The pKa of sulfuric acid (H₂SO₄) is -10.
Understanding Sulfuric Acid's Acidity
Sulfuric acid is renowned as one of the strongest acids. The pKa value serves as a quantitative measure of an acid's strength, indicating its tendency to dissociate and release a proton (H⁺) in a solution. A remarkably low pKa value, such as -10, signifies an exceptionally strong acid that undergoes virtually complete dissociation in water, making it a very powerful proton donor.
| Acid | pKa Value |
|---|---|
| Sulfuric Acid | -10 |
Why is Sulfuric Acid So Strong?
The exceptional acidity of sulfuric acid, leading to its pKa of -10, can be attributed to a combination of structural factors that facilitate the ionization of its first proton:
- High Electronegativity of Oxygen Atoms: The oxygen atoms directly bonded to the central sulfur atom are highly electronegative. They effectively pull electron density away from the O-H bond, significantly weakening it and making it easier for the hydrogen to dissociate as a proton.
- Resonance Stabilization of the Conjugate Base: Upon losing its first proton, sulfuric acid transforms into the bisulfate ion (HSO₄⁻). This conjugate base benefits from extensive resonance stabilization, where the negative charge can be delocalized over multiple oxygen atoms. The greater the stability of the conjugate base, the stronger the parent acid. This robust stabilization makes HSO₄⁻ an extremely weak conjugate base, a hallmark of sulfuric acid's profound ability to donate its proton.
- Inductive Effect: The remaining hydroxyl (-OH) group further contributes to the inductive withdrawal of electron density, which enhances the acidity of the first proton.
Due to its extreme strength and complete dissociation in aqueous solutions, sulfuric acid is an indispensable chemical in various industrial applications, including the production of fertilizers, petroleum refining, and the synthesis of numerous chemicals.
