Write the Ka and Kb reactions of NaHCO3
The correct answer and explanation is:
Sodium bicarbonate (NaHCO₃) is a weak acid and can undergo both acidic and basic reactions in water. Its acid dissociation constant (Ka) and base dissociation constant (Kb) represent the equilibrium reactions that occur when NaHCO₃ interacts with water.
- Ka (Acid Dissociation) Reaction:
Sodium bicarbonate can act as a weak acid and donate a proton (H⁺) to water. The Ka expression for this reaction is: NaHCO₃(aq)⇌H⁺(aq)+CO₃²⁻(aq)\text{NaHCO₃} (aq) \rightleftharpoons \text{H⁺} (aq) + \text{CO₃²⁻} (aq) In this reaction, sodium bicarbonate dissociates into hydrogen ions (H⁺) and carbonate ions (CO₃²⁻). The Ka represents the strength of sodium bicarbonate as an acid. Sodium bicarbonate’s Ka is relatively low because it is a weak acid, meaning the dissociation is limited. - Kb (Base Dissociation) Reaction:
Sodium bicarbonate can also act as a weak base by accepting a proton (H⁺) from water. The Kb expression for this reaction is: HCO₃⁻(aq)+H₂O(l)⇌H₂CO₃(aq)+OH⁻(aq)\text{HCO₃⁻} (aq) + \text{H₂O} (l) \rightleftharpoons \text{H₂CO₃} (aq) + \text{OH⁻} (aq) Here, the bicarbonate ion (HCO₃⁻) acts as a base by accepting a proton from water, forming carbonic acid (H₂CO₃) and hydroxide ions (OH⁻). This reaction shows that NaHCO₃ can increase the pH of a solution, acting as a weak base. The Kb value of NaHCO₃ is also relatively small, reflecting its weak basicity.
Explanation:
The Ka and Kb reactions of NaHCO₃ reflect its amphoteric nature, meaning it can act as both an acid and a base depending on the conditions. The acid dissociation (Ka) occurs when NaHCO₃ donates a proton to water, while the base dissociation (Kb) occurs when the bicarbonate ion accepts a proton. Both equilibria are crucial in understanding the buffering capacity of NaHCO₃ in aqueous solutions. The relatively low values of Ka and Kb indicate that sodium bicarbonate only weakly dissociates into ions, making it an effective buffer in maintaining pH stability in various biological and chemical systems.