Is H2SO4 an acid or base? Strong vs Weak - Conjugate base

Sulfuric acid is colorless and odorless made up of the elements hydrogen, oxygen, and sulfur with the chemical formula H₂SO₄. This acid is also known as oil of vitriol and it is soluble in water at any concentration. It is corrosive to metal and tissue.

In this article, we will discuss Is H₂SO₄ an acid or base? Is it strong or weak? Conjugate acid-base of H₂SO₄, etc.

So, Is H₂SO₄ an acid or base? H₂SO₄ is a strong acid. In an aqueous solution, it dissociates into 2H⁺ and SO₄^2-, So, it has 2H⁺ ions that are replaceable that correspondingly showing its acidic nature, although, H₂SO₄ is one of the strongest acids, but, in the presence of superacids, it can act as a base also.

The pH value of H₂SO₄ is around 1 to 3 but it depends on its concentration.

When reacted with other elements or compounds it can release a huge amount of energy due to its high reactivity properties.

Name of Molecule	Sulfuric acid
Chemical formula	H₂SO₄
pH value	1 to 3
Conjugate base	HSO₄–
Nature	Strong acid
Acidity (pK_a)	-2.8, 1.99
Solubility in water	miscible

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Why H2SO4 acts as an acid?

H₂SO₄ acts as an acid because it dissociates into two ions on dissolving in an aqueous solution and liberates proton or H⁺ ion.

As per various acid theories, an acid is something that releases protons or H⁺ ions when dissolved in an aqueous solution. Since H₂SO₄ releases H⁺ ions in an aqueous solution, it acts as an acid.

⇒ H₂SO₄ → 2H⁺ + SO₄ – –

We have two important theories to check whether H₂SO₄ acid or not. (a). Arrhenius theory (b). Bronsted-Lowry theory

1. Arrhenius’s theory for acid:

According to Arrhenius’s concept of acid, an acid is a substance that increases the concentration of H+ ions in an aqueous solution. In the case of H₂SO₄, it dissociates into two ions(2H⁺ and SO₄– -) on dissolving in an aqueous solution. Hence, it acts as an Arrhenius acid.

2. Bronsted-Lowry theory for acid:

This theory state that acid is the substance that donates the proton to other species and itself forms the conjugate base. So, in the case of H₂SO₄, when it reacts with H₂O then it donates the proton to H₂O and itself forms HSO₄– conjugate base.

As you see in the above reaction, H₂SO₄ donates the one H⁺ ion to H₂O and itself forms the conjugate base HSO₄^–. Hence, as per Bronsted-Lowry theory, H₂SO₄ acts as an acid whereas H2O acts as a Bronsted-Lowry base.

Is Sulfuric acid (H2SO4) strong or weak?

For knowing whether H₂SO₄ is a strong or weak acid, first, we have to understand what is a strong acid and weak acid.

A strong acid is a compound that is ionized 100% in an aqueous solution and produced a high concentration of H⁺ ion in the solution. More is the concentration of H⁺ ion in an aqueous solution, and less is the pH value of that compound.

Examples of strong acids – are HClO₄, HBr, HNO₃, HCl, HI, etc.

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And weak acid is a compound that partially dissociates to yield H⁺ ions which means some fraction of the compound is ionized but most of it remains undissociated in the solution.

Almost all organic acids like HNO₂, HSO₃, etc. are weak acids and their pH value lies between 5 to 7.

Some more examples of weak acids:- Hydrocyanic acid(HCN), Acetic acid(CH₃COOH), Ammonium ion (NH₄⁺), and Phosphoric acid(H₃PO₄), HNO₂, HF, HCOOH, H₂CO₃, etc.

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Now, Is H₂SO₄ strong acid or weak acid? Sulfuric acid (H₂SO₄) is a strong acid. Because when it is dissolved in an aqueous solution, it completely dissociates into H⁺ and HSO₄^– ions in the solution. There is no undissociated H₂SO₄ present in the solution, it completely breaks off into the ions.

Sulfuric acid is also called the “king of chemicals or acid” because of its strong acid and more reactive nature.

The presence of more H⁺ ions in the solution defines sulfuric acid as a good conductor of electricity and a strong electrolyte.

To further understand Why H₂SO₄ is a strong acid? we have to focus on two reactions that occur after dissolving sulfuric acid in the aqueous solution.

In the first reaction, as you see, a single arrow is used that shows only a forward reaction will occur because H₂SO₄ completely dissociates into H⁺ and HSO₄^–. And this reaction is irreversible means, it strongly favors the right side.

HSO₄^– is a weak acid and stable conjugate base of H₂SO₄, hence, it will not reform with H⁺ to form H₂SO₄ again in an aqueous solution.

In the second reaction, HSO₄^– is a weak acid that also partially dissociates and releases one proton and forms SO₄^2-. The double arrow in the second reaction shows that both backward and forward reactions will occur at equilibrium.

(H⁺ and SO₄^2-) will reform and produce the HSO₄^– again in solution but there is no effect on the first reaction. Hence, this is why H₂SO₄ is a strong acid in nature as it fully dissociates into ions in one shot.

So, as you see, in the above figure, when H₂SO₄ is dissolved in an aqueous solution, it completely dissociates into two ions(H⁺ and HSO₄^–), hence, there is no undissociated H₂SO₄ molecule present in its aqueous solution that correspondingly shows its very strong acid nature.

Here’s the list of some common strong and weak acids.

Also Read:
How to determine if a weak acid or strong acid?
How to tell if something is acid or base or salt or neutral?

Is H2SO4 also act as base?

Yes, sometimes H₂SO₄ can also act as a base if it reacts with superacids. “A superacid is an acid with an acidity greater than that of 100% pure sulfuric acid” For example – CF₃SO₃H and HSO₃F, both are thousand times stronger than sulfuric acid.

A compound is said to be base when it accepts the proton or donates the OH^– in an aqueous solution.

So, when sulfuric acid reacts with superacids then it will accept the proton from super acid, hence, acting as a base.

Super acid always dominates the reacting compound and strongly liberates the H⁺ ions, so reacting compound has no choice left and has to accept the proton of superacid.

H₂SO₄ acts as a base:

Is H2SO4 lewis acid or base?

Lewis acid is a compound that accepts the pair of electrons and lewis base which donates the pair of electrons to other compounds.

So, Is H₂SO₄ lewis acid or base? H₂SO₄ is a lewis acid. Because it has an H⁺ ion that can accept the electron pairs from a compound donor. Almost all simple cation is lewis acid, for example- H⁺, Na⁺, Mg⁺², etc. Some exceptional cases NH₄⁺, PH₄⁺, etc. are not lewis acid.

When H₂SO₄ is dissolved in an aqueous solution it gives H⁺ ions that can accept the pair of electrons from surrounding atoms to complete the deficiency of electrons.

To be a Lewis acid, the atom needs an empty orbital in its valence shell. H⁺ ion has empty orbital as one electron of it stripped from orbital due to positive charge.

⇒ Lewis acid → electron pair acceptor

⇒ Lewis base → electron pair donator

Note: Lewis acid is a superset of Bronsted-Lowry acids which means every compound that falls into the category of Bronsted-Lowry acid is Lewis acid.

“A Lewis acid is a molecule that accepts pair of electrons, a Bronsted acid is a molecule that donates a proton. The acid will have both of these characteristics.”