Answer the following questions in detail :

1. Explain giving example, Arrhenius acid-base theory, Mention the limitations of this theory.

2. Discuss Bronsted-Lowry Acid-base theory.

3. Discuss methods to measure the pH of the aqueous solution.

1. Arrhenius acid is a hydrogen-containing substance which produces hydrogen ion (H ⁺ ) in its aqueous solution.

Examples of such acids are Nitric acid and hydrochloric acid.

HNO₃→ H ⁺ + NO₃^-

HCl → H ⁺ + CL^-

Both contain Hydrogenand produce H ⁺

Arrhenius base is hydroxide containing substance and which produces OH- ion in its aqueous solution.

Examples of two such bases are sodium Hydroxide and potassium hydroxide.

NaOH → Na ⁺ + OH^-

KOH → K ⁺ + OH^-

Both contain Hydroxide and produces OH^-

Limitations of this theory are as follows:

•It is applicable to aqueous solutions only.

•It could not explain the basicity of ammonia as ammonia doesn’t contain hydroxide ion.

•It said that H ⁺ is responsible for the acidity while H ⁺ is very unstable and readily combines with water to form hydronium ion.

2. The substances which donate protons [H ⁺ ] to the other substance are called bronsted-lowry acids. The species responsible for acidity is not [H ⁺ ] but [H₃O ⁺ ].

Examples of bronsted-lowry acids are: HCl, HNO₃

The bronsted-lowry base is a substance which accepts a proton from other substance. For example ammonia.

When ammonia is dissolved in the water, it accepts a proton from water and thus behaves as a bronsted-lowry base.

3. There are two types of methods to measure the pH of a solution. Approximate methods are used to get the general idea about the acidity and basicity of a given solution and exact method is used to know the exact numeric value of pH.

Approximate methods:

1. Litmus paper: if the colour of litmus paper changes from blue to red it indicates the acidic nature and if the colour of litmus paper changes from red to blue it indicates basic nature. Thus, it is an approximate method.

2. pH paper: pH paper is dipped in the solution whose pH is to be determined. There occurs a colour change. By observing the colour of the pH paper, we can get a general idea of the pH of the solution.

Exact method:

1. pH-meter: pH-meter is an instrument used for exact pH value measurement. The pH-meter has two electrodes which are dipped in the solution whose pH is to be determined. The electrodes are attached through a circuit to an indicating box where the pH of the solution is read.