Acids and Bases

With our understanding of strong, weak, and non-electrolytes we can now examine the Arrhenius Definition of Acids and Bases:

Acids:

Substance that produces H⁺ ions when it is dissolved in H₂O.

Bases:

Substance that produces OH^- ions when it is dissolved in H₂O.

For example, HCl is an acid,

and NaOH is a base,

Acids and Bases that are strong electrolytes are called Strong Acids and Strong Bases, respectively. Acids and Bases that are weak electrolytes are called Weak Acids and Weak Bases, respectively.

Strong Acids

HCl is an example of a strong acid:

Other examples include HBr, HI, HClO₄, HClO₃, H₂SO₄, and HNO₃.

Weak Acids

HF is an example of a weak acid:

Other examples include HC₂H₃O₂, H₂CO₃, H₂SO₃, H₃PO₃, and H₃PO₄. The last four are examples of polyprotic acids. These are acids that can produce more than one H⁺ ions when dissolved in water. H₂CO₃ and H₂SO₃ are called diprotic acids, and H₃PO₃ and H₃PO₄ are called triprotic acids. HF, HCl, HBr, and HC₂H₃O₂ are examples of monoprotic acids. The dissociation of polyprotic acids usually occurs in steps. For example, only after H₃PO₄ loses its first H⁺ ion will it lose its second H⁺ ion, and then it will lose its third.

In this example, all three species H₃PO₄, H₂PO₄^-, and HPO₄^2- are weak electrolytes so H₃PO₄ is considered to be a weak acid. H₂SO₄ is another example of a diprotic acid. In the case of H₂SO₄ the first H⁺ ion is produced readily so the species H₂SO₄ is considered to be a strong electrolyte (i.e. strong acid). The species HSO₄^-, however, is a weak electrolyte.

Strong Bases

NaOH is an example of a strong Base:

Other examples include LiOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂.

Weak Bases

Ammonia, NH₃, is a good example of a weak base.

Other examples include C₆H₅N (pyridine) and C₆H₅NH₂ (aniline).