How does H2SO4 dissociate in water?

Learn everything about the H₂SO₄ dissociation, how it happens, the hidden chemistry behind it, and its diverse applications.

Introduction：

Goes by the name oil of vitriol, sulfuric acid or H₂SO₄ is unarguably one of the most commonly known chemicals that have significant importance in our lives. It is a diprotic acid which means that it can give away two H⁺ ions per molecule when it is converted into its ions. It is characterized as a strong mineral acid (has a very high Ka value) and exists as colorless, odorless, and viscous when in its pure form. Its strong acidic nature comes in handy for today’s countless industrial processes and poses a very significant role due to its versatile chemical properties.

Like any other acid, sulfuric acid is also diluted with water to be used in different concentrations for various suitable applications. When this dilution is carried out, the ions of this acid are generated in two steps process. Below is a complete process of H₂SO₄ dissociation:

As mentioned before, sulfuric acid is a diprotic acid containing two hydrogen atoms and can donate two protons when in an ionic state. It achieves its ionic state when it is introduced in water where interactions with water molecules break its molecule. This breaking happens in two stages. In the first stage, the H⁺ ions depart the sulfuric acid molecule combine with a water molecule and resulting in a hydronium ion and hydrogen sulfate ions. The first step of this reaction is stated as:

H₂SO₄   ⇌  H⁺ + HSO₄⁻ 

Note that one of the protons from sulfuric acid is shifted to hydrogen sulfate. Also, they combine with water to form hydronium ions H₃O⁺ (mentioned below). In the second step, this hydrogen sulfate also decays into its ions against water giving away its proton and remaining as sulfate ions. This step can be stated as follows:

HSO₄⁻  ⇌  H⁺ + SO₄²⁻ 

The overall reaction for the H₂SO₄ dissociation in water can be represented as follows:

H₂SO₄(aq)  ⇌  2H⁺(aq) + SO₄²⁻ (aq)

Or

HSO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺ (aq)+ SO₄²⁻ (aq)

These two protons (H+ ions) and previously generated protons combine with water molecules to form hydronium ions H₃O⁺. It should be noted that the splitting of ions in the second stage is much weaker as compared to the first step and the SO₄²⁻ will be much less as compared to HSO₄⁻ ions in the diluted solution of sulfuric acid with water.

The reasons behind this are two: first, the dissociation of sulfuric acid in water strongly favors the formation of ions (H⁺ and HSO₄⁻) rather than the undissociated acid molecules. The strong nature of sulfuric acid favors the first step as explained above rather than the second step. 

Another reason is that HSO⁴⁻ ions are much more stable as compared to SO₄²⁻ ions and therefore when you add water to sulfuric acid, the resultant diluted solution will have significantly more concentration of H⁺ and HSO₄⁻ ions as compared to H⁺ and SO₄²⁻produced from the second step of sulfuric acid dissociation.

Applications:

This two-step process of H₂SO₄ dissociation has many applications in analytical chemistry and for reactions involving industrial synthesis. The HSO₄⁻ produced when mixing with water is used as a component of buffer solutions.

The release of hydronium ions H₃O⁺ ions in its dissociation is used for regular pH of solutions which is vital in water treatment plants and during the manufacturing of common chemicals we use every day. The dissociation of this acid is also used in the titration process where it helps determine the endpoint of titration reactions facilitating the concentration of the entity being measured.

Conclusion:

The H₂SO₄ dissociation involves two steps; in the first, the acid turns into HSO⁴⁻ and H⁺ ions, and in the second step, the HSO⁴⁻ dissociates further into SO₄²⁻ ions. The equilibrium in this process first step more as compared to the second one which results in fewer SO₄²⁻ ions as compared to HSO⁴⁻ in a solution of sulfuric acid and water. The result of dissociation is hydronium ions (H₃O⁺) and sulfate ions (HSO₄⁻ and SO₄²⁻). This dissociation in water has significant use in various industrial processes and analytical chemistry.