Science Class 11 - Hydrogen Notes

Hydrogen

Welcome to the chapter on Hydrogen for Class 11. In this chapter, you will learn about the position of hydrogen in the periodic table, its occurrence, properties, preparation, uses, and compounds. By the end of this chapter, you will understand why hydrogen is called the "fuel of the future" and its importance in chemistry and industry.

Introduction

Hydrogen is the first element in the periodic table and the lightest known element. Its symbol is H and its atomic number is 1. Hydrogen is a colorless, odorless, tasteless, and highly flammable gas.

Position in the Periodic Table

• Placed at the top of Group 1 (alkali metals), but also shows similarities with Group 17 (halogens).
• Electronic configuration: 1s¹
• Can lose or gain one electron, forming H⁺ or H^-.

Occurrence

• Most abundant element in the universe.
• Exists as diatomic molecules (H₂).
• Found in water (H₂O), acids, organic compounds, and living organisms.

Isotopes of Hydrogen

• Protium (¹H): Most common, no neutrons.
• Deuterium (²H or D): One neutron, used in heavy water.
• Tritium (³H or T): Two neutrons, radioactive.

Preparation of Hydrogen

• Laboratory preparation: Reaction of dilute acids with metals (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂↑).
• Industrial preparation: Electrolysis of water, steam reforming of hydrocarbons.

Physical Properties

• Colorless, odorless, tasteless gas.
• Lightest element (density much less than air).
• Insoluble in water.
• Highly combustible.

Chemical Properties

• Burns in air to form water: 2H₂ + O₂ → 2H₂O
• Reduces metal oxides to metals (e.g., CuO + H₂ → Cu + H₂O).
• Forms covalent compounds with non-metals (e.g., HCl, NH₃).

Uses of Hydrogen

• Used in the manufacture of ammonia (Haber process).
• Used in hydrogenation of oils.
• Used as rocket fuel and in fuel cells.
• Used in the production of methanol and hydrochloric acid.

Compounds of Hydrogen

• Water (H₂O): Essential for life.
• Hydrogen peroxide (H₂O₂): Used as a bleaching agent and antiseptic.
• Hydrides: Compounds of hydrogen with metals and non-metals.

Practice Questions

1. Name the three isotopes of hydrogen and write one use of each.
2. How is hydrogen prepared in the laboratory?
3. Why is hydrogen called the lightest element?
4. Write two chemical properties of hydrogen.
5. List two uses of hydrogen in industry.

Challenge Yourself

• Explain why hydrogen can be placed in both Group 1 and Group 17 of the periodic table.
• Describe the role of hydrogen in the fuel cell and its importance for the future.

Did You Know?

• Hydrogen is the most abundant element in the universe, making up about 75% of its mass.
• The sun and other stars are mainly made of hydrogen.

Glossary

• Isotope: Atoms of the same element with different numbers of neutrons.
• Hydride: A compound of hydrogen with another element.
• Electrolysis: Breaking down a compound using electricity.
• Fuel cell: A device that converts hydrogen and oxygen into electricity and water.

Answers to Practice Questions

1. Protium (used in ordinary water), Deuterium (used in heavy water for nuclear reactors), Tritium (used in luminous paints and nuclear fusion).
2. By reacting a metal like zinc with dilute sulphuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂↑
3. It has the lowest atomic mass (1 u) among all elements.
4. It burns in air to form water; it reduces metal oxides to metals.
5. Manufacture of ammonia, hydrogenation of oils, used as rocket fuel.

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Hydrogen is the element of the future—clean, powerful, and essential for life and industry!