List the 2 pKa’s for H2SO4
The Correct Answer and Explanation is:
The two pKa values of sulfuric acid (H₂SO₄) are:
- First pKa ≈ –3
- Second pKa ≈ 1.99
Explanation:
Sulfuric acid (H₂SO₄) is a diprotic acid, meaning it can donate two protons (H⁺) in aqueous solution. Each of these proton donations is associated with a specific acid dissociation constant (Ka), which is commonly expressed as pKa, the negative logarithm of Ka. The lower the pKa value, the stronger the acid.
First Dissociation:
H2SO4→H++HSO4−\text{H}_2\text{SO}_4 \rightarrow \text{H}^+ + \text{HSO}_4^-H2SO4→H++HSO4−
This step is a strong acid dissociation, meaning it occurs almost completely in water. Because of this, the first pKa is very low—approximately –3. In aqueous solution, virtually all H₂SO₄ molecules dissociate to form hydrogen ions (H⁺) and hydrogen sulfate ions (HSO₄⁻), making sulfuric acid one of the strongest known mineral acids.
Second Dissociation:
HSO4−⇌H++SO42−\text{HSO}_4^- \rightleftharpoons \text{H}^+ + \text{SO}_4^{2-}HSO4−⇌H++SO42−
This is a weaker acid dissociation compared to the first step. The hydrogen sulfate ion (HSO₄⁻) is amphiprotic—it can act as both an acid and a base. The equilibrium for this second dissociation lies less completely toward the right, with a pKa of approximately 1.99. This value indicates that HSO₄⁻ is still a relatively strong acid, though significantly weaker than H₂SO₄.
Understanding the two pKa values is critical in acid–base chemistry, especially in titration curves, buffer systems, and industrial applications where sulfuric acid is used. In many practical contexts, only the first dissociation is considered when dealing with strong acids, because it is complete. However, in reactions where the pH is closer to neutral or in more dilute solutions, the second dissociation step becomes relevant.
