The Theory of Acids and Bases.
February 10, 2021, Chemistry
An acid is any hydrogen that contains a substance capable of donating a proton to another substance, whereas, a base is an ion or molecule that is able to accept a hydrogen ion from and acid.
Acids and bases are recognized by taste. Acids are sour in taste. For example – lemon or orange juice. Whereas, the bases are bitter. For example – alkaline batteries, baking soda. There are certain indicators which are helpful in determining the acid solution and base solution. For example, in an acid solution, the blue litmus paper turns red while red litmus paper does not show any change in colour. In the basic solution, the red litmus paper turns blue while blue litmus paper does not show any change in colour. From this observation we can say acids and bases neutralize the action of each other. In the neutralization reaction, acids and bases react with each other to form salt and water. Acids and bases can be defined with the help of various theories.
Arrhenius acid-base theory:
In 1884, the Swedish chemist Svante Arrhenius proposed two specific classifications of compounds, termed acids and bases.
Acids are defined as a compound or element that releases hydrogen (H+) ions into the solution (mainly water).
HNO3 (aq) + H2O(l) → H3O+ (aq) + NO3−(aq)
In this reaction nitric acid (HNO3) dissociates into hydrogen (H+) and nitrate (NO3–) ions when dissolved in water. Bases are defined as a compound or element that releases hydroxide (OH–) ions into the solution.
LiOH(s) → Li+(aq) + OH−(aq)
In this reaction lithium hydroxide (LiOH) dissociates into lithium (Li +) and hydroxide (OH –) ions when dissolved in water.
Bronsted-Lowry acid-base theory:
In 1923, chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently developed definitions of acids and bases based on the compounds’ abilities to either donate or accept protons (H+ ions). In this theory, acids are defined as proton donors; whereas bases are defined as proton acceptors. A compound that acts as both a Brønsted-Lowry acid and base together is called amphoteric.
For example, consider the following chemical equation:
HCl(aq) + NH3(aq) → NH4+(aq) + Cl –(aq)
Here, hydrochloric acid (HCl) “donates” a proton (H+) to ammonia (NH3) which “accepts” it, forming a positively charged ammonium ion (NH4+) and a negatively charged chloride ion (Cl –). Therefore, HCl is a Brønsted-Lowry acid (donates a proton) while the ammonia is a Brønsted-Lowry base (accepts a proton). Also, Cl – is called the conjugate base of the acid HCl and NH4+ is called the conjugate acid of the base NH3.
Lewis acid-base theory:
In 1923, G.N. Lewis proposed an alternate theory to describe acids and bases. Lewis’ theory used electrons instead of proton transfer and specifically stated that an acid is a species that accepts an electron pair while a base donates an electron pair.
A Lewis Base (NH3) donates its electrons to a Lewis Acid (BF3) resulting in a coordinate covalently bonded compound, also known as an adduct.
BF3 + :NH3 → H3N → BF3
The reaction of a Lewis acid and a Lewis base will produce a coordinate covalent bond. A coordinate covalent bond is just a type of covalent bond in which one reactant gives its electron pair to another reactant. In this case the lewis base donates its electrons to the Lewis acid. When they do react this way the resulting product is called an addition compound, or more commonly an adduct.