Arrange the following in the order of property indicated against each set:

(i) F₂, Cl₂, Br₂, I₂ (increasing bond dissociation enthalpy)

(ii) H₂O, H₂S, H₂Se, H₂Te (increasing acidic character)

(b) A colourless gas ‘A’ with a pungent odour is highly soluble in water and its aqueous solution is weakly basic. As a weak base it precipitates the hydroxides of many metals from their salt solution. Gas ‘A’ finds application in detection of metal ions. It gives a deep blue colouration with copper ions. Identify the gas ‘A’ and write the chemical equations involved in the following:

(i) Gas ‘A’ with copper ions

(ii) Solution of gas ‘A’ with ZnSO₄ solution.

(i) The order of increasing bond dissociation enthalpy is given as:

I₂ < F₂ < Br₂ < Cl₂

Explanation:

⇒ Bond dissociation enthalpy should decrease as the bond distance increases from F₂ to I₂ due to the corresponding increase in the size of the atom as we move from F to I.

⇒ However, the F—F bond dissociation enthalpy is smaller than that of Cl—Cl bond and even smaller than that of Br—Br.

⇒ This is due to reason that the F atom is very small and hence the three lone pair of electrons on each F atom repel the bond air holding the F-atom in F₂ molecule.

(ii) H₂O, H₂S, H₂Se, H₂Te (increasing acidic character)

The order of increasing acidic character of the given hydrides is:

H₂O < H₂S < H₂Se < H₂Te

Explanation:

⇒ The increase in acidic strength can be easily explained on the basis of their bond dissociation energy.

⇒ As the atomic size increase down the group, the bond length increase and hence the bond strength decreases.

⇒ Consequently, the cleavage of E—H bond (E= O, S, Se, Te) becomes easier.

⇒ As a result, the tendency to release hydrogen as proton increase down the group.

(b) As gas ‘A’ have a pungent odour and is highly soluble in water and its aqueous solution is weakly basic. As a weak base it precipitates the hydroxides of many metals from their salt solution. Gas ‘A’ finds application in detection of metal ions. It gives a deep blue colouration with copper ions.

Hence, Gas ‘A’ is ammonia (NH₃)

(i) Gas ‘A’ (NH₃) with copper ions

Cu²⁺ + 4NH₃→ [Cu(NH₃)₄]²⁺

(ii) Solution of gas ‘A’ (NH₃) with ZnSO₄ solution.

ZnSO₄ + 2NH₄OH → Zn(OH)₂ + (NH₄)₂SO₄