Hydrogen and Its Compounds

Hydrogen is the lightest and simplest element in the periodic table.

i. Symbol: H

ii. Atomic number: 1

iii. Electronic configuration: 1s¹

It is a colourless, odourless, and tasteless gas.

Hydrogen is unique because it can behave like both:

i. Alkali metals (Group I) → loses 1 electron

ii. Halogens (Group VII) → gains 1 electron

Occurrence of Hydrogen

Hydrogen is the most abundant element in the universe, but it is not found as a free element on Earth. It occurs mainly in the combined state in:

i. Water (H₂O)

ii. Hydrocarbons (petroleum, natural gas)

ii. Acids and organic compounds

Laboratory Preparation of Hydrogen

Hydrogen is commonly prepared by reacting metals with dilute acids:

Zn + 2HCl → ZnCl2 + H2

Apparatus used:

• Conical flask

• Delivery tube

• Water trough (for collection)

Industrial Preparation of Hydrogen

(a) Electrolysis of Water

Water is decomposed into hydrogen and oxygen using electricity.

(b) Steam Reforming

Methane reacts with steam at high temperature:

CH4 + H2O  → CO + 3H2

Physical Properties of Hydrogen

i. Colourless, odourless gas

ii. Very light (least dense gas)

iii. Slightly soluble in water

iv. Highly combustible

Chemical Properties of Hydrogen

(a) Combustion

Hydrogen burns in oxygen to form water:

2H2 + O2  → 2H2O

(b) Reducing Property

Hydrogen reduces metal oxides:

CuO + H2  → Cu + H2O

(c) Reaction with Non-metals

i. With chlorine → HCl

ii. With nitrogen → NH₃

Uses of Hydrogen

i.  Manufacture of ammonia (Haber Process)

ii. Hydrogenation of oils (margarine production)

iii. Fuel (rocket fuel, clean energy)

iv. Welding (oxy-hydrogen flame)

 

Activity Series of Hydrogen

This is an arrangement of metals based on their ability to displace hydrogen from cold water, steam or acids. The activity series helps us predict whether a metal can displace hydrogen from acids.

Activity Series (Simplified)

K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au

Key Idea:

1. Metals above hydrogen → react with dilute acids to produce hydrogen gas

2. Metals below hydrogen → do NOT produce hydrogen gas

Examples:

• Zinc + Acid → Hydrogen produced 

• Copper + Acid → No reaction 

Important Reactions

Zn + H2SO4  → ZnSO4 + H2

Cu + HCl  →  (No reaction)

Compounds of Hydrogen

1.  Water (H₂O)

Properties:

i. Colourless liquid

ii. Neutral (pH ≈ 7)

iii. Universal solvent

Uses:

i. Drinking

ii. Industrial cooling

iii. Chemical reactions

2. Hydrogen Chloride (HCl)

i. It is a colourless gas with pungent smell

ii. It is forms hydrochloric acid in water

Uses:

i. Cleaning metals

ii. Laboratory reagent

3. Ammonia (NH₃)

i. It is a colourless gas with sharp pungent smell

ii. It is highly soluble in water

Uses:

i. It is used in manufacturing Fertilizers

ii. It is used as aRefrigeration

iii. It is used as Cleaning agents

4.  Hydrogen Sulphide (H₂S)

i. Colourless gas with rotten egg smell

ii. Toxic and poisonous

Test:

Turns lead(II) ethanoate paper black

5.  Methane (CH₄)

i.  Simplest hydrocarbon

ii. Main component of natural gas

Uses:

i. Fuel

ii. Chemical production

6.  Safety Precautions

i. Hydrogen is highly flammable → avoid open flames

ii. H₂S is toxic → handle in well-ventilated areas

iii. Always test gases properly in the lab

 Summary

i. Hydrogen is the lightest element and very reactive

ii. It acts as a reducing agent

iii. Metals above hydrogen in the activity series produce H₂ gas

iv. Important compounds include water, ammonia, HCl, H₂S,