Sodium cyanide (\(\text{NaCN}\)) is a white, water-soluble compound used in applications like gold mining and chemical synthesis. While classifying simple acids and bases is straightforward, determining the nature of a salt like \(\text{NaCN}\) when dissolved in water can be confusing. To understand its behavior, we must examine its chemical origin and how it interacts with water.
The Definitive Classification
Sodium cyanide is not categorized as a strong acid or a strong base; it is a salt. A salt is the product formed from the neutralization reaction between an acid and a base. When \(\text{NaCN}\) is dissolved in an aqueous solution, the resulting liquid is basic, or alkaline.
This means the solution has a \(\text{pH}\) value greater than 7. Although \(\text{NaCN}\) is a salt, its effect on the \(\text{pH}\) of water causes it to be referred to as a basic salt. The final classification depends on the chemical strengths of the acid and base that combined to create it.
Understanding Chemical Composition
The chemical formula \(\text{NaCN}\) indicates the compound is made up of a positively charged sodium ion (\(\text{Na}^+\)) and a negatively charged cyanide ion (\(\text{CN}^-\)). To determine the nature of the salt, chemists identify the parent acid and parent base that reacted to form it. Sodium cyanide is formed from the reaction between sodium hydroxide (\(\text{NaOH}\)) and hydrogen cyanide (\(\text{HCN}\)).
Sodium hydroxide (\(\text{NaOH}\)) is recognized as a strong base because it completely dissociates into ions in water. Hydrogen cyanide (\(\text{HCN}\)), however, is a weak acid, meaning it does not fully ionize in a solution. When a salt is formed from a strong base and a weak acid, the resulting aqueous solution will adopt the properties of the stronger parent, which in this case is the base. This principle explains why the \(\text{NaCN}\) solution exhibits basic characteristics.
The Role of Hydrolysis
The basic nature of the \(\text{NaCN}\) solution is due to a process called hydrolysis, which is the reaction of a salt’s ions with water. When \(\text{NaCN}\) dissolves, it separates into its constituent ions, \(\text{Na}^+\) and \(\text{CN}^-\). The sodium ion (\(\text{Na}^+\)) originates from the strong base (\(\text{NaOH}\)) and does not react with water, essentially acting as a spectator ion.
The cyanide ion (\(\text{CN}^-\)), which comes from the weak acid (\(\text{HCN}\)), is the ion that actively participates in hydrolysis. The \(\text{CN}^-\) ion acts as a weak base by attracting a hydrogen ion (\(\text{H}^+\)) from a water molecule (\(\text{H}_2\text{O}\)). This reaction forms hydrogen cyanide (\(\text{HCN}\)) and leaves behind a hydroxide ion (\(\text{OH}^-\)). The presence of these excess hydroxide ions raises the concentration of \(\text{OH}^-\) in the solution, which is the defining factor that makes the solution basic.