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Hydrogen Exercise Solutions | KalviGavani

Samacheer Kalvi 11th Chemistry Hydrogen exercise solutions — isotopes, hydrides, water hardness, heavy water, hydrogen economy, MCQs, full answers.

Updated 15 June 202617 min read

Exercise Solutions Available

Step-by-step answers for all textbook questions in this chapter

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How to Use This Page

Welcome to the exercise solutions for Chapter 4 — Hydrogen. Here is how to use this resource effectively:

1. Try every question on your own first before looking at the answers. 2. Compare your response with the detailed solution provided. 3. Focus on reaction equations — they carry significant marks in board exams. 4. Use the Key Concepts tables for quick memorisation before the exam. 5. Check the Exam Checklist to ensure complete preparation.

Key Concepts (Quick Recap)

Position of Hydrogen in the Periodic Table

Hydrogen is unique — it resembles both alkali metals (Group 1) and halogens (Group 17):

Resemblance to Alkali MetalsResemblance to Halogens
Has 1 valence electron (1s¹)Needs 1 electron to complete octet
Forms H⁺ ion (like Na⁺)Forms H⁻ ion (like Cl⁻)
Forms H₂O (like Na₂O)Forms NaH (like NaCl)
Is a diatomic gas (H₂)Is a diatomic gas (Cl₂, F₂)
Is a non-metalIs a non-metal

Because of this dual nature, hydrogen is placed separately at the top of the periodic table.

Isotopes of Hydrogen

IsotopeSymbolMass NumberNatural AbundanceKey Use
Protium¹H or H199.985%Most common hydrogen
Deuterium²H or D20.015%Heavy water (D₂O)
Tritium³H or T3Trace (radioactive)Nuclear fusion research

Types of Hydrides

TypeFormed byBondingExampleProperties
Ionic (Saline)Group 1 & 2 metalsIonic (M⁺H⁻ or M²⁺H₂²⁻)NaH, CaH₂Crystalline, high melting, react with water
Covalent (Molecular)p-block elementsCovalent sharingCH₄, NH₃, H₂OVolatile, low melting, molecular
Metallic (Interstitial)d- and f-block elementsInterstitialTiH₂, ZrH₂Conduct electricity, non-stoichiometric

Water Hardness

TypeCauseRemoval
TemporaryBicarbonates of Ca and MgBoiling
PermanentChlorides and sulphates of Ca and MgIon exchange / washing soda

Heavy Water (D₂O)

  • Molecular mass: 20 (vs 18 for H₂O)
  • Used as moderator and coolant in nuclear reactors
  • Higher boiling point (101.4 °C) and density than ordinary water
  • Produced by electrolytic enrichment of normal water

I. Choose the Best Answer (MCQs)

1. Hydrogen resembles alkali metals because it
Answer

(a) Has one valence electron (b) Is a metal (c) Forms H⁻ ion (d) Has high ionisation energy

Answer(a) Has one valence electron
ExplanationHydrogen has the electronic configuration 1s¹, similar to alkali metals (ns¹). Like alkali metals, it can lose its one valence electron to form H⁺ ion. However, hydrogen is a non-metal and does not share all properties with alkali metals.
2. The isotope of hydrogen used as a tracer in organic reactions is
Answer

(a) Protium (b) Deuterium (c) Tritium (d) None of these

Answer(b) Deuterium
ExplanationDeuterium (D) is used as a tracer in organic reactions because it behaves chemically like protium but can be distinguished by mass spectrometry or other analytical techniques due to its higher mass.
3. Which of the following is a covalent hydride?
Answer

(a) NaH (b) CaH₂ (c) NH₃ (d) TiH₂

Answer(c) NH₃
ExplanationNH₃ (ammonia) is a covalent hydride formed by a p-block element (nitrogen). NaH and CaH₂ are ionic hydrides formed by s-block metals. TiH₂ is a metallic hydride.
4. Temporary hardness of water is due to
Answer

(a) Chlorides of Ca and Mg (b) Sulphates of Ca and Mg (c) Bicarbonates of Ca and Mg (d) Nitrates of Ca and Mg

