Hydrogen
1. Introduction
Hydrogen (H₂) is the LIGHTEST element. Atomic number 1. It is a colourless, odourless, tasteless gas.
'About 75% of the UNIVERSE's mass is hydrogen. It is the MOST abundant element in the universe, but on Earth it is mostly found in COMBINED form — in water (H₂O), hydrocarbons, and living things.'
2. Laboratory Preparation of Hydrogen
Reactants: Zinc granules + Dilute Sulphuric Acid (or Dilute Hydrochloric Acid)
Equation: Zn + H₂SO₄ → ZnSO₄ + H₂↑
Apparatus:
- Woulfe's bottle (or flat-bottomed flask) as the reaction vessel
- Thistle funnel (for adding acid drop by drop)
- Delivery tube
- Gas jar (for collection)
- Trough of water
Collection: By DOWNWARD DISPLACEMENT of WATER (hydrogen is INSOLUBLE in water and LIGHTER than air).
3. Purification
The hydrogen gas produced is IMPURE. It may contain:
- H₂S (hydrogen sulphide — due to impurities in zinc)
- HCl vapour (from hydrochloric acid)
- Water vapour
Purification: Pass through:
- LEAD NITRATE or LEAD ACETATE solution (removes H₂S)
- CONCENTRATED H₂SO₄ (drying agent — removes water vapour)
- SILVER NITRATE solution (removes HCl vapour)
4. Physical Properties
| Property | Description |
|---|---|
| Colour | COLOURLESS |
| Odour | ODOURLESS |
| Taste | TASTELESS |
| Density | 0.09 g/L — 14 times LIGHTER than air (LIGHTEST known gas) |
| Solubility | VERY SLIGHTLY soluble in water |
| Liquefaction | Very DIFFICULT to liquefy (BP —252.9°C) |
5. Chemical Properties
Combustion (Burns in Air)
2H₂ + O₂ → 2H₂O
Hydrogen burns with a PALE BLUE flame. The product is WATER.
Pop Test: A burning splinter near hydrogen gas makes a 'POP' sound — this is a TEST for hydrogen.
Reaction with Non-Metals
H₂ + Cl₂ → 2HCl (hydrogen burns in chlorine — forms hydrogen chloride) N₂ + 3H₂ → 2NH₃ (Haber's process — ammonia synthesis)
Reaction with Metal Oxides (REDUCING AGENT)
Hydrogen ACTS as a reducing agent — it REMOVES oxygen from metal oxides.
CuO + H₂ → Cu + H₂O (black CuO turns BROWN — copper metal) Fe₂O₃ + 3H₂ → 2Fe + 3H₂O WO₃ + 3H₂ → W + 3H₂O
'In these reactions, hydrogen GAINS oxygen (oxidation of H₂) and the metal oxide LOSES oxygen (reduction).'
6. Uses of Hydrogen
| Use | Details |
|---|---|
| MANUFACTURE of ammonia | Haber's process — N₂ + 3H₂ → 2NH₃ (fertilisers) |
| MANUFACTURE of HCl | H₂ + Cl₂ → 2HCl |
| HYDROGENATION of oils | Vegetable oils → Vanaspati ghee (solid fats) |
| ROCKET fuel | Liquid hydrogen + liquid oxygen |
| WELDING | Atomic hydrogen welding (very high temperature) |
| METALLURGY | Reducing agent to extract metals from oxides |
| FUEL CELLS | Generate electricity (clean — only water byproduct) |
7. Hydrogen as a Clean Fuel — The Future
'Hydrogen is being hailed as the FUEL OF THE FUTURE because it BURNS CLEANLY — producing ONLY WATER as a byproduct, with ZERO carbon emissions.'
Advantages as a fuel:
- HIGH energy content per unit mass
- ZERO pollution (only water vapour)
- Can be produced from WATER (abundant source)
- Can be used in FUEL CELLS for vehicles and power generation
Challenges:
- Production is ENERGY-INTENSIVE (electrolysis needs electricity)
- STORAGE is difficult (hydrogen is very light → needs high pressure)
- TRANSPORT and infrastructure are expensive
- SAFETY concerns (highly FLAMMABLE)
Green Hydrogen: Hydrogen produced using RENEWABLE energy (solar, wind) to power electrolysis — CARBON NEUTRAL.
8. Comparison: Hydrogen vs Other Fuels
| Property | Hydrogen | Petrol | Natural Gas |
|---|---|---|---|
| Energy per kg (MJ/kg) | 142 | 44 | 54 |
| CO₂ emissions | ZERO | HIGH | MEDIUM |
| Byproduct | Water only | CO₂, NOₓ, SO₂ | CO₂ |
| Renewable? | YES (via electrolysis) | NO | NO |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Hydrogen is collected by upward displacement of air' | It CAN be, but DOWNWARD displacement of WATER is safer and gives PURER hydrogen |
| 'Pop test proves it is hydrogen' | A 'POP' with a burning splinter is a CONFIRMATORY test for hydrogen |
| 'H₂ burns with a yellow flame' | Pure H₂ burns with a PALE BLUE flame, not yellow. Yellow indicates IMPURITIES |
| 'Zn + dil HNO₃ gives H₂' | Nitric acid (HNO₃) is an OXIDISING agent — it does NOT produce hydrogen with metals. Use H₂SO₄ or HCl |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Physical properties of hydrogen
- 3-mark questions: Laboratory preparation with diagram
- 4-mark questions: Hydrogen as a reducing agent — reactions
- 6-mark questions: Uses of hydrogen and its potential as a clean fuel
Self-Test
Q1. How is hydrogen collected in the laboratory? Why? A1. By DOWNWARD displacement of water. Hydrogen is INSOLUBLE in water and LIGHTER than air.
Q2. Write the equation for the reaction of zinc with dilute sulphuric acid. A2. Zn + H₂SO₄ → ZnSO₄ + H₂↑
Q3. Describe the pop test for hydrogen. A3. Bring a BURNING splinter near the gas. If it is hydrogen, it burns with a 'POP' sound (the hydrogen combines explosively with oxygen in the air).
Q4. Why is hydrogen called a reducing agent? A4. Hydrogen REMOVES oxygen from metal oxides. Example: CuO + H₂ → Cu + H₂O. Hydrogen GAINS oxygen (oxidised itself) and the metal LOSES oxygen (reduced).
Q5. Name two uses of hydrogen in industry. A5. (1) Manufacture of ammonia (Haber's process) for fertilisers. (2) Hydrogenation of vegetable oils to make vanaspati ghee.
Q6. What is 'green hydrogen'? A6. Green hydrogen is hydrogen produced by ELECTROLYSIS of water using electricity from RENEWABLE sources (solar, wind). It has ZERO carbon footprint.
