CBSE Class 11 Chemistry Chapter 9 Hydrogen Notes

Key Definitions

Hydrogen

The first element in the periodic table (atomic number 1), lightest element, existing as diatomic molecule H₂.

Protium (¹H)

The most common isotope of hydrogen, containing one proton and no neutron.

Deuterium (²H or D)

An isotope of hydrogen containing one proton and one neutron, also known as heavy hydrogen.

Tritium (³H or T)

A radioactive isotope of hydrogen containing one proton and two neutrons.

Hydrides

Binary compounds formed by hydrogen with other elements.

Heavy Water (D₂O)

Water in which hydrogen atoms are replaced by deuterium atoms, used as a moderator in nuclear reactors.

Temporary Hardness

Hardness of water caused by the presence of bicarbonates of calcium and magnesium, which can be removed by boiling.

Permanent Hardness

Hardness of water caused by the presence of chlorides and sulfates of calcium and magnesium, which cannot be removed by boiling.

Hydrogen Economy

A proposed future energy system where hydrogen is used as a major energy carrier.

Position of Hydrogen in the Periodic Table

Hydrogen holds a unique and somewhat ambiguous position in the periodic table. It resembles alkali metals (Group 1) because it has one electron in its outermost shell (1s¹ configuration) and can lose this electron to form a unipositive ion (H⁺). Like alkali metals, it forms oxides, halides, and sulfides. However, unlike alkali metals, it has a very high ionization enthalpy and does not show metallic character under normal conditions.

Conversely, hydrogen also resembles halogens (Group 17). It requires only one electron to complete its duplet configuration (like halogens need one electron to complete their octet), forming a uninegative ion (H⁻), known as a hydride ion. Similar to halogens, it forms diatomic molecules (H₂), is a non-metal, and forms covalent compounds with many elements. However, halogens are strong oxidizing agents, while hydrogen is a reducing agent.

Due to these dual similarities and distinct differences, hydrogen is usually placed separately at the top of the periodic table, emphasizing its unique character. Its ability to both lose and gain an electron, along with its small size and high ionization energy, differentiates it significantly from both Group 1 and Group 17 elements. Understanding this dual nature is key to comprehending its reactivity.

Preparation Methods of Dihydrogen (H₂)

1. Laboratory Preparation — 1. From Acids: Reaction of active metals (Zn, Mg, Fe) with dilute acids. Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) 2. From Alkalies: Reaction of certain metals (Zn, Al, Si) with strong alkalies. 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate)
2. Industrial Preparation — 1. Electrolysis of Acidified Water: Pure dihydrogen is obtained. 2H₂O(l) --(electrolysis)--> 2H₂(g) + O₂(g) 2. From Brine (Chlor-alkali process): Dihydrogen is a byproduct. 2NaCl(aq) + 2H₂O(l) --(electrolysis)--> 2NaOH(aq) + Cl₂(g) + H₂(g) 3. From Hydrocarbons/Coke (Steam Reforming): Water gas shift reaction. CH₄(g) + H₂O(g) --(Ni catalyst, 1270K)--> CO(g) + 3H₂(g) (Syngas or Water Gas) CO(g) + H₂O(g) --(FeCrO₄ catalyst, 673K)--> CO₂(g) + H₂(g) (Water-gas shift reaction to increase H₂ yield)

Types of Hydrides

Worked Example: Hardness of Water

• {"title":"Removing Temporary Hardness","bodyMarkdown":"Q: How can temporary hardness due to Mg(HCO₃)₂ be removed by boiling?\nA: When water containing Mg(HCO₃)₂ is boiled, the soluble bicarbonate decomposes to insoluble magnesium hydroxide, which precipitates out, thereby removing the hardness.\nMg(HCO₃)₂(aq) --(heat)--> Mg(OH)₂(s) ↓ + 2CO₂(g)"}

Key Points to Remember

• Hydrogen has three isotopes: Protium (¹H), Deuterium (²H or D), and Tritium (³H or T). Tritium is radioactive.
• Dihydrogen is a powerful reducing agent, especially at high temperatures.
• Hydrogen forms three main types of hydrides: ionic, covalent, and metallic.
• Water is a highly polar molecule with strong hydrogen bonding, leading to its unusual properties (high boiling point, specific heat, etc.).
• Hardness of water is due to dissolved calcium and magnesium salts. Temporary hardness is due to bicarbonates, permanent due to chlorides and sulfates.
• Hydrogen peroxide (H₂O₂) has an open book-like, non-planar structure. It acts as both an oxidizing and reducing agent.
• Heavy water (D₂O) is used as a moderator in nuclear reactors and in exchange reactions.
• Hydrogen can act as an oxidizing agent with highly electropositive metals (forming H⁻) and as a reducing agent with non-metals.
• The 'hydrogen economy' proposes using hydrogen as a clean fuel to reduce pollution.

Exam Tip: Balancing Hydrogen Peroxide Reactions

When dealing with reactions involving hydrogen peroxide (H₂O₂), pay close attention to whether it's acting as an oxidizing agent or a reducing agent. In acidic medium, H₂O₂ acts as an oxidant and gets reduced to H₂O. In basic medium, it can act as a reductant and gets oxidized to O₂. Always write balanced half-reactions or full redox reactions for H₂O₂ to avoid losing marks. Remember its decomposition: 2H₂O₂(l) → 2H₂O(l) + O₂(g).

Practice Questions with Solutions

• Q1: Why is hydrogen placed separately in the periodic table? A1: Due to its unique properties, resembling both alkali metals (loses e⁻ to form H⁺) and halogens (gains e⁻ to form H⁻), yet differing significantly from both.
• Q2: Name two industrial methods for the preparation of dihydrogen. A2: Electrolysis of acidified water and Steam reforming of hydrocarbons/coke (Water-gas shift reaction).
• Q3: What causes temporary hardness of water, and how is it removed by boiling? A3: Caused by bicarbonates of Ca and Mg. Boiling converts soluble bicarbonates into insoluble carbonates/hydroxides, which precipitate out.
• Q4: Describe the structure of hydrogen peroxide. A4: H₂O₂ has a non-planar, open book-like structure. The two H atoms lie in different planes, resulting in a dihedral angle (111.5° in gas phase, 90.2° in solid phase).

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