What diprotic means
Two protons can be transferred in separate steps.
Diprotic does not mean the two steps have the same equilibrium constant. H₂SO₄ and HSO₄⁻ are chemically different proton donors.
Molecular acid · diprotic oxoacid
Sulfuric acid
H₂SO₄
Sulfuric acid is H₂SO₄, a diprotic oxoacid of sulfur. In water it can donate two protons stepwise. The first proton transfer is extremely favorable; the second, from hydrogen sulfate, is a distinct equilibrium and should not be treated as identical.
Concentration matters: concentrated sulfuric acid and dilute aqueous sulfuric acid have very different compositions and practical behaviour. “H₂SO₄” identifies the compound, not the concentration of a solution.
Essentials
Two acidic O–H groups make sulfuric acid diprotic.
A common structural drawing, HO–S(=O)₂–OH, makes the two O–H groups visible. It is a useful bookkeeping representation, while more advanced bonding descriptions distribute electron density beyond a single Lewis picture.
Detail
The two acid-dissociation steps are not equivalent.
In dilute water the first proton transfer is effectively complete for ordinary introductory chemistry. Hydrogen sulfate can then donate a second proton, but this second step is a reversible equilibrium.
Species in solution
A solution is not made of one formula repeated unchanged.
Depending on concentration, water activity and conditions, sulfuric-acid solutions contain water plus several protonation states and strongly solvated ions.
Understand
Mixing sulfuric acid and water is strongly exothermic.
Hydration and ion formation release substantial heat. That thermal effect is a property of the mixing and ion-solvation process, not a visual feature of the H₂SO₄ formula itself.
Safety follows chemistry
Concentration controls how much heat can be released locally.
Because the mixing is highly exothermic, dilution procedure matters in laboratory practice. This page explains the chemistry rather than serving as an operating protocol.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | H₂SO₄ |
|---|---|
| CAS Registry Number | 7664-93-9 |
| PubChem CID | 1118 |
| Standard InChI | InChI=1S/H2O4S/c1-5(2,3)4/h(H2,1,2,3,4) |
| Canonical SMILES | OS(=O)(=O)O |
Advanced
Push past the first model without losing the simple picture.
Are the S=O double bonds literal localized double bonds?
The familiar Lewis structure is useful for electron counting, but modern bonding descriptions involve substantial charge separation and delocalization. A single Lewis drawing should not be mistaken for a complete wavefunction-level description.
Why is the second proton less strongly acidic?
After the first proton is lost, the species is already negatively charged. Removing another proton creates a dianion, so electrostatic and solvation effects differ strongly from the first step.
Is concentrated sulfuric acid just 'very acidic water'?
No. At high concentration the solvent environment and speciation differ markedly from dilute aqueous acid; simple dilute-solution intuition becomes incomplete.
Constituent elements