ACID-BASE NEUTRALISATION-SALTS

When aqueous solutions of hydrochloric acid and sodium hydroxide are mixed in the proper proportion, a reaction takes place to form sodium chloride and water.

HCl(aq) + NaOH(aqr) ↔  NaCl(aq) + H₂O(l)

                                  Sodium chloride

Such a reaction is termed neutralisation because both acidic (H⁺) and basic (OH^-) properties are eliminated during the reaction. The hydrogen ion, which is responsible for the acidic properties, has reacted with the hydroxyl ion which is respon­sible for the basic properties, producing neutral water. The Na⁺ and CI^- ions have undergone no chemical change and appear in the form of crystalline sodium chloride upon evapo­ration of the solution. Sodium chloride is an example of the class of compounds called salts.

HCl(aq) + Cl^-(aq) + Na⁺(aq) ↔  H₂O(l) + Na⁺(aq) + Cl^-(aq)

        or       H⁺(aq) + OH^-(aq)  H₂O(l)

Thus, the neutralisation of a base with an acid involves the interaction between OH^-and H⁺ ions.

                                                or

The reaction between an acid and a base to form salt and water is termed neutralisation.

The process of neutralization does not produce the resulting solution always neutral; no doubt it involves the interaction of H⁺ and OH^- ions. The nature of the resulting solution depends on the particular acid and a particular base involved in the reaction. The following examples illustrate this point when equivalent amounts of acids and bases are reacted in aqueous solution.

(i) A strong acid plus a strong base gives a neutral solution because both are completely ionised and the reaction goes to completion.

              H⁺ + CI^- + Na⁺ + OH^- ↔ H₂O + Na⁺ + CI-

(ii) A strong acid plus a weak base gives an acidic solution as the weak base is not completely ionised. The reaction does not go to completion and there is an excess of hydrogen ions in solution.

              H⁺ + CI^- + NH₄OH ↔  H₂O + NH₄⁺ + Cl^-

(iii) A weak acid plus a strong base gives a basic solution as the weak acid is not completely ionised. The reaction does not go to completion and there is an excess of hydroxyl ions in solution.

             CH₃COOH + Na⁺ + OH^- ↔ H₂O + CH₃COO^- + Na⁺

(iv) A weak acid plus a weak base gives an acidic or a basic or a neutral solution depending on the relative strength of acid and base. In case both have equal strength, the resulting solution is neutral in nature.

             CH₃COOH + NH₄OH  H₂O + NH₄⁺ + CH₃COO^-