Acids – Concept of Acid

In our everyday lives, we often deal with compounds that are known as acids. Ranging from grape juice and Pineapple juice to lemon juice, a lot of substances that we deal with every day are acidic. Acids are a very important class of compounds since they are used in manufacturing a huge number of substances through neutralisation reactions. Generally, acids are sour to taste.

Properties of acids:

1. Acids are generally sour to taste. Example: Vinegar is a harmless acid. It is sour to taste.
2. The aqueous solution of acids are electrolytes. In other words, their aqueous solution can conduct electricity. This is because, in an aqueous solution, they dissociate to form ions.
3. Acids can change blue Litmus Paper to red. Acids can turn phenolphthalein colourless.
4. Acids react with bases to form corresponding salts. This is also known as a neutralisation reaction. 
5. The pH value of strong acids is between 1-2, whereas the pH of weak acids is between 3-7.

Definitions:

There are quite a lot of definitions associated with acids. Different Scientists give different definitions of acids. Also, the definition is updated from time to time as new compounds and their properties are established.

Some of the most widely accepted definitions of acids are as follows:

Arrhenius’s concept of acid: 

According to Arrhenius, an acid is a substance containing Hydrogen that has the capacity to donate a proton or hydrogen ions to another substance known as a base.

In other words, acids are those substances that can give H+ ions in the aqueous medium. With water, it forms H3O+, also known as the hydronium ion.

Limitations of Arrhenius definitions:

There are certain limitations in Arrhenius’ definition of acids. They are as follows:

1. This definition is valid only when the substance can be dissolved in water. So, the definition is restricted to a limited number of substances only.
2. It cannot define the acid uniquely. Take the example of H2SO4. This is known as Sulphuric Acid. However, even though it can donate protons in Toluene as per the definition, it is not an acid because the medium is not water.

Arrhenius  reaction of acid and water: 

Any acid will furnish H+ ions in water to give hydronium ions.

H+ + H2O→H3O+

Bronsted Lowry’s theory of acids:

According to the Bronsted Lowry theory of acids, acids are those substances that can donate protons. This is a broader definition of an acid. It also introduces the concept of conjugate acids and bases. A conjugate acid is formed when a proton is added to an acid and a conjugate base is formed when Proton is removed from it. 

Limitations of Bronsted Lowry’s theory:

1. It fails to explain the acidic properties of compounds such as  BF3, AlCl3, etc.
2. It cannot explain all the properties of acids.

Lewis’s concept of acid:

This is the most widely accepted definition of acids. As per the definition, an acid is a species that can accept an electron pair. The advantage of this theory is that it does not involve the concept of H+ ions and hence it can be applied to almost all kinds of species, whether water-soluble or insoluble.

Lewis acids are electrophilic. In other words, they are attracted to electrons. 

Factors affecting the strength of Lewis acid:

1. The nuclear charge on the central atom: The greater the nuclear charge on the central atom, the greater force of attraction offered by the central atom to the negatively charged electrons. Hence it implies greater acidic strength.
2. Number of electronegative atoms attached to the central atoms:

For example, compare S02 and SO3. SO3 has more acidic strength because it has more electronegative atoms (oxygen atoms).

3. The charge on the ion: Greater magnitude of positive charge signifies stronger acidic strength.

Example: Strength of Fe3+ > Fe2+

Note: All Bronsted acids are Lewis acids, but not all Lewis acids are Bronsted acids. 

Uses of acids:

1. Vinegar is a dilute solution of Acetic Acid. It has a wide range of applications in cooking.
2. Citric and malic acid, which are obtained from foods, are an integral part of our diets.
3. Sulphuric acid is known as the king of all chemicals. This is because it is used for the manufacturing of hundreds of chemicals that are industrially important. The Development Index of a country is measured by its consumption of sulphuric acid.
4. Phosphoric acid is used in many soft drinks.
5. Many other acids are used in explosives, paints, fertilisers etc.

Conclusion:

Acids are a class of compounds that can furnish H+ in an aqueous solution. They are sour to taste. As per the Bronsted concept, acid is a substance that can donate protons. As per Lewis’ definition, an acid is a substance that can donate a pair of electrons. The strength of a Lewis acid depends on several factors like nuclear charge on the central atom, several electronegative atoms attached to the central atom, the charge on the ion, etc. Acids find a wide scale of applications in both the laboratory and in industry. They are widely used in manufacturing explosives, paints, fertilisers, dyes, etc.