Explain the following observations giving appropriate reasons:

(i) Ozone is thermodynamically unstable with respect to oxygen. .

(ii) The HEH bond angle of the hydrides of group 15 elements decrease as we move down the group.

(iii) Bleaching effect of chlorine is permanent.

(i)

• Ozone is thermodynamically unstable with respect to oxygen because its decomposition into oxygen results in the liberation of heat.

• Ozone consists of 3 molecules of O and thus in it’s in an unstable state . Hence in order to get stable it gives up its one molecule of oxygen to restore diatomic state.

• In this process eventually large amount of heat is released therefore it is an endothermic reaction .

3O₂→ 2O₃ ΔH⁰_298K = +142KJmol^-1.

(ii)

• As moving down the group 15 , the central atom of the elements becomes reluctant to hybridization (due to larger size) Hence, it is closer to pure p character for the orbitals on E associated with E-H bond.

• Also as one move down the group , the electronegativity also decreases steadily due to increase in size and lone pair – bond pair repulsion becomes more and more effective hence bond angle decreases down the group in the order of NH₃ > PH₃ > AsH₃> SbH₃ > BiH₃ .

(iii)

• Bleaching effect of chlorine is permanent unlike other bleaching agents like SO₂ whose bleaching property is based on reduction using nascent hydrogen (H).

• Bleaching action of Cl₂ is actually based on oxidation of the compounds which requires nascent oxygen (O) .

• This nascent oxygen (O) is a more powerful oxidizing agent it is solely responsible for the bleaching action of Cl in presence of moisture which i left a permanent decolouration on the substance.

Equations:

Cl₂(g) + H₂O(l) → HCl(aq) + HOCl(aq)

Both HCl and HOCl makes Acidic solution.

Then:

Dye(Coloured) + HOCl(aq) → HCl(aq) + {Dye + [O]}(colourless)