Answer(c) Bicarbonates of Ca and Mg
ExplanationTemporary hardness is caused by bicarbonates of calcium and magnesium. It is called "temporary" because it can be removed by boiling, which decomposes the bicarbonates to insoluble carbonates.
5. Heavy water is
Answer

(a) H₂O₂ (b) D₂O (c) H₂¹⁸O (d) T₂O

Answer(b) D₂O
ExplanationHeavy water is deuterium oxide (D₂O), where both hydrogen atoms are replaced by deuterium (²H). It has a molecular mass of 20 (compared to 18 for normal water) and is used as a moderator in nuclear reactors.
6. The reaction of sodium with water produces
Answer

(a) Na₂O + H₂ (b) NaOH + H₂ (c) Na₂O₂ + H₂ (d) NaH + O₂

Answer(b) NaOH + H₂
ExplanationSodium reacts vigorously with cold water to form sodium hydroxide and hydrogen gas:
2Na + 2H₂O → 2NaOH + H₂↑
7. Hydrogen gas is prepared in the laboratory by the action of dilute H₂SO₄ on
Answer

(a) Copper (b) Zinc (c) Silver (d) Gold

Answer(b) Zinc
ExplanationZinc reacts with dilute sulphuric acid to produce hydrogen gas:
Zn + H₂SO₄(dil) → ZnSO₄ + H₂↑

Copper, silver, and gold are below hydrogen in the reactivity series and cannot displace hydrogen from acids.

II. Fill in the Blanks

1. The three isotopes of hydrogen are Protium, Deuterium, and Tritium.

2. The radioactive isotope of hydrogen is Tritium (³H or T).

3. Heavy water is chemically D₂O (deuterium oxide).

4. The hydride formed by sodium (NaH) is an ionic / saline hydride.

5. Temporary hardness of water can be removed by boiling.

6. The atomic number of hydrogen is 1.

7. Hydrogen has 1 valence electron in its 1s orbital.

8. The density of heavy water is greater than that of ordinary water.

9. The process of removing hardness of water using resins is called ion exchange method.

10. Permanent hardness is caused by chlorides and sulphates of calcium and magnesium.

11. Water which forms lather easily with soap is called soft water.

12. The reaction Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂ occurs when water is boiled (removing temporary hardness).

13. Hydrides of d-block elements are called metallic / interstitial hydrides.

14. The product of reaction between calcium hydride and water is Ca(OH)₂ and H₂.

15. Hydrogen economy refers to the use of hydrogen as a clean fuel energy carrier.

III. True or False (with Corrections)

1. Statement: Hydrogen is placed in Group 1 as well as Group 17 of the periodic table.

AnswerFalse. Correction — Hydrogen is placed separately at the top of the periodic table, not in any group. It shows similarities to both Group 1 (alkali metals) and Group 17 (halogens), but it does not perfectly fit in either group and is given a unique position.

2. Statement: All isotopes of hydrogen have the same chemical properties.

AnswerTrue. Isotopes have the same electronic configuration (1s¹), so they have identical chemical properties. However, they differ in physical properties due to mass differences.

3. Statement: Temporary hardness of water can be removed by adding washing soda.

AnswerTrue. Washing soda (Na₂CO₃) removes both temporary and permanent hardness by precipitating calcium and magnesium ions as carbonates:
CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl

4. Statement: Metallic hydrides are formed by s-block elements.

AnswerFalse. Correction — Metallic hydrides are formed by d-block and f-block elements (transition metals). S-block elements form ionic (saline) hydrides like NaH and CaH₂.

5. Statement: Heavy water is used as a moderator in nuclear reactors.

AnswerTrue. Heavy water (D₂O) slows down fast-moving neutrons in nuclear reactors, making them more effective for sustaining a chain reaction. It also serves as a coolant.

