Hydrogen

This is a long answer type question as classified in NCERT Exemplar

Mass of H₂O₂ = 68 g.

Mol. Mass of H₂O₂=34gmol^-1

∴1L of M solution of H₂O₂ will contain H₂O₂=34*5 g

∴2L of 5 M solution will contain H₂O₂=34*5*2=340g

or 200 mL of 5 solution will contain H₂O₂ = $\frac{340}{2000}$ $*$  200 = 34 g

Now 2H₂O₂ → 2H₂O+O₂

           64 g  32g

Now 68 g of H₂O₂ on decomposition will give O₂=32g

∴34g of H₂O₂ on decomposition will give O₂ = $\frac{34}{68}$ $*$    34 =16 g

(i) Industrial preparation: H₂O₂ is prepared by the auto-oxidation of 2- alkylanthraquinols

2-ethylanthraquinol ↔ H₂O₂⁺ oxidised product

2Fe₂+ (aq)+2H+ (aq)+H₂O₂ → Fe3 (aq)+2H₂O (l)

PbS (s)+4H₂O₂ (l) → PbSO4 (s)+4H₂O (l)

(ii) Reducing action of hydrogen peroxide 

2MnO₄^-+6H⁺+5H₂O₂→ 2Mn²⁺+8H₂O+5H₂

HOCl+H₂O₂ → H₃O⁺+Cl^-+O₂

Oxidising action of hydrogen peroxide 

2Fe²⁺+H₂O₂→ 2Fe³⁺+2OH^-

Mn²⁺+H₂O₂ → Mn⁴⁺+2OH^-

Acidic properties of H₂O₂

I₂+H₂O₂+2OH^-→ 2I^-+2H₂O+H₂

2MnO₄^-+3H₂O₂→ 2MnO₂+3O₂+2H₂O+2OH^-

This is a long answer type question as classified in NCERT Exemplar

D₂O can be prepared by prolonged electrolysis of water. Due to high molecular mass D₂O differs from water.

NaOH+D₂O → NaOD+HOD

HCl+D₂O → DCl+ HOD

NH₄Cl+D₂O → NH₃DCl+HOD

Physical Properties of H₂O and D₂O

This is a long answer type question as classified in NCERT Exemplar

Atomic hydrogen is highly reactive, whereas molecular hydrogen is rather inert. Bond dissociation enthalpy determines the chemical behaviour of dihydrogen (and, for that matter, any molecule) to a great extent. For a single bond between two atoms of any element, the H-H bond dissociation enthalpy is the highest. As a result, molecular hydrogen reacts only with a few elements.

This is a long answer type question as classified in NCERT Exemplar

Atomic hydrogen is highly reactive, whereas molecular hydrogen is rather inert. Bond dissociation enthalpy determines the chemical behaviour of dihydrogen (and, for that matter, any molecule) to a great extent. For a single bond between two atoms of any element, the H-H bond dissociation enthalpy is the highest. As a result, molecular hydrogen reacts only with a few elements.

9.56. Dihydrogen is prepared from water by the action of alkali metals like Na and K which is a strong reducing agent.

2Na + 2H₂O → 2NaOH + H₂

2K + 2H₂O → 2KOH + H₂

9.55. 1. High heat of combustion.

2. It is pollution free.

9.54. Water which does not produce lathers with soap is known as hard water. Hardness is due to the presence of bicarbonates, sulphates and chlorides of Ca²⁺ and Mg²⁺.

On boiling, the bicarbonates of calcium and magnesium decompose to insoluble carbonate which can be removed by filtration.

9.53. In water molecule, O is SP³ hybridized. Due to stronger lone pair-lone pair repulsion than bond pair-bond pair repulsions, the H-O-H bond angle decreases from 109.5° to 104.5°. Thus, water is bent molecule.