9.1 Hydrogen, Oxygen, and Ozone

• The simplest and lightest element in the periodic table.
• The most abundant element in the known universe.
• Discovered by Henry Cavendish (1776) by passing steam through iron.
• Named inflammable air, then.
• Later, Lavoisier named it hydrogen as it produced water when burnt.
• Exists as a diatomic molecule (H₂) under standard conditions.

9.1.1 Chemistry of atomic and nascent hydrogen

Atomic hydrogen

• Consists of individual hydrogen atoms (H)
• Generally produced by passing molecular hydrogen through an electric arc between two tungsten electrodes at about 3000 °C.
• Exceptionally strong reducing agent, more powerful than nascent hydrogen; can reduce oxides and chlorides of metals like silver, mercury, copper, and lead.

It can reduce BaSO₄ to BaS and CO to formaldehyde (Methanal).

Uses of atomic hydrogen:

• As a reducing agent
• In an atomic hydrogen torch for welding metals of very high melting points (refractory metals, e.g., tungsten, molybdenum).

Nascent Hydrogen

• Nascent hydrogen refers to freshly generated hydrogen at the moment of formation.
• Ordinary molecular hydrogen gas, when passed through acidified KMnO₄, K₂Cr₂O₇ & FeCl₃ solutions separately, doesn’t reduce them.
• But nascent hydrogen produced in situ, in the medium itself containing the oxidising agents, reduces them. This shows nascent hydrogen is a more powerful reducing agent than ordinary hydrogen gas.

• Earlier assumed that nascent hydrogen is atomic hydrogen.
• But at present, it is thought that a part of the heat change of the chemical reaction is imparted to the hydrogen in situ, making it more energetic.
• The internal pressure of the tiny bubbles formed in the solution also contributes to its higher reactivity.

Uses of nascent hydrogen:

• As a reducing agent.

9.1.2 Isotopes of hydrogen and their uses

There are three isotopes of hydrogen, viz. Protium, Deuterium, and Tritium.

Protium, ₁H¹,   Deuterium, ₁H², D,   Tritium, ₁H³, T

These isomers differ from one another in the number of neutrons in the nucleus, as shown above. Protium does not have any neutrons. Deuterium has 1 neutron, and tritium has 2 neutrons in the nucleus. Their mass numbers are 1, 2, and 3, respectively.

The three isotopes have the same atomic number and electronic configuration. Hence, they have the same chemical properties.

Due to different atomic masses, their physical properties and rate of reactions differ.

The relative abundances of protium, deuterium, and tritium are 99.985 %, 0.015 %, and 10⁻¹⁵ %, respectively.

Uses of Hydrogen (or Protium)

• In the industrial synthesis of ammonia by Haber’s process.
• In the hydrogenation of vegetable oil into vegetable ghee (margarine).
• In rockets and missiles as a fuel.
• In metallurgical processes as a reducing agent.
• Promising future clean fuel: produces only water on combustion, with zero carbon emissions, if water could be decomposed economically by using solar or other renewable energy.
• In filling balloons and airships

Uses of Heavy hydrogen (deuterium)

• As a tracer in the study of the reaction mechanism.
• In the transformation of elements (i.e., nuclear reactions)

Uses of Tritium (Radioactive isotope)

• As a radioactive tracer in biology, chemistry, medicine, etc.
• In nuclear reactions to produce energy.
• In exit signs in dark halls.

9.1.3 Application of hydrogen as fuel

Because of its high inflammability, pollution-free combustion, and high enthalpy of combustion, hydrogen is used as a fuel and considered a very promising future fuel. It can be used in fuel cells and internal combustion engines.

A wide variety of research is currently being conducted on this topic.

Hydrogen is currently used as fuel in rockets and missiles.

It could be used as our future fuel without polluting the environment if water could be decomposed economically using solar or other renewable energy.

Advantages of hydrogen as a fuel:

• High energy efficiency
• Mitigation of greenhouse gases (green fuel)
• Reduction of petroleum use
• Reduction of air pollution
• Zero carbon emissions

Challenges in the use of hydrogen as a fuel

• High cost of hydrogen, its storage, transportation, and distribution technology.
• Takes up more space to store in the fuel tank
• Difficult to store in a high-pressure tank or cryogenic tank.

9.1.4 Heavy water and its applications

Deuterated water (D₂O or ₁H²₂O) or deuterium oxide is called heavy water. It is obtained in residual water after electrolysis of a large amount of water. This is employed

• As a moderator in nuclear reactors to slow down the neutrons, as well as a coolant at the same time.
• In the preparation of heavy hydrogen (deuterium, D₂) and other deuterated compounds.
• In scientific research involving isotopic labeling.

• Discovered by Joseph Priestley in 1772 and by Scheele in 1773.
• Named by Antoine Lavoisier after its detailed study.
• 3^rd most abundant element in the Earth’s crust & the human body.

• Oxygen has 3 isotopes ₈O¹⁶, ₈O¹⁷, ₈O¹⁸, their relative abundances are 99.76%, 0.03% and 0.204% respectively.

9.2 Allotropes of Oxygen

The property by virtue of which an element exists in different forms with different physical properties and the same chemical properties is called allotropy. Such different forms of the same element are called allotropes.