IV. Match the Following

Column AColumn B
1. Protium(a) Radioactive isotope of hydrogen
2. Deuterium(b) Most abundant isotope (99.985%)
3. Tritium(c) Heavy hydrogen, used in D₂O
4. Ionic hydride(d) NaH
5. Covalent hydride(e) NH₃
6. Metallic hydride(f) TiH₂
Answer

1 — (b): Protium is the most abundant isotope with 99.985% natural abundance 2 — (c): Deuterium is called heavy hydrogen and is used to make heavy water (D₂O) 3 — (a): Tritium is the radioactive isotope of hydrogen (beta emitter, half-life ~12.3 years) 4 — (d): NaH is an ionic hydride formed by an s-block metal 5 — (e): NH₃ is a covalent hydride formed by a p-block element 6 — (f): TiH₂ is a metallic (interstitial) hydride formed by a d-block element

V. Answer Briefly (2-mark)

1. Why is hydrogen placed separately in the periodic table?
Answer

Hydrogen is placed separately because it shows dual nature — it resembles both alkali metals and halogens:

  • Like alkali metals: it has one valence electron (1s¹), forms H⁺ ion, and forms an oxide (H₂O like Na₂O).
  • Like halogens: it needs one electron to complete its valence shell, forms H⁻ ion, and exists as a diatomic molecule (H₂ like Cl₂).

Since hydrogen does not perfectly fit into either group, it is placed separately at the top of the periodic table.

2. Differentiate between temporary and permanent hardness of water.
Answer
FeatureTemporary HardnessPermanent Hardness
CauseBicarbonates of Ca and MgChlorides and sulphates of Ca and Mg
Removal by boilingYes — bicarbonates decompose on boilingNo — salts are not decomposed by boiling
Removal methodBoilingIon exchange / washing soda / distillation
ExampleCa(HCO₃)₂, Mg(HCO₃)₂CaCl₂, MgSO₄, CaSO₄
3. What is heavy water? State two uses.
Answer

Heavy water is deuterium oxide (D₂O), where both hydrogen atoms are replaced by deuterium (²H or D). Its molecular mass is 20 (compared to 18 for normal water).

Uses: 1. Used as a moderator and coolant in nuclear reactors to slow down fast neutrons. 2. Used as a tracer in the study of reaction mechanisms and metabolic pathways.

4. Name the three types of hydrides with one example each.
Answer
TypeExampleBonding
Ionic (Saline) hydrideNaHIonic (Na⁺ and H⁻)
Covalent (Molecular) hydrideNH₃Covalent (shared electron pair)
Metallic (Interstitial) hydrideTiH₂Hydrogen occupies interstitial sites

VI. Answer in Detail (5-mark)

1. Explain the preparation, properties, and uses of hydrogen gas.

Preparation of Hydrogen

Laboratory Method:

Hydrogen is prepared by the action of dilute hydrochloric acid or dilute sulphuric acid on zinc:

Zn + H₂SO₄(dil) → ZnSO₄ + H₂↑ Zn + 2HCl(dil) → ZnCl₂ + H₂↑

Pure hydrogen is obtained by reacting zinc with dilute acid; impure hydrogen can be obtained from granulated zinc and warm dilute acid.

Industrial Methods:

1. Steam reforming of natural gas (most common):

CH₄ + H₂O → CO + 3H₂ (at 1100 °C, Ni catalyst) CO + H₂O → CO₂ + H₂ (water-gas shift reaction)

2. Electrolysis of water:

2H₂O → 2H₂↑ + O₂↑ (using platinum electrodes)

3. Lane's process (steam over heated iron):

3Fe + 4H₂O → Fe₃O₄ + 4H₂↑

Physical Properties

  • Colourless, odourless, tasteless gas.
  • Lightest of all gases (density ≈ 0.09 g/L at STP).
  • Slightly soluble in water.
  • Boiling point: -252.8 °C.

Chemical Properties

1. Combustion (burns in air):
2H₂ + O₂ → 2H₂O (pale blue flame)
2. Reaction with halogens:
H₂ + Cl₂ → 2HCl (in sunlight) H₂ + F₂ → 2HF (explosive, even in dark)
3. Reaction with nitrogen (Haber's process):
N₂ + 3H₂ → 2NH₃ (Fe catalyst, 450°C, 200 atm)
4. Reaction with metals (forms hydrides):
2Na + H₂ → 2NaH (ionic hydride) Ca + H₂ → CaH₂
5. Reducing agent:
CuO + H₂ → Cu + H₂O (hydrogen reduces metal oxides) Fe₃O₄ + 4H₂ → 3Fe + 4H₂O

Uses of Hydrogen

1. Manufacture of ammonia (Haber's process). 2. Hydrogenation of vegetable oils (converting unsaturated oils to saturated fats). 3. As a rocket fuel (liquid hydrogen + liquid oxygen). 4. As a reducing agent in metallurgy. 5. Fuel cells for generating electricity (hydrogen economy). 6. Welding (oxy-hydrogen torch).

