Sodium cyanide (\(\text{NaCN}\)) is a base when dissolved in water. This chemical behavior is a direct consequence of how the substance interacts with water molecules. \(\text{NaCN}\) is an ionic salt. The solution’s basic nature is determined by the relative strengths of the acid and base that originally formed the salt.
Ionic Components: Dissociation in Water
When solid \(\text{NaCN}\) is placed in water, it dissolves completely through dissociation. The compound separates into its constituent ions: the sodium cation (\(\text{Na}^+\)) and the cyanide anion (\(\text{CN}^-\)). The reaction is represented as \(\text{NaCN} \rightarrow \text{Na}^+ + \text{CN}^-\).
The sodium ion (\(\text{Na}^+\)) originates from the strong base sodium hydroxide (\(\text{NaOH}\)). Since \(\text{NaOH}\) fully dissociates, the \(\text{Na}^+\) ion does not react with water molecules. It is classified as a spectator ion and does not participate in determining the solution’s \(\text{pH}\).
The cyanide ion (\(\text{CN}^-\)) is derived from hydrocyanic acid (\(\text{HCN}\)), which is a weak acid. Unlike the sodium ion, \(\text{CN}^-\) is chemically active and reacts with the water solvent. This difference in reactivity makes the \(\text{NaCN}\) solution basic.
Understanding Basicity: The Conjugate Base Principle
The cyanide ion acts as a base according to the Brønsted-Lowry theory, which defines a base as a species capable of accepting a proton (\(\text{H}^+\)). The cyanide ion (\(\text{CN}^-\)) is the conjugate base of hydrocyanic acid (\(\text{HCN}\)).
\(\text{HCN}\) is classified as a weak acid because it only partially dissociates in water. The principle of conjugate pairs states that a weak acid must have a relatively strong conjugate base. Therefore, the cyanide ion (\(\text{CN}^-\)) is a relatively strong base.
This strength drives the chemical action in the water. The \(\text{CN}^-\) ion has a significant affinity for protons and accepts a proton from a surrounding water molecule (\(\text{H}_2\text{O}\)). This proton-transfer process increases the solution’s alkalinity.
The Resulting Solution: Hydroxide Ion Production
The interaction between the cyanide ion and water is called hydrolysis. The cyanide ion abstracts a proton from a water molecule, reforming the weak acid \(\text{HCN}\). This process leaves behind a hydroxide ion (\(\text{OH}^-\)).
The chemical equation for this equilibrium reaction is: \(\text{CN}^- (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{HCN} (aq) + \text{OH}^- (aq)\). The production of hydroxide ions is the defining characteristic of a basic solution. This excess of \(\text{OH}^-\) ions pushes the solution’s \(\text{pH}\) above 7.
Because hydrolysis is an equilibrium reaction where only a fraction of cyanide ions react, sodium cyanide is classified as a weak base. The resulting \(\text{pH}\) is lower than that of a strong base solution, such as \(\text{NaOH}\), at the same concentration.