Oxygen shows allotropy. Oxygen (O₂) and ozone (O₃) are allotropes.

9.2.2 Types of oxides

Binary compounds of other elements with oxygen are called oxides. These can be classified on a different basis as discussed below.

Classification based on acidic or basic behaviour

a) Basic oxides

The oxides that form a base in combination with water are called basic oxides. Generally, metal oxides are basic. e.g.

The basic oxides react with an acid to form a salt and water.

b) Acidic oxides

The oxides that form acids in combination with water are called acidic oxides. Generally, oxides of non-metals are acidic. E.g.

Acidic oxides react with bases to form salt and water.

c) Neutral oxides

Neutral oxides do not react with an acid or a base to form salt, e.g., NO, CO, H₂O, N₂O, etc.

d) Amphoteric oxides

These are oxides that react with acids as well as bases to form salts. Oxides of Al, Zn, Sn, and Pb are amphoteric oxides.

Classification of oxides based on structure

a) Peroxides

These oxides contain the group (–O–O–)²⁻, in which the oxidation number of oxygen is −1. E.g., H₂O₂, Na₂O₂, BaO₂, R–O–O–R (ROOR), etc.

b) Mixed oxides

These oxides behave as if they were the mixtures of two simpler oxides of the same element.

e.g. Fe₃O₄ (FeO + Fe₂O₃) – ferroso ferric oxide, Pb₃O₄ (PbO + PbO₂)

Summary Table: Types of Oxides

9.2.3 Applications of hydrogen peroxide

• H₂O₂ is an important peroxide with strong oxidising properties.
• Pale blue liquid that decomposes readily into water and oxygen.

Applications

• As a bleaching agent for wool, pulp, paper, silk, cotton, wool, etc.
• To produce different chemicals by oxidation reaction (as an oxidising agent).
• As an antiseptic, to clean wounds & in some sanitisers also
• To prepare mouthwash (for dental hygiene – due to antiseptic properties)
• To produce organic peroxides (ROOR), some of which are explosives too
• As a propellant in rockets

9.2.4 Medical and industrial applications of oxygen

It is the most essential element for life on Earth. Apart from being used in respiration and combustion. It is used in

1. In the Manufacture of steel
2. As an oxidising and bleaching agent in different chemical industries
3. Oxy-hydrogen and oxy-acetylene flames in the cutting and welding of metals.
4. Artificial respiration in hospitals

All living cells need a constant supply of oxygen for aerobic respiration. It is used by the cells to oxidise the food to produce energy.

Lack of oxygen for a few minutes leads to cell death. Therefore, in a wide variety of medical conditions (hypoxemia, chronic obstructive pulmonary disease, pneumonia, heart attack, respiratory diseases, during surgeries, etc.), oxygen should be given to the patients.

5. Mountaineering, diving, and space travelling.
6. Fuel for rockets and missiles.

9.3.1 Occurrence

The ozone layer lies in the stratosphere, 12 to 24 km above the Earth’s surface. Ozone is naturally formed in the stratosphere as O₂ molecules absorb the high-energy ultraviolet rays of the sun. UV radiation splits O₂ molecules into oxygen atoms that combine with oxygen molecules to form ozone.

9.3.2 Preparation of ozone from oxygen

It is prepared in the laboratory by passing silent electric discharge (i.e., without any spark) or ultraviolet radiation in a spherically designed instrument called an ozonizer. It is an endothermic reaction.

9.3.3 Structure of ozone

The ozone molecule has an angular (bent) shape with a bond angle of approximately 116.8°.

The two O–O bond lengths are equal (1.28 Å), intermediate between an O–O single bond and an O=O double bond. This is because ozone is best described as a resonance hybrid of two equivalent Lewis structures:

But it is found to have a dipole moment of 0.53 D. Hence, its structure may be represented as:

9.3.4 Test for ozone

The test of ozone may be based on one of its following properties.

1. It is a pale blue gas with a distinctively pungent fishy smell.
2. When ozone is passed into a mixture of KI solution and starch solution, blue colouration appears. The involved reaction may be shown as;
   O₃  →  O₂ + O

   2 KI + O  →  K₂O + I₂

   K₂O + H₂O  →  2 KOH

   2 KI + H₂O + O₃  →  2 KOH + O₂ + I₂

   I₂ + starch  →  Blue coloration
3. Tailing of mercury test: Mercury does not stick to the glass surface. However, when exposed to the atmosphere of ozone, it loses its meniscus and sticks to the glass surface. This is due to the formation of mercurous oxide as shown below.
   2 Hg + O₃  →  Hg₂O + O₂

9.3.5 Ozone layer depletion (causes, effects, and control measures)

🌍 The Ozone Layer: Our Planetary Shield

9.3.6 Uses of Ozone

1. As a germicide and as a disinfectant for sterilising water.
2. To improve the atmosphere of crowded places.
3. As an oxidising agent in different chemical industries.
4. In food processing and cold store sanitation.
5. For bleaching oils, ivory articles, and other soft materials.
6. In the ozonolysis of unsaturated hydrocarbons.