Hydrogen
Welcome to HSLC Guru. This page presents complete English-medium notes and question answers for ASSEB Class 11 Chemistry Chapter 9 — Hydrogen. Hydrogen is the simplest and the most abundant element in the universe and occupies a unique position in the Periodic Table because of its dual character. The chapter explores its position, isotopes, methods of preparation, properties, hydrides, the chemistry of water and heavy water, hydrogen peroxide, hard water and its softening, and the role of hydrogen as a future fuel. The notes follow the ASSEB syllabus and are designed for quick revision and exam preparation.
Summary
Position and isotopes: Hydrogen has the electronic configuration 1s¹. It resembles alkali metals (Group 1) in losing one electron to form H⁺ and resembles halogens (Group 17) in gaining an electron to form H⁻. Because of this dual character, its position is anomalous; it is generally placed at the top of Group 1. Hydrogen has three isotopes — protium (¹H), deuterium (²H or D) and tritium (³H or T). Protium is the most abundant; tritium is radioactive and emits β-particles. The isotopes have the same chemical properties but differ in physical properties and rates of reaction.
Preparation: Industrially, hydrogen is prepared by (i) Bosch’s process — passing steam over red-hot coke gives water gas (CO + H₂); the CO is then converted to CO₂ using steam over Fe₂O₃/Cr₂O₃ catalyst and absorbed in water under pressure to leave pure H₂; (ii) Lane’s process — steam passed over heated iron at 1023 K gives Fe₃O₄ + H₂; the oxide is reduced back by water gas; (iii) Electrolysis of acidified or alkaline water gives very pure hydrogen at the cathode. In the laboratory, hydrogen is prepared by the action of dilute H₂SO₄ on granulated zinc.
Properties and uses: Hydrogen is a colourless, odourless, tasteless, highly combustible gas. It reacts with oxygen to form water, with halogens to form hydrogen halides, with nitrogen to form ammonia (Haber’s process), and reduces metal oxides to metals. It forms three classes of hydrides — ionic (saline) hydrides like NaH, CaH₂; covalent (molecular) hydrides like CH₄, NH₃, H₂O; and metallic (interstitial) hydrides formed by d- and f-block metals. Hydrogen is used in the manufacture of ammonia, methanol and vanaspati ghee, in oxy-hydrogen and atomic-hydrogen welding, in metallurgy as a reducing agent, and as a clean fuel.
Water, hard water, H₂O₂ and heavy water: Water is a polar molecule with a bent structure (∠H–O–H ≈ 104.5°) and shows extensive hydrogen bonding which accounts for its high boiling point, high specific heat and the lower density of ice. In ice, each water molecule is hydrogen-bonded to four others giving an open tetrahedral cage-like structure. Hard water contains soluble Ca²⁺/Mg²⁺ salts; temporary hardness (bicarbonates) is removed by boiling or Clark’s process; permanent hardness (sulphates/chlorides) is removed by washing soda, the permutit (zeolite) process, or ion-exchange resins. Hydrogen peroxide (H₂O₂) is prepared by Merck’s process, by electrolysis of 50% H₂SO₄, or industrially by the auto-oxidation of 2-ethyl anthraquinol; it has an open-book / skew structure and acts both as an oxidising and a reducing agent. Heavy water (D₂O) is obtained by prolonged electrolysis of ordinary water and is used as a moderator in nuclear reactors. Because of high calorific value and clean combustion, hydrogen is regarded as the fuel of the future.
Short Answer Questions (1 Mark)
Q1. Write the electronic configuration of hydrogen.
Answer: The electronic configuration of hydrogen is 1s¹.
Q2. Name the three isotopes of hydrogen.
Answer: Protium (¹H), Deuterium (²H or D) and Tritium (³H or T).
Q3. Which isotope of hydrogen is radioactive?
Answer: Tritium (³H) is radioactive and emits β-particles with a half-life of about 12.33 years.
Q4. What is water gas?
Answer: The mixture of CO and H₂ obtained by passing steam over red-hot coke at about 1270 K is called water gas. C + H₂O → CO + H₂.
Q5. Give one example each of an ionic and a covalent hydride.
Answer: Ionic hydride — NaH; Covalent hydride — CH₄ (methane).
Q6. What is heavy water?
Answer: Heavy water is deuterium oxide, D₂O, in which the hydrogen atoms of ordinary water are replaced by deuterium.
Q7. Name the process used to obtain heavy water.
Answer: Heavy water is obtained by the prolonged (exhaustive) electrolysis of ordinary water containing a little alkali.
Q8. What is the bond angle in a water molecule?
Answer: The H–O–H bond angle in a water molecule is about 104.5°.
