How to know if compound is acid or base or neutral or salt? - Nature of solution

The best way to tell whether the compound is acid or base is by applying the three different definitions of acid and base given by Arrhenius, Bronsted-Lowry, and Lewis’s theory.

Arrhenius theory:  According to this, a compound is an acid when it furnishes an H⁺ ion on dissolving in an aqueous solution or increases the concentration of hydrogen ion in the solution.

And a compound is said to be base when it furnishes OH^– ions on dissolving in an aqueous solution or increases the concentration of OH^– ions in solution.

Example – HCl, HCN, CH₃COOH, HNO₃, HNO₂, HClO₄, HCOOH, H₃PO₄, HI, HBr, etc. All these compounds are Arrhenius acid in nature as they release H⁺ ions when dissolved in an aqueous solution, hence, increasing the concentration of H⁺ ions.

Also check-

• Acidic nature of HCN
• Acidic nature of HNO3
• Acidic nature of HClO4
• Acidic nature of HI
• Acidic nature of HBr
• Acidic nature of HCOOH
• Acidic nature of H3PO4
• Acidic nature of NH4+
• Acidic nature of HNO2

And the compound such as NaOH, KOH, LiOH, Ba(OH)₂, Ca(OH)₂, etc. is the Arrhenius base as they release OH^– ions when dissolved in an aqueous solution, hence, increasing the concentration of OH^– ions.

Also check:

• Basic nature of KOH
• Basic nature of LiOH
• Basic nature of Ca(OH)2
• Basic nature of Ba(OH)2
• Basic nature of NH4OH
• Basic nature of CH3NH2

Note: – Arrhenius’s theory is limited to the aqueous solution, which means, the presence of water solution is a must for knowing the acidic or basic nature of the substance.

So, for overcoming this problem, another theory is proposed named the Bronsted-Lowry theory which can explain the nature of the compound in both aqueous and non-aqueous solvents.

Bronsted-Lowry theory: This theory is a generalization of the Arrhenius theory. According to this, the substance which donates the proton to another species is classified as a Bronsted-Lowry acid whereas the substance which accepts the proton from another species is classified as a Bronsted-Lowry base.

• The Bronsted-Lowry theory is an acid-base reaction theory that means a substance can only act as an acid in presence of a base similarly a substance can only act as a base in presence of an acid.
• When acid and base react with each other, the acid compound donates its proton and forms a base, called the conjugate base of the acid, Similarly, the base compound accepts the proton and forms an acid, called the conjugate acid of the base.

The reaction defines the concept of the Bronsted-Lowry theory.

⇒ Acid + Base ⇌ Conjugate Base + Conjugate Acid

Let’s understand it with an example of HCl reacting with NH3. 

So, HCl is acid and NH3 is a base, therefore, the Bronsted-Lowry theory is applicable here.

So, HCl as an acid donates its proton and makes a conjugate base(Cl^–), and NH₃ as a base accept the proton and makes a conjugate acid(NH₄⁺).

Therefore, we can say, HCl acts as a Bronsted-Lowry acid, and NH₃ acts as a Bronsted-Lowry base when reacts with each other.

Let’s take another example, CH₃COOH reacting with the H₂O.

When CH₃COOH reacts with water, it donates the proton to the water molecule and makes a conjugate base(CH₃COO^–) whereas H₂O accepts the proton and makes the conjugate acid(H₃O⁺).

Therefore, CH3COOH acts as a Bronsted-Lowry acid, and H₂O acts as a Bronsted-Lowry base when they react with each other.

That’s it, you can apply this theory in a reaction that contains both acid and base and can easily predict their conjugate acid and base.

Limitation of Bronsted-Lowry theory: – 

“Bronsted Lowry’s theory is limited to explaining the acidity and basicity of only protons containing compounds.”

There is no absolute acid and base defined by this theory, the substance can only act as the acid in presence of a base similarly the substance can only act as a base in presence of acid.

The same compound that acts as an acid in one reaction can act as a base in another reaction, it all depends on the strength of the reactive compound.

As we know, HCl is a strong acid and it can easily donate the proton when reacts with bases like NH₃, NaOH, etc., or weak acidic like CH₃COOH, HCO₃^–, etc. Hence, HCl act here as a Bronsted-Lowry acid.

But when HCl reacts with superacids like Fluoroantimonic acid, then it will act as a base, because superacids are able to donate the proton thousands of times better than HCl, H₂SO₄, etc.

So, Please consider the strength of the substance when applying the Bronsted-Lowry theory.

In short, you have to remember is that no compound is absolute acid or base, all these can act as an acid or base depending on the circumstances.

Lewis theory: This theory used electrons instead of proton transfer. According to the lewis theory, a compound is said to be acid when it accepts the electron pair and a compound is said to be base when it donates the electron pair.

Lewis acid → lone pair acceptor

Lewis base → lone pair donator

⇒ Lewis acid: They are electron-loving compounds and are also known as electrophilic compounds that accept the electron pair from Lewis bases.

Some examples of Lewis acids that can accept the pair of electrons from another species.

1. The molecules or ions with an incomplete octet of electrons. Example – BF₃, BCl₃, AlF₃, etc.
2. The molecule that central atom has an empty d-orbital. Example- SiCl₄ (1s²2s²2p⁶3s²3p² 3d⁰).
3. The compounds that the central atom formed multiple bonds with the adjacent atoms. Example – SiO₂ (O = Si = O), CO₂, SO₂, etc.
4. Simple cations like H⁺, Na⁺, Mg⁺², Al⁺³, etc. are lewis acids since they are able to accept the electrons. ( Exception NH₄⁺, PH₄⁺ are not a lewis acid)

⇒ Lewis base: They are electron-rich species and also known as nucleophile compounds that donate the electron pairs to the Lewis acids.

Some examples of Lewis bases that can donate the pair of electrons to other species.

1. The negative ion such as H^–, OH^–, F^–, etc.
2. Species that have lone pair of electrons. Example – NH₃, H₂O, CH₃^–, etc.

Let’s understand it with the help of an example-

The electrons in the hydrogen-fluorine bonds attracted towards the fluoride ion because of it high electronegativity leaving hydrogen slightly positive and fluorine negative.

The lone pair on the nitrogen of NH₃ molecule attracted towards hydrogen atom in HF molecule.

As it approaches it, the electrons in the hydrogen-fluorine bond are repelled still further towards the fluorine.

Eventually, a co-ordinate bond is formed between the nitrogen and the hydrogen, and the fluorine breaks away as a fluoride ion.

The whole HF molecule acts as Lewis acid as it accept the lone pair from nitrogen atom, and in this process it breaks up. 

So, HF accepts the lone pair of the electron, therefore, it is Lewis acid and NH₃ donates the lone pair of the electron, therefore, it is Lewis base.