Arrhenius Acid: Definition, Basicity, Examples and Sample Questions

Arrhenius acid is referred to as a substance that is dissolved in the water to create hydrogen atoms through dissociation. An example of the same is when we dissolve hydrochloric acid into the water, it then creates chlorine atoms and hydronium atoms. In the same way, acetic acid also acts in a similar manner. In this article, we will further explore the Arrhenius acid, its basicity, examples, and sample questions.

Read Also : Equilibrium

What is Arrhenius acid?

An acid and a base are distinct chemical compounds that have specific properties that render them useful in certain situations. For instance, acids are used in the production of detergents and fertilizers while bases such as Calcium Hydroxide are used to manufacture bleaching powder. We can define acid and a base on the basis of many theories such as:

• Lewis Theory
• Bronsted-Lowry definition
• Arrhenius acid and base theory

An acid or a substance which releases hydrogen atoms easily in the solution is called an Arrhenius acid. The principle is known as the Arrhenius principle. The H⁺ ion that is released is not freely floating in the solution. Instead, it comes in contact with the water molecule to form Hydronium ions (H₃O⁺). Some common examples of Arrhenius acid include sulphuric acid (H₂SO₄), Hydrochloric Acid (HCl), Nitric Acid (HNO₃), etc.

Arrhenius acid-base theory

In the year 1884, Swante Arrhenius, a Swedish scientist proposed acidic and basic compounds on the basis of an acid-base theory. According to him, a substance that gives the proton or a hydrogen ion in their water solution is called an acid. For a better understanding of this theory, let us see some examples.

1. HCl(aq) → H⁺(aq) + Cl^–(aq)

Here in the above example, we see that Hydrochloric acid with water dissolves the hydrogen and chlorine ions. 

2. HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl^–(aq)

When we add some water externally into the hydrochloric acid, it will dissolve with the water and release the hydronium ion.

3. NaOH(aq) → Na⁺(aq) + OH^–(aq) 

In this example, the given solution itself produces sodium ions and hydroxide ions.

Read more : Acid Bases and Salts

In the same way, the Arrhenius property is also followed by many other acids such as:

• HF (Hydrofluoric acid)
• HCLO₄ (Perchloric acid)
• H₂CO₃ (Carbonic acid)
• H₂SO₄ (sulphuric acid)
• H₃PO₄ (Phosphoric acid)

Various acid-base theories

Basicity of an Arrhenius acid 

Basically, the basicity of an acid is measured on the basis of how many H⁺ ions have been released. So, acids are categorized into three categories on the basis of their basic value.

1. Acids with the basicity values 1 are called monobasic acids.

Example: HPO₄^2– → H⁺ + PO₄^3–

2. Acids with the basicity value 2 are called dibasic acids.

Example: H₂PO₄^– → H⁺ + HPO₄^2–

3. Acids with the basicity values 3 are called tribasic acids.

Examples: H₃PO₄ → H⁺ + H₂PO₄^–

There are many more examples of monobasic, dibasic, and tribasic acids on the basis of their basicity values which are mentioned in the below table. 

Arrhenius acid as Strong Electrolytes

Swedish scientist Arrhenius was trying to find out the reason behind the conductivity of certain solutions and found out that ions are most responsible for the conductivity of a solution. He found that whenever Arrhenius acids are dissolved in the water, they release an ion that acts as strong electrolytes due to the ionization reactions that take place in water. On the basis of the inferences, the acids were classified into two categories named strong acids and weak acids. 

1. Strong acids:- A strong acid means acid has a strong capability of dissolving in the water and releasing maximum H⁺ ions. The acid dissociation constant (Ka) is higher for strong acids. Hence, we can observe that an acid’s strength is directly proportional to Ka, the acid dissociation constant.

HX⁺ → H⁺ + X^- 

2. Weak acids:- Weak acids are the acids that do not have the capability of dissolving completely in the water, so they are half dissolved. Hence, they can’t release maximum H⁺ ions For weak acids, the acid dissociation constant is lower. 

HCl acid as a strong electrolyte

Examples of Arrhenius Acid

Things to remember based on Arrhenius Acid

• An acid or a substance which releases hydrogen atoms easily in the solution is called an Arrhenius acid.
• According to the Arrhenius Principle, a substance that gives the proton or a hydrogen iron in their water solution is called an acid. 
• On the basis of their basicity values, acids are classified as monobasic, dibasic, and tribasic acids.
• Whenever Arrhenius acids are dissolved in the water, they release an ion and act as strong electrolytes due to the ionization reactions in water.
• Examples: HCl, HBr, H₂SO₄, etc

Sample Questions based on Arrhenius Acid

Ques. What is Arrhenius acid? 1 mark

Ans. An acid that dissociates to give H⁺ ions in water is known as an Arrhenius acid.

Ques. In the following equation, identify the acid and base: 1 mark 

HF + H₂O → H₃O⁺ + F^- 

(a) H₂O is the base and HF is its conjugate acid.

(b) H₂O is acid and HF is the conjugate base.

(c) HF is acid and F^- is its conjugate base.

(d) HF is a base and F^- is its conjugate acid.

Ans. c) HF is an acid and F^- is its conjugate base

Read more : Lewis Acids

Ques. Which of these is the weakest acid? 1 mark 

(a) H₃PO₄

(b) H₂PO₄^-

(c) HPO₄^2-

(d) HNO₃

Ans. c) HPO₄^2-

Ques. What are Hydronium ions? How are they formed? 2 marks

Ans. Arrhenius acids release hydrogen atoms when dissolved in a solution. The H⁺ ion that is released is not freely floating in the solution. Instead, it comes in contact with the water molecule to form Hydronium ions (H₃O⁺).

Ques. Differentiate between weak acids and strong acids. 2 marks

Ans. On the basis of their strength of dissociation, acids can be classified as: