Class 11th

9.21. Ice has crystalline structure which is highly ordered due to hydrogen bonding. It has hexagonal form at atmospheric pressure and cubic form at low temperature. Each O atom has tetrahedral geometry and is surrounded by 4 oxygen atoms each at a distance of 276 pm.

9.20.  (i) PbS(s) +4H₂O₂(aq) → PbSO₄(s) + 4H₂O(l)

(ii) 2MnO₄^– (aq) +H₂O₂(aq) + 6H⁺(aq) → 2Mn (aq) + 8H₂O(l) + 5O₂(g)

(iii) CaO(s) + H₂O(g) → Ca(OH)₂(aq)

(iv) AlCl₃(aq) + 3H₂O(l) → Al(OH)₃(s) + 3HCl (aq)

(v) Ca₃N₂(s) + H₂O(l) → 3Ca(OH)₂(aq) + 2NH₃(aq)

(a) Hydrolysis reactions, (iii) (iv) and (v)

(b) Redox reactions (i) and (ii)

9.19. 2F₂ (ag) + 2H₂O (l)? O₂ (g) + 4H⁺ (aq) + 4F (aq)
In this reaction water acts as a reducing agent and itself gets oxidised to O₂ while F₂ acts as an oxidising agent and hence itself reduced to F^– ions.

9.18. Auto-protolysis means self-ionisation of water. It may be represented as

2H₂O(l) + H₂O(l) ? H₃O⁺(aq) + OH^-(aq)

Acid 1       Base 2     Acid 2        Base 1
Due to auto-protolysis nature of water, it can act as an acid as well as base, i.e. amphoteric in nature.

9.17. In water, O is sp³ hybridized. Due to stronger lone pair-lone pair repulsions than bond pair-bond pair repulsions, the HOH bond angle decreases from 109.5° to 104.5°. Thus, water molecule has a bent structure.

H₂O₂ has a non-planar structure. The O—H bonds are in different planes. Thus, the structure of H₂O₂ is like an open book.

9.16.  (i) BeH₂< TiH₂ < CaH₂

(ii) LiH

(iii) F—F < HH < DD

(iv) H₂O < MgH₂ 

9.15. No. Because if saline hydrides react with water the reaction will be highly exothermic thus the hydrogen evolved in this case can catch fire. CO₂ cannot be used as fire extinguisher because CO₂ will get absorbed in alkali metal hydroxides.

9.14. HF is expected to have the highest magnitude of hydrogen bonding since, fluorine is most electronegative. Therefore, HF is the most polar.

9.13. When hydrogen is burnt in oxygen the reaction is highly exothermic, it produces very high temperature nearly 4000°C which is used for cutting and welding purposes.

9.12. Metallic hydrides are useful for ultra-purification of dihydrogen and as dihydrogen storage media. In metallic hydrides, hydrogen is adsorbed as H-atoms. Due to the adsorption of H atoms the metal lattice expands and become unstable. Thus, when metallic hydride is heated, it decomposes to form hydrogen and finely divided metal. The hydrogen evolved can be used as fuel.