to form hydrogen cyanide molecule

Let us consider the acid-base reactions

$C{N}^{-}(aq)+H_{2}O(l)\underset{}{\overset{}{\rightleftharpoons }}\mathrm{H</}$