2. Explain water hardness — types, causes, and methods of removal.

Definition

Hard water is water that does not form lather easily with soap due to the presence of dissolved salts of calcium and magnesium. Soft water forms lather easily.

Types of Hardness

1. Temporary Hardness:
Answer
  • Caused by bicarbonates of calcium and magnesium.
  • Can be removed by boiling.
  • When boiled, bicarbonates decompose to insoluble carbonates:
Ca(HCO₃)₂ →(heat) CaCO₃↓ + H₂O + CO₂↑ Mg(HCO₃)₂ →(heat) Mg(OH)₂↓ + CO₂↑ (MgCO₃ is slightly soluble)
2. Permanent Hardness:
Answer
  • Caused by chlorides and sulphates of calcium and magnesium.
  • Cannot be removed by boiling.
  • Removed by chemical methods (washing soda, ion exchange).

Methods of Removing Hardness

1. Boiling (for temporary hardness only):
Ca(HCO₃)₂ →(heat) CaCO₃↓ + H₂O + CO₂↑
Answer

The insoluble CaCO₃ precipitates and is filtered out.

2. Clark's method (for temporary hardness):
Answer

Adding slaked lime (Ca(OH)₂) to hard water:

Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O
3. Washing soda method (for both types):
Answer

Adding sodium carbonate (Na₂CO₃):

CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl MgSO₄ + Na₂CO₃ → MgCO₃↓ + Na₂SO₄
4. Ion exchange method (for both types):
Answer

Hard water is passed through an ion exchange resin (zeolite or synthetic resin). The resin exchanges Na⁺ ions for Ca²⁺ and Mg²⁺ ions:

2NaZ + Ca²⁺ → CaZ₂ + 2Na⁺ (Where NaZ = sodium zeolite)

The resin can be regenerated by passing concentrated NaCl solution:

CaZ₂ + 2NaCl → 2NaZ + CaCl₂
5. Calgon method:
Answer

Adding sodium hexametaphosphate (Na₆P₆O₁₈), called Calgon, which complexes with Ca²⁺ and Mg²⁺ ions, preventing them from forming insoluble salts with soap.

Degree of Hardness

Hardness is measured in ppm (parts per million) of CaCO₃:

1 ppm = 1 mg of CaCO₃ per litre of water
3. Explain the types of hydrides with examples, properties, and reactions.
Answer

Hydrides are binary compounds of hydrogen with other elements. They are classified into three types based on the nature of bonding:

1. Ionic (Saline) Hydrides

Formation: s-block metals (Group 1 and 2) react with hydrogen to form ionic hydrides.

2Na + H₂ → 2NaH (sodium hydride) Ca + H₂ → CaH₂ (calcium hydride)

Properties:

  • Crystalline solids with high melting and boiling points.
  • Conduct electricity in molten state (electrolytes).
  • React vigorously with water to produce hydrogen:
NaH + H₂O → NaOH + H₂↑ CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂↑
  • H⁻ ion is a strong reducing agent and a strong base.
  • Saline hydrides are stoichiometric (definite composition).

Uses:

  • CaH₂ is used as a drying agent (desiccant) called "hydrolith."
  • NaH is used as a reducing agent in organic chemistry.
  • LiAlH₄ (lithium aluminium hydride) is a powerful reducing agent.

2. Covalent (Molecular) Hydrides

Formation: p-block elements form covalent hydrides by sharing electrons with hydrogen.

C + 2H₂ → CH₄ (methane) N₂ + 3H₂ → 2NH₃ (ammonia) 2H₂ + O₂ → 2H₂O (water)

Properties:

  • Exist as discrete molecules with weak Van der Waals forces.
  • Low melting and boiling points (many are gases at room temperature).
  • Non-conductors of electricity.
  • Molecular hydrides can be further classified:
  • Electron-deficient (e.g., BH₃, B₂H₆) — fewer electrons than needed for 2-centre-2-electron bonds.
  • Electron-precise (e.g., CH₄, SiH₄) — exactly enough electrons for all bonds.
  • Electron-rich (e.g., NH₃, H₂O, HF) — have lone pairs on the central atom.

