Relative Strength of Acids and Bases

Different acids and bases have different strength depending on their nature. Since there are different ways of defining acids and bases, there are different ways of comparing their relative strengths also.

According to Arrhenius Concept

According to Arrhenius concept, strong electrolytes (like HCl) which dissociate completely in aqueous solutions and produce H⁺ (or H₃O⁺) ions are called strong acids. Other examples of strong acids are H₂SO₄, HBr, HI, HNO₃ and HClO₄.

Weak electrolytes like CH₃COOH whose ionization is not complete (because the process is reversible) and produce H⁺ (or H₃O⁺) ions are called weak acids.

Similarly, strong bases are defined as the electrolytes with complete ionization and weak bases as the electrolytes with incomplete ionization. NaOH and NH₃ are examples of strong and weak bases respectively.

According to Brønsted-Lowry Concept

According to Brønsted-Lowry concept an acid is a species that donates a protons while a base is a species that accepts a protons. The tendency of a substance to donate a proton also depends on the species accepting the proton. A given acid may have different strengths in different solvents of varying basic strength or proton accepting tendencies.