Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
Hydrogen is the most abundant element in the universe and the simplest atom. This chapter covers its unique position in the periodic table, its isotopes, types of hydrides, properties of water and heavy water, and the chemistry of hydrogen peroxide. Board exams consistently pick 3–5 mark questions from this chapter.
Key Concepts and Definitions
- Protium (¹H): Most abundant isotope; 99.985% natural abundance
- Deuterium (D, ²H): Heavy hydrogen; used in making D₂O
- Tritium (T, ³H): Radioactive; used in nuclear fusion research
- Hydrides: Compounds of hydrogen with other elements
- Ionic hydrides: Formed with active metals (e.g., NaH)
- Covalent hydrides: Formed with non-metals (e.g., H₂O, NH₃)
- Metallic hydrides: Formed with d-block metals; non-stoichiometric
- Syngas: CO + H₂ mixture; used in industrial synthesis
- Ortho-hydrogen: H₂ with parallel nuclear spins (predominant at room temperature)
- Para-hydrogen: H₂ with antiparallel nuclear spins (more stable at low T)
Formulae and Key Facts
| Property | Value |
|---|---|
| Bond dissociation energy of H₂ | 435.8 kJ/mol |
| Density of H₂ | Lightest gas |
| Boiling point of H₂O | 100°C (due to H-bonding) |
| Boiling point of D₂O | 101.4°C |
| Strength of H₂O₂ | Expressed as volume strength |
| 10 vol H₂O₂ | Gives 10 L O₂ per litre |
Types of Hydrides
| Type | Bonding | Examples | Properties |
|---|---|---|---|
| Ionic | Ionic | NaH, CaH₂ | High melting, conduct when molten |
| Covalent | Covalent | H₂O, HCl, NH₃ | Molecular; gas or liquid |
| Metallic | Metallic | TiH₁.₇₃ | Non-stoichiometric; good conductors |
Water and Heavy Water
Anomalous properties of water:
- Maximum density at 4°C (ice less dense than liquid water)
- High boiling point and specific heat due to hydrogen bonding
- Good solvent ("universal solvent") due to polarity
Heavy water (D₂O):
- Slightly higher boiling point, density, and viscosity than H₂O
- Slows down biological reactions
- Used as moderator in CANDU nuclear reactors
Hydrogen Peroxide (H₂O₂)
Preparation: Na₂O₂ + H₂SO₄ → H₂O₂ + Na₂SO₄ (lab method) Structure: Non-planar; O–O bond; book-like open structure Reactions:
- Oxidising agent: 2KI + H₂O₂ → I₂ + 2KOH
- Reducing agent: PbO₂ + H₂O₂ → PbO + H₂O + O₂
- Bleaching agent (oxidises coloured matter)
Volume strength: 10 vol H₂O₂ means 1 L releases 10 L of O₂ at STP
Important Questions
One Mark (5)
- Name the three isotopes of hydrogen.
- What is heavy water? Give its one use.
- Which type of hydride is NaH?
- State one anomalous property of water.
- What is syngas?
Short Answer 3-mark (3)
- Distinguish between ortho and para hydrogen.
- Explain the structure of hydrogen peroxide.
- Why does water have an unusually high boiling point compared to H₂S?
Long Answer 5-mark (2)
- Describe the physical and chemical properties of hydrogen peroxide. Why is it stored in dark bottles?
- Explain the types of hydrides with examples and properties.
Common Mistakes
- Thinking H₂O₂ is a planar molecule — it is non-planar (open book structure).
- Confusing volume strength with molarity.
- Stating that H₂O has maximum density at 0°C — it's at 4°C.
- Writing heavy water formula as H₂O² instead of D₂O.
Board Exam Tips
- H₂O₂ structure and uses are a consistent 3-mark question.
- Anomalous properties of water may come as 5-mark in some years.
- Know the industrial preparation of hydrogen (steam reforming: CH₄ + H₂O → CO + 3H₂).
- NEET: Focus on hydride types and H₂O₂ as oxidising/reducing agent.
Revision Checklist
- [ ] Three isotopes with mass and neutron count noted
- [ ] Types of hydrides with one example each written
- [ ] Structure of H₂O₂ drawn
- [ ] Volume strength formula practised with a sum
- [ ] Anomalous properties of water listed (5 points)
Frequently Asked Questions
Hydrogen has a unique dual character — it resembles alkali metals (loses one electron) and halogens (gains one electron). It is placed at the top of Group 1 but treated separately.