Q9. Give the chemical formula of washing soda.
Answer: Washing soda is Na₂CO₃·10H₂O.
Q10. Why is hydrogen called the fuel of the future?
Answer: Because it has very high calorific value (about 150 kJ/g), produces only water on combustion (no pollution) and its source — water — is virtually unlimited.
Q11. What is the chemical formula of hydrolith?
Answer: Hydrolith is calcium hydride, CaH₂.
Q12. Name the catalyst used in Bosch’s process for the conversion of CO into CO₂.
Answer: A mixture of Fe₂O₃ and Cr₂O₃ (chromium-iron oxide catalyst) at 770 K.
Short Answer Questions (2–3 Marks)
Q1. Justify the position of hydrogen in the Periodic Table.
Answer: Hydrogen has only one electron (1s¹). Like alkali metals, it loses this electron to form H⁺ and shows +1 oxidation state, electropositive character, and forms halides (HCl) and oxides (H₂O). Like halogens, it can gain an electron to form H⁻ (hydride ion), is a non-metal, and exists as a diatomic molecule H₂. Because of this dual character, its position is debatable; conventionally it is placed at the top of Group 1, although some tables place it above Group 17 as well.
Q2. Explain Bosch’s process for the preparation of hydrogen.
Answer: Steam is passed over white-hot coke at about 1270 K to give water gas: C + H₂O → CO + H₂. The water gas is mixed with twice its volume of steam and passed over Fe₂O₃ + Cr₂O₃ catalyst at 770 K, when CO is oxidised to CO₂: CO + H₂O → CO₂ + H₂. The CO₂ is removed by passing the mixture through water at 25–30 atm pressure (CO₂ is soluble; H₂ is not), and any residual CO is absorbed in ammoniacal cuprous chloride. Pure hydrogen is left behind.
Q3. What is Lane’s process? Write the reactions involved.
Answer: In Lane’s process superheated steam is passed over scrap iron heated to 1023 K, when hydrogen is liberated: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. The Fe₃O₄ formed is reduced back to iron by passing water gas (CO + H₂) over it: Fe₃O₄ + 4CO → 3Fe + 4CO₂; Fe₃O₄ + 4H₂ → 3Fe + 4H₂O. The two stages are repeated alternately, so the iron acts essentially as a catalyst.
Q4. Distinguish between hard water and soft water.
Answer: Soft water readily forms lather with soap and contains no soluble salts of calcium or magnesium. Hard water does not produce lather easily because the Ca²⁺/Mg²⁺ ions form an insoluble scum (calcium/magnesium stearate) with soap. Hardness may be temporary (due to bicarbonates of Ca/Mg) or permanent (due to chlorides and sulphates of Ca/Mg).
Q5. Why does ice float on water?
Answer: In ice each water molecule is hydrogen-bonded tetrahedrally to four other molecules, producing an open cage-like structure with large empty spaces. This makes ice less dense (≈ 0.917 g cm⁻³) than liquid water (≈ 1.0 g cm⁻³ at 4 °C). Therefore ice floats on water.
Q6. Write a short note on the structure of H₂O₂.
Answer: H₂O₂ has a non-planar, open-book (skew) structure. The two O–H bonds lie in two different planes that meet along the O–O bond, like the two pages of a half-open book. In the gas phase, the O–O bond length is 147.5 pm, the O–H bond length is 95 pm, the H–O–O angle is 94.8°, and the dihedral angle between the two planes is 111.5°. In the solid state the dihedral angle is 90.2°.
Q7. Mention any three important uses of hydrogen.
Answer: (i) Manufacture of ammonia by Haber’s process, which is then used to prepare urea and other fertilisers. (ii) Hardening of vegetable oils into vanaspati ghee by catalytic hydrogenation in the presence of finely divided nickel. (iii) In oxy-hydrogen and atomic-hydrogen torches for cutting and welding metals because of the very high temperatures produced (about 4000 K).
Q8. Why is hydrogen peroxide stored in coloured (wax-lined) bottles?
Answer: Hydrogen peroxide is unstable and decomposes readily in the presence of light, heat, dust or alkali, giving water and oxygen: 2H₂O₂ → 2H₂O + O₂. Coloured glass bottles cut off light and the wax lining prevents alkali from the glass surface from catalysing the decomposition; a little urea or phosphoric acid is also added as a stabiliser.
Long Answer Questions (5–7 Marks)
Q1. Discuss the position of hydrogen in the Periodic Table and the dual character of hydrogen with examples.
Answer: Hydrogen is the lightest element, with the configuration 1s¹. Its position has remained anomalous because it shows resemblance to two different families of elements.
Resemblance with alkali metals (Group 1): (i) same valence-shell configuration (ns¹); (ii) electropositive character — loses one electron to give a unipositive ion (H⁺, Na⁺); (iii) forms similar oxides (H₂O, Na₂O), halides (HCl, NaCl), and sulphides (H₂S, Na₂S); (iv) is a strong reducing agent.
Resemblance with halogens (Group 17): (i) it is one electron short of the noble-gas configuration like halogens; (ii) it is a non-metal and exists as a diatomic molecule H₂ like Cl₂; (iii) ionisation enthalpy is high and comparable to halogens; (iv) it forms covalent compounds and an electrovalent hydride containing H⁻ (e.g., NaH) like Cl⁻ in NaCl.
Differences: Unlike alkali metals, hydrogen is a non-metal, has high ionisation enthalpy, and does not have the metallic lustre or conductivity. Unlike halogens, it has only one electron in the K-shell, no lone pairs, and is much less electronegative.
Because of this dual character, hydrogen is best placed separately at the top of the Periodic Table, although IUPAC conventionally places it at the head of Group 1.
Q2. Describe the preparation of hydrogen by (a) Bosch’s process, (b) Lane’s process, and (c) electrolysis of water.
Answer: (a) Bosch’s process: Steam is passed over red-hot coke at about 1270 K giving water gas, C + H₂O → CO + H₂. Water gas is mixed with about twice its volume of steam and passed over heated Fe₂O₃ + Cr₂O₃ catalyst at 770 K, when CO is converted to CO₂: CO + H₂O → CO₂ + H₂. The CO₂ is removed by dissolving in water at 25–30 atm pressure; traces of CO are absorbed in ammoniacal cuprous chloride solution. Pure hydrogen is obtained.
(b) Lane’s process: Superheated steam is passed over heated iron at 1023 K. Two alternating stages occur — Steam stage: 3Fe + 4H₂O → Fe₃O₄ + 4H₂↑; Reduction stage: Fe₃O₄ + 4CO → 3Fe + 4CO₂ (or with H₂ from water gas). The cycle is repeated and the same charge of iron yields hydrogen continuously.
(c) Electrolysis: Acidified water (a few drops of H₂SO₄) or alkaline water (with 15–20% NaOH) is electrolysed between nickel/iron electrodes. At the cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻; at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻. Pure hydrogen is liberated at the cathode and oxygen at the anode in the ratio 2:1 by volume.
Q3. What are hydrides? Classify them with examples and discuss their main characteristics.
Answer: Binary compounds of hydrogen with other elements are called hydrides. They are classified into three types.
1. Ionic (saline) hydrides: Formed by highly electropositive s-block metals of Group 1 and the heavier members of Group 2 (Ca, Sr, Ba). Examples — NaH, KH, CaH₂, BaH₂. They contain the H⁻ ion, are crystalline, non-volatile, non-conducting solids in the solid state but conduct electricity in the molten state, liberating H₂ at the anode.
2. Covalent (molecular) hydrides: Formed by p-block elements with non-metals (and a few metalloids). Examples — CH₄, NH₃, H₂O, HF, B₂H₆. They are usually volatile liquids or gases with low melting and boiling points. They are sub-classified into electron-deficient (B₂H₆), electron-precise (CH₄, SiH₄), and electron-rich hydrides (NH₃, H₂O, HF) depending on whether the central atom has fewer than, exactly, or more than the required number of electrons for normal covalent bonding.
3. Metallic (interstitial) hydrides: Formed by many d- and f-block metals (e.g., Pd, Ti, Zr, La). Hydrogen occupies interstitial sites in the metal lattice. They are often non-stoichiometric (e.g., LaH₂.₈₇, TiH₁.₇₃), retain metallic conductivity and lustre, and are useful for hydrogen storage.
Q4. What is hard water? Describe how temporary and permanent hardness can be removed.
Answer: Water that does not produce lather readily with soap because of the presence of soluble salts of calcium and magnesium is called hard water. Hardness is of two types.
Temporary hardness is due to soluble bicarbonates of Ca and Mg, e.g., Ca(HCO₃)₂ and Mg(HCO₃)₂. Removal: (i) Boiling — the bicarbonates decompose: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂; Mg(HCO₃)₂ → Mg(OH)₂↓ + 2CO₂. (ii) Clark’s process — calculated quantity of slaked lime is added: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O.
Permanent hardness is due to soluble chlorides and sulphates of Ca and Mg, e.g., CaCl₂, MgSO₄. Removal: (i) Washing-soda process — Na₂CO₃ converts the salts to insoluble carbonates: CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl. (ii) Permutit (zeolite) process — sodium aluminium silicate (Na₂Al₂Si₂O₈·xH₂O) exchanges its Na⁺ for Ca²⁺/Mg²⁺ ions; the exhausted permutit is regenerated by treatment with brine. (iii) Ion-exchange resin process — water is passed through a cation-exchanger (R–H) and an anion-exchanger (R–OH); all cations are replaced by H⁺ and all anions by OH⁻ giving deionised (demineralised) water.
Q5a. Compare the physical properties of ordinary water (H₂O) and heavy water (D₂O).
Answer: Although chemically very similar, the two differ noticeably in physical constants because of the mass difference between H and D.
(i) Molar mass: H₂O = 18 g mol⁻¹; D₂O = 20 g mol⁻¹.
(ii) Density at 20 °C: H₂O = 0.998 g cm⁻³; D₂O = 1.105 g cm⁻³ (heavy water is denser).
(iii) Melting point: H₂O = 273.0 K; D₂O = 276.8 K (heavy water freezes at a higher temperature).
(iv) Boiling point: H₂O = 373.0 K; D₂O = 374.4 K (heavy water boils at a higher temperature).
(v) Temperature of maximum density: H₂O = 277 K (4 °C); D₂O = 284.2 K (11.2 °C).
(vi) Heavy water is harmful to plant and animal life in high concentration. It is widely used as a moderator in nuclear reactors and as a tracer in studying reaction mechanisms.
Q5. Discuss the preparation, structure, properties and uses of hydrogen peroxide.
Answer: Preparation: (i) Merck’s process (Laboratory): a paste of hydrated barium peroxide is added to ice-cold dilute H₂SO₄ — BaO₂·8H₂O + H₂SO₄ → BaSO₄↓ + H₂O₂ + 8H₂O. (ii) Industrial preparation by electrolysis of 50% H₂SO₄ at low temperature gives peroxydisulphuric acid which on hydrolysis yields H₂O₂: 2H₂SO₄ → H₂S₂O₈ + 2H⁺ + 2e⁻; H₂S₂O₈ + 2H₂O → 2H₂SO₄ + H₂O₂. (iii) Auto-oxidation of 2-ethylanthraquinol (modern industrial route) — anthraquinol is oxidised in air to anthraquinone and H₂O₂; the anthraquinone is then reduced back catalytically by H₂.
Structure: H₂O₂ has a non-planar open-book structure with O–O = 147.5 pm, O–H = 95 pm, H–O–O = 94.8°, and dihedral angle 111.5° (gas) / 90.2° (solid).
Properties: Pure H₂O₂ is a colourless, syrupy liquid (b.p. 423 K, density 1.44 g cm⁻³). It is miscible with water in all proportions. It is a weak acid (Ka ≈ 1.5 × 10⁻¹²). It acts both as an oxidising agent (PbS + 4H₂O₂ → PbSO₄ + 4H₂O) and as a reducing agent (2KMnO₄ + 5H₂O₂ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 5O₂). It decomposes in light or in the presence of impurities: 2H₂O₂ → 2H₂O + O₂.
Uses: as an antiseptic and disinfectant (3% solution); as a bleaching agent for hair, silk, wool, and ivory; in rocket propellant systems; in the manufacture of sodium perborate (used in detergents); for restoring the colour of old oil paintings; and in the laboratory as a versatile redox reagent.
Multiple Choice Questions (MCQ)
Q1. The most abundant isotope of hydrogen is —
(a) Tritium (b) Deuterium (c) Protium (d) None of these
Answer: (c) Protium.
Q2. Which of the following is a saline hydride?
(a) CH₄ (b) NaH (c) NH₃ (d) PdH₀.₆
Answer: (b) NaH.
Q3. The H–O–H bond angle in water is approximately —
(a) 90° (b) 104.5° (c) 109.5° (d) 120°
Answer: (b) 104.5°.
Q4. Heavy water is —
(a) H₂O at 4 °C (b) D₂O (c) T₂O (d) H₂O₂
Answer: (b) D₂O.
Q5. Permanent hardness of water is due to the presence of —
(a) Bicarbonates of Ca and Mg (b) Carbonates of Na (c) Chlorides and sulphates of Ca and Mg (d) Nitrates of K
Answer: (c) Chlorides and sulphates of Ca and Mg.
Q6. Hydrogen peroxide acts as —
(a) Only oxidising agent (b) Only reducing agent (c) Both oxidising and reducing agent (d) Neither
Answer: (c) Both oxidising and reducing agent.
Q7. Water gas is a mixture of —
(a) CO₂ + H₂ (b) CO + H₂ (c) CH₄ + H₂ (d) N₂ + H₂
Answer: (b) CO + H₂.
