Laboratory Preparation

By the reaction of granulated zinc with dilute hydrochloric acid.

Zn + 2H+ → Zn₂⁺ + H₂

By the reaction of zinc with aqueous alkali.

Zn + 2NaOH → Na₂ZnO₂ + H₂

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Commercial Preparation

Electrolysis of acidified water using platinum electrodes gives hydrogen

2H₂O(l) → 2H₂(g) + O₂(g)

High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes

Chemical Properties

The H–H bond
dissociation enthalpy is the highest for a single bond between two atoms of any
element.

Reaction with halogens, dioxygen, metal, dinitrogen

Uses of Dihydrogen

1.      Synthesis of ammonia, methanol, HCl

2.     Manufacture of vanaspati fat

3.     In metallurgical processes

4.     Fuel cells for generating electrical energy

5.     Cutting and welding purposes.

6.     Rocket fuel in space research.

Hydrides

The hydrides are classified into three categories:

• Ionic or saline or salt-like hydrides
• Covalent or molecular hydrides
• Metallic or non-stoichiometric hydride

Most of the s-block elements

LiH, BeH₂ and MgH₂

LiH, BeH₂ are covalent character

Lithium hydride is rather unreactive

Rest of are ionic hydrides are crystalline, non-volatile and non-conducting in solid state. However, their melts conduct electricity.

Covalent or Molecular Hydride

1.       p-block elements form this types of hydride

2.      Most familiar examples are CH₄, NH₃, H₂O and HF

3.      Being covalent, they are volatile compounds

(i) Electron-deficient

1.       In fact all elements of group 13 will form electron-deficient compounds.

2.      They act as Lewis acids

3.      Diborane (B₂H₆) is an example.

(ii) Electron-precise

1.       All elements of group 14 form such compounds (CH₄)

2.      Electron-precise compounds have the required number of electrons to write their conventional Lewis structures.

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(iii) Electron-rich hydrides

1.       Electron-rich hydrides have excess electrons which are present as lone pairs.

2.      Elements of group 15-17 form such compounds. (NH₃, H₂O and HF, has one, two and there lone pair respectively).

3.      They will behave as Lewis bases

Metallic or Non-stoichiometric (or Interstitial ) Hydrides

1.       These are formed by many d-block and f-block elements.

2.      The metals of group 7, 8 and 9  do not form hydride.

3.      From group 6, only Chromium forms CrH.

4.      They are almost always nonstoichiometric, being deficient in hydrogen

5.      They were termed as interstitial hydride

6.      The property of absorption of hydrogen on transition metals is widely used in catalytic reduction / hydrogenation reactions

LaH_2.87 YbH_2.55 YiH_1.5-1.8 ZrH_1.3-1.75

Water

Human body has about 65% and some plants have as much as 95% water

Extensive hydrogen bonding

High freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to H₂S and H₂Se

Heavy Water (D₂O):

It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms.

It can be prepared by exhaustive electrolysis of water.

Dielectric constant of H₂O is more than D₂O

How many hydrogen-bonded water molecule(s) are associated in CuSO₄.5H₂O

Density of ice is less than that of water. Therefore, an ice cube floats on water.

Ice has a highly ordered three dimensional hydrogen bonded structure

Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm

Chemical Properties of Water

1.       Amphoteric Nature

2.      Redox Reactions

3.      Hydrolysis

4.      Hydrates

Hard and Soft Water

Hard water:

Presence of calcium and magnesium salts in the form of

hydrogen carbonate, chloride and sulphate in water makes water ‘hard’.

Hard water does not give lather with soap.

Soft water

Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.

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Hardness of water is of two types:

(i) Temporary hardness       (ii) Permanent hardness.

Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates.

It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.

Hydrogen Peroxide (H₂O₂)

Preparation

Barium peroxide

BaO₂.8H₂O(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O₂(aq) + 8H₂O(l)

Peroxo Disulphate:

2HSO^–₄(aq) – Electrolysis → HO₃SOOSO₃H (aq) – Hydrolysis → 2HSO^–₄(aq) + 2H⁺ (aq) + H₂O₂ (aq)

Industrial method: by oxidation of 2-alklylanthraquinols.

2 – ethylanthraquinol ⇌ H₂O₂ + Oxidised Product