Acids and bases are a class of substances that exhibit certain properties that are characteristic of them. For example, acids are typically sour to the taste whereas bases are usually bitter to taste. Furthermore, acids turn blue litmus paper red whereas bases turn red litmus paper blue. Neutralization reactions are a class of chemical reactions in which acids and bases combine to form salts. A brief introduction to the concept of acids and bases is provided in this article.
The Arrhenius Theory of Acids and Bases
In the year 1884, Svante Arrhenius, a Swedish chemist, proposed that substances could be classified as acids and bases based on the way they behave in aqueous solutions. He suggested that acids are substances that increase the concentration of H+ ions in the solutions they are dissolved in and that bases are substances that increase the concentration of hydroxide ions in their solutions. The H+ ions in water typically combine with other H2O molecules to form H3O+ ions, commonly referred to as hydronium ions.
One of the greatest limitations of the Arrhenius concept of acids and bases is that it suggests that acids must contain H+ ions and bases must contain OH– ions. However, this is not always true. For example, ammonia, despite not containing any hydroxide ions, is a weak base that increases the hydroxide ion concentration of water when it is dissolved in it.
The Bronsted-Lowry Concept of Acids and Bases
As per the Bronsted-Lowry theory of acids and bases, acids can be regarded as proton donors or H+ ions donors. Similarly, bases can be regarded as proton acceptors or H+ ion acceptors. As per this theory, substances like ammonia can be classified as bases since it can accept protons from water to form ammonium cations (NH4+ ions).
Ionization of Acids as per the Bronsted-Lowry Theory
As per the Bronsted-Lowry theory, the dissociation of an acid can be represented as follows:
HA ⇌ H++ A–
Where HA is the acid, H+ is a proton, and A– is the conjugate base of the acid. Here, an acid acts as a proton donor by dissociating into a proton and a conjugate base.
Ionization of Bases as per the Bronsted-Lowry Theory
As per the Bronsted-Lowry theory, the dissociation of a base can be represented as follows:
B + H2O ⇌ BH++ OH–
Where B is the base, H2O is water, BH+ is the conjugate acid of the base, and OH– is a hydroxide ion. Here, base acts as a proton acceptor by accepting a proton from water to form a conjugate acid.
The pH Scale
pH (which can be regarded as an abbreviation of power of hydrogen or potential of hydrogen) is a quantity that can be used to express the acidity or the basicity of a substance. The pH value of a substance is equal to the negative of the logarithm (with base 10) of its H+ ion concentration. The values on the pH scale range from 1 to 14, where 1 is the most acidic and 14 is the most basic value. Click here to learn more about pH chemistry at BYJU’S.
