What is the acid dissociation constant expression (Ka) for the reaction of H2CO3
The Correct Answer and Explanation is:
The acid dissociation constant expression (Ka) for the dissociation of carbonic acid (H2CO3) can be written as follows:
H2CO3 (aq) ⇌ H+ (aq) + HCO3- (aq)
The Ka expression is given by:
Ka = [H+][HCO3-] / [H2CO3]
Here’s a breakdown of the components:
- H2CO3 is the carbonic acid, which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).
- [H+], [HCO3-], and [H2CO3] represent the molar concentrations of the respective species in solution.
Explanation:
The acid dissociation constant (Ka) measures the strength of an acid in solution. A higher Ka value indicates a stronger acid that dissociates more easily into ions, whereas a lower Ka value indicates a weaker acid. For carbonic acid, the dissociation is represented by the equilibrium equation:
H2CO3 (aq) ⇌ H+ (aq) + HCO3- (aq)
This equilibrium shows that carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-) in water. The Ka expression quantifies this dissociation. The concentrations of the products (H+ and HCO3-) are in the numerator, while the concentration of the reactant (H2CO3) is in the denominator.
Carbonic acid is a weak acid, so the dissociation does not proceed to completion, meaning there will always be some undissociated H2CO3 molecules present in the solution. The value of Ka depends on the temperature and concentration of the acid in solution.
For carbonic acid, the first dissociation has a Ka value of approximately 4.3 × 10⁻⁷, which means it’s a weak acid. However, it’s important to note that carbonic acid undergoes two dissociation steps, so it has two different Ka values for each step:
- Ka1 ≈ 4.3 × 10⁻⁷ (first dissociation: H2CO3 to H+ and HCO3-)
- Ka2 ≈ 5.6 × 10⁻¹¹ (second dissociation: HCO3- to H+ and CO3²⁻)
The Ka values for each dissociation help determine the extent of dissociation and the pH of the solution.
