Neutralisation & Salts

Neutralisation

The reaction between an acid and a base to form salt and water is termed neutralisation. Thus, the neutralisation involves the interaction between OH^-and H⁺ ions.

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.

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.

?Refer to the following video for Neutralization

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^-

Salts

The salts are classified into the following classes:

(i)  Simple salts:

The salt formed by the neutralization process, i.e., interaction between acid and base, is termed as simple salt. These are of three types:

(a)  Normal salts: The salts formed by the loss of all
possible protons (replaceable hydrogen atoms as H⁺) are called normal salts. Such a salt does not contain either a replaceable hydrogen or a hydroxyl group.

Examples are: NaCl, NaNO₃, K₂SO₄, Ca₃(PO₄)₂, Na₃BO₃, Na₂HPO₃ (one H atom is not replaceable as H₃PO₃ is a dibasic acid), NaH₂PO₂ (both H atoms are not replaceable as H₃PO₂ is a monobasic acid), etc.

(b)  Acid salts:  Salts formed by incomplete neutralization of poly-basic acids are called acid salts. Such salts still contain one or more replaceable hydrogen atoms. These salts when neutralised by bases form normal salts.

Examples are: NaHCO₃, NaHSO₄, NaH₂PO₄, Na₂HPO₄, etc.,

(c)  Basic salts:  Salts formed by incomplete neutralization of poly acidic bases are called basic salts. Such salts still contain one or more hydroxyl groups. These salts when neutralised by acids form normal salts.

Examples are: Zn(OH)Cl, Mg(OH)Cl, Fe(OH)₂Cl, Bi(OH)₂Cl, etc.

(ii)  Double salts:

The addition compounds formed by the combination of two simple salts are termed double salts. Such
salts are stable in solid state only.

Examples are: Ferrous ammonium sulphate, FeS0₄-(NH₄)₂SO_4.6H₂O, Potash alum, K₂SO₄Al₂(SO₄)₃.24H₂O, and other alums.

Properties:

(a) When dissolved in water, it furnishes all the ions present in the simple salts from which it has been constituted.

(b) The solution of double salt shows the properties of the simple salts from which it has been constituted.

(iii) Complex salts:

 These are formed by combination of simple salts or molecular compounds. These are stable in solid state as well as in solutions.

                   →  K₄Fe(CN)₆ + K₂SO₄

                     Simple salt           Complex salt

      CoSO₄ +      6NH₃  →   Co(NH₃)₆SO₄

      Simple salt Molecular   Complex salt

                      compound 

Properties of Complex salts: 

(a) On dissolving in water it furnishes a complex ion.

       K₄F₂(CN)₆  ↔ 4K⁺ + [Fe(CN)₆]^4-

     Cu(NH₃)₄SO₄ ↔ [Cu(NH₃)₄]²⁺ +  SO₄^-2

                          Complex ion

(b) The properties of the solution are different from the properties of the substances from which it has been constituted.

(iv) Mixed salts:

The salt which furnishes more than one cation or more than one anion when dissolved in water is called a mixed salt.

Examples are:

You can also Refer to

• JEE  Physical Chemistry Syllabus,

• Reference books of  Chemistry for IIT JEE

• Buffer Solutions

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