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Class 11 Chemistry Notes

Hydrogen Class 11 Notes

Complete, exam-ready notes on hydrogen: the simplest element, its isotopes, the debate over its position in the periodic table, methods of preparation and properties of dihydrogen, hydrides and their types, heavy water, and hydrogen peroxide — written for CBSE, JEE and NEET revision.

Class11SubjectChemistryCoversCBSE · JEE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What is the position of hydrogen in the periodic table?

Hydrogen (1s¹) is placed uniquely above Group 1 because it resembles both the alkali metals (loses an electron to H⁺) and the halogens (gains one to form H⁻).

Hydrogen and Its Isotopes

Isotopes of hydrogen

1H  (protium),2H  (deuterium D),3H  (tritium T){}^1H\;(\text{protium}),\quad {}^2H\;(\text{deuterium D}),\quad {}^3H\;(\text{tritium T})

All three isotopes have one proton: protium (no neutron), deuterium (one neutron) and tritium (two neutrons). They have identical chemical properties but different physical properties and masses.

  • Protium is the most abundant (99.98%), then deuterium (~0.016%), tritium is radioactive.
  • Deuterium replaces hydrogen in heavy water D₂O.
  • Physical properties differ: D₂O freezes at 3.8 °C and boils at 101.4 °C.

Position of Hydrogen in the Table

  • Like alkali metals: one valence electron, forms H⁺, strong reducing agent in some contexts, diatomic H₂.
  • Like halogens: needs one electron to complete its shell, forms hydride ion H⁻ and diatomic covalent molecules.
  • Unlike both: very low ionisation enthalpy is misleading — its IE₁ is actually higher than alkali metals; it is a unique element, so it is placed separately above Group 1.

Preparation and Properties of Dihydrogen

Preparations

Zn+2H+→Zn2++H2,2Na+2H2O→2NaOH+H2Zn + 2H^+ \to Zn^{2+} + H_2,\qquad 2Na + 2H_2O \to 2NaOH + H_2

Laboratory: zinc with dilute hydrochloric or sulphuric acid. Industrial: steam-hydrocarbon reforming (C + H₂O → CO + H₂ at high temperature), electrolysis of water, and from water-gas shift.

  • Very light, colourless, odourless gas; burns with a pale blue flame to give water.
  • Acts as a reducing agent: reduces metal oxides (CuO → Cu) and metal ions.
  • Combines with halogens, nitrogen (Haber process) and metals to form hydrides.
  • Used in the Haber process, hydrogenation of oils, rocket fuel, and as a clean fuel with only H₂O as product.

Hydrides and Their Types

Ionic, covalent and metallic hydrides

Ionic: NaH,Covalent: CH4,Metallic: TiH2\text{Ionic: } NaH,\quad \text{Covalent: } CH_4,\quad \text{Metallic: } TiH_2

Ionic (saline) hydrides form with s-block metals (NaH, CaