Consider the weak acids, HCN and H 2CO 3.
a. Which acid has the larger Ka?
b. Which acid is stronger?
c. Which acid has the stronger conjugate base?
d. Which acid has the weaker conjugate base?
e. When each acid is dissolved in water, for which acid does the equilibrium lie further to the
right?
The correct answer and explanation is :
Let’s examine the two weak acids: HCN (hydrocyanic acid) and H₂CO₃ (carbonic acid), and answer each question based on their acid dissociation constants (Ka) and the nature of their conjugate bases.
a. Which acid has the larger Ka?
The acid dissociation constant (Ka) measures the strength of an acid in solution. A larger Ka indicates a stronger acid. H₂CO₃ has a larger Ka compared to HCN. This is because carbonic acid is a diprotic acid, meaning it can donate two protons (H⁺), and its first dissociation step is relatively stronger than the dissociation of HCN. The dissociation of carbonic acid forms bicarbonate (HCO₃⁻), which is a weak base, but it still dissociates more readily than HCN, which has a smaller Ka value.
b. Which acid is stronger?
H₂CO₃ (carbonic acid) is the stronger acid. This is because it dissociates more readily in solution than HCN, as evidenced by its larger Ka value. A stronger acid has a higher Ka, indicating that it more efficiently donates protons to the solution, making H₂CO₃ a stronger acid than HCN.
c. Which acid has the stronger conjugate base?
The conjugate base of a weak acid is the species that remains after the acid donates a proton. HCN dissociates to form the cyanide ion (CN⁻), while H₂CO₃ dissociates to form bicarbonate (HCO₃⁻). The conjugate base of HCN (CN⁻) is stronger because HCN is a weaker acid. In general, the weaker the acid, the stronger its conjugate base, and since HCN is weaker, CN⁻ is a stronger base than HCO₃⁻.
d. Which acid has the weaker conjugate base?
Since H₂CO₃ is the stronger acid, it will have the weaker conjugate base. The conjugate base of H₂CO₃ is HCO₃⁻, which is weaker than CN⁻ because H₂CO₃ dissociates more readily, leaving HCO₃⁻ with a lesser tendency to accept a proton.
e. When each acid is dissolved in water, for which acid does the equilibrium lie further to the right?
The equilibrium lies further to the right for H₂CO₃. This is because H₂CO₃ has a larger Ka and is a stronger acid, meaning it dissociates more completely in water compared to HCN. As a result, the concentration of dissociated ions (H⁺ and HCO₃⁻) will be higher for carbonic acid at equilibrium, shifting the equilibrium further to the right for H₂CO₃.
In summary:
- Larger Ka: H₂CO₃
- Stronger acid: H₂CO₃
- Stronger conjugate base: CN⁻ (from HCN)
- Weaker conjugate base: HCO₃⁻ (from H₂CO₃)
- Equilibrium further to the right: H₂CO₃