Question

Why is an aqueous solution of sodium carbonate alkaline ?

Hint:

A simple rule for salt hydrolysis: The "stronger" parent dictates the nature of the solution. For \(Na_2CO_3\), the parent base (\(NaOH\)) is strong and the parent acid (\(H_2CO_3\)) is weak. The strong base wins, making the solution basic (alkaline).

Answer 1

An aqueous solution of sodium carbonate (\(Na_2CO_3\)) is alkaline (basic) due to the phenomenon of salt hydrolysis. \begin{enumerate} \item Nature of the Salt: Sodium carbonate is a salt formed from the reaction of a strong base, Sodium Hydroxide (\(NaOH\)), and a weak acid, Carbonic Acid (\(H_2CO_3\)).
\item Dissociation in Water: When \(Na_2CO_3\) is dissolved in water, it completely dissociates into its constituent ions: \[ Na_2CO_3(s) \xrightarrow{H_2O} 2Na^+(aq) + CO_3^{2-}(aq) \] \item Hydrolysis of Ions: The resulting ions interact with water: \[\begin{array}{rl} \bullet & \text{The sodium ion (\(Na^+\)) is the conjugate acid of a strong base (\(NaOH\)). It is a spectator ion and does not react with water.} \\ \bullet & \text{The carbonate ion (\(CO_3^{2-}\)) is the conjugate base of a weak acid (\(H_2CO_3\)). It is strong enough to accept a proton from a water molecule, leading to hydrolysis.} \\ \end{array}\] \item Production of OH\(^-\) Ions: The reaction of the carbonate ion with water is as follows: \[ CO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HCO_3^-(aq) + OH^-(aq) \] This reaction produces an excess of hydroxide ions (\(OH^-\)) in the solution.
\item Conclusion: An increase in the concentration of hydroxide ions makes the solution alkaline, resulting in a pH greater than 7.
\end{enumerate}