Trends:

  • Thermal stability decreases down the group (HF > HCl > HBr > HI).
  • Reducing character increases down the group.
  • Acidic character increases down the group (HF < HCl < HBr < HI).

3. Metallic (Interstitial) Hydrides

Formation: d-block and f-block elements absorb hydrogen into interstitial sites in their crystal lattice.

Ti + H₂ → TiH₂ (titanium hydride) Zr + H₂ → ZrH₂ (zirconium hydride)

Properties:

  • Hydrogen atoms occupy interstitial sites in the metal lattice.
  • Often non-stoichiometric (e.g., LaH₂.₇₆, TiH₁.₇₃).
  • Retain metallic properties — conduct electricity, have metallic lustre.
  • Can absorb and release hydrogen reversibly (useful for hydrogen storage).
  • Palladium can absorb up to 900 times its volume of hydrogen.

Uses:

  • Hydrogen storage in fuel cells and batteries.
  • Palladium hydride is used for hydrogen purification.
  • TiH₂ and ZrH₂ are used in nuclear reactors as moderators.

Comparison Table

Property | Ionic | Covalent | Metallic -----------------|-----------|---------------|------------- Formed by | s-block | p-block | d/f-block Bonding | Ionic | Covalent | Interstitial State at RT | Solid | Gas/Liquid | Solid Melting point | High | Low | High Electrical cond. | Yes (melt)| No | Yes Stoichiometry | Fixed | Fixed | Variable Example | NaH | NH₃ | TiH₂

Common Mistakes to Avoid

1. Confusing ionic and covalent hydrides: Ionic hydrides are formed by metals (s-block); covalent hydrides are formed by non-metals (p-block). Do not mix them up.

2. Assuming all hydrides react with water: Covalent hydrides like CH₄ do not react with water. Only ionic hydrides react vigorously with water to produce H₂ gas.

3. Confusing temporary and permanent hardness: Temporary hardness is caused by bicarbonates and is removed by boiling. Permanent hardness is caused by chlorides/sulphates and cannot be removed by boiling.

4. Forgetting that heavy water is D₂O, not H₂O₂: Heavy water is deuterium oxide (D₂O). Hydrogen peroxide (H₂O₂) is a completely different compound.

5. Incorrectly placing hydrogen in Group 1 or Group 17: Hydrogen is placed separately. It resembles both groups but does not fully belong to either. Do not state it belongs to Group 1 alone.

6. Writing incorrect equations for hardness removal: Make sure you write balanced equations. For example, Clark's method: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O (not CaO or other products).

7. Confusing electron-deficient and electron-rich hydrides: BH₃ is electron-deficient (fewer electrons than needed for all bonds). NH₃ is electron-rich (has a lone pair on nitrogen). CH₄ is electron-precise (exact number of electrons for all bonds).

8. Forgetting that tritium is radioactive: Unlike protium and deuterium, tritium (³H) is radioactive with a half-life of about 12.3 years. It undergoes beta decay.

Exam Checklist

  • [ ] Can explain why hydrogen is placed separately in the periodic table
  • [ ] Can list and compare the three isotopes of hydrogen (H, D, T)
  • [ ] Can define and classify hydrides (ionic, covalent, metallic) with examples
  • [ ] Can write balanced equations for reactions of hydrogen with halogens, nitrogen, and metals
  • [ ] Can differentiate between temporary and permanent hardness with causes and removal methods
  • [ ] Can write balanced equations for hardness removal (boiling, Clark's method, washing soda, ion exchange)
  • [ ] Can explain the preparation, properties, and uses of heavy water (D₂O)
  • [ ] Can describe the preparation of hydrogen in the laboratory and industrially
  • [ ] Can explain the concept of hydrogen economy and its significance
  • [ ] Can compare ionic, covalent, and metallic hydrides in terms of properties and reactions

Frequently Asked Questions

Protium (¹H or H), Deuterium (²H or D), and Tritium (³H or T).