Q8. Which process is used for the removal of permanent hardness using zeolite?
(a) Clark’s process (b) Permutit process (c) Bosch’s process (d) Lane’s process
Answer: (b) Permutit process.
Q9. The structure of H₂O₂ is best described as —
(a) Linear (b) Planar (c) Open-book / skew (d) Tetrahedral
Answer: (c) Open-book / skew.
Q10. Heavy water is mainly used in nuclear reactors as —
(a) Coolant only (b) Fuel (c) Moderator (d) Shield
Answer: (c) Moderator.
Fill in the Blanks
Q1. The electronic configuration of hydrogen is __________.
Answer: 1s¹.
Q2. The radioactive isotope of hydrogen is __________.
Answer: Tritium (³H).
Q3. Temporary hardness of water can be removed by __________.
Answer: Boiling (or by Clark’s process using slaked lime).
Q4. The dihedral angle in gaseous H₂O₂ is __________.
Answer: 111.5°.
Q5. The hydride NaH is an example of an __________ hydride.
Answer: Ionic (saline) hydride.
True or False
Q1. Hydrogen has the same valence-shell configuration as alkali metals.
Answer: True.
Q2. Ice is denser than liquid water at 4 °C.
Answer: False — ice is less dense than water at 4 °C.
Q3. CaH₂ is known as hydrolith.
Answer: True.
Q4. H₂O₂ acts only as an oxidising agent.
Answer: False — H₂O₂ acts both as an oxidising and as a reducing agent.
Q5. Heavy water is used as a moderator in nuclear reactors.
Answer: True.
Important Reactions to Remember
1. Laboratory preparation: Zn + dil. H₂SO₄ → ZnSO₄ + H₂↑
2. Water gas: C + H₂O → CO + H₂ (at 1270 K)
3. Lane’s process (steam stage): 3Fe + 4H₂O → Fe₃O₄ + 4H₂
4. Combustion: 2H₂ + O₂ → 2H₂O (highly exothermic)
5. Haber’s process: N₂ + 3H₂ ⇌ 2NH₃ (Fe catalyst, 200 atm, 700 K)
6. With chlorine: H₂ + Cl₂ → 2HCl (in sunlight)
7. Reduction of metal oxide: CuO + H₂ → Cu + H₂O
8. Decomposition of H₂O₂: 2H₂O₂ → 2H₂O + O₂
9. H₂O₂ as oxidant: 2KI + H₂O₂ → 2KOH + I₂
10. H₂O₂ as reductant: Cl₂ + H₂O₂ → 2HCl + O₂
11. Clark’s process: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O
12. Permutit (zeolite): Na₂Ze + CaCl₂ → CaZe + 2NaCl (regenerated by NaCl).
Glossary
| Term | Meaning |
|---|---|
| Protium (¹H) | The lightest, non-radioactive and most abundant isotope of hydrogen. |
| Deuterium (²H, D) | The heavy, stable isotope of hydrogen with one proton and one neutron. |
| Tritium (³H, T) | The radioactive isotope of hydrogen; emits β-particles. |
| Water gas | An equimolar mixture of CO and H₂ obtained by passing steam over red-hot coke. |
| Bosch’s process | Industrial method of preparing pure hydrogen from water gas using Fe₂O₃ + Cr₂O₃ catalyst. |
| Lane’s process | Method of preparing hydrogen by alternately oxidising and reducing iron with steam and water gas. |
| Hydride | A binary compound of hydrogen with another element. |
| Ionic hydride | Saline hydride containing the H⁻ ion, e.g., NaH, CaH₂. |
| Covalent hydride | Molecular hydride formed by non-metals, e.g., CH₄, NH₃, H₂O. |
| Metallic hydride | Interstitial, often non-stoichiometric hydride formed by transition metals. |
| Hard water | Water containing soluble Ca²⁺/Mg²⁺ salts that does not lather easily with soap. |
| Temporary hardness | Hardness due to bicarbonates of Ca/Mg; removed by boiling or Clark’s process. |
| Permanent hardness | Hardness due to chlorides/sulphates of Ca/Mg; removed by Na₂CO₃, permutit or ion-exchange. |
| Permutit | Sodium aluminium silicate used to soften hard water by ion-exchange. |
| Hydrogen peroxide | H₂O₂; a pale-blue liquid with open-book structure that acts as both oxidising and reducing agent. |
| Heavy water | Deuterium oxide, D₂O; used as a moderator in nuclear reactors. |
| Hydrogen as fuel | Use of H₂ as a high-calorific, pollution-free fuel for vehicles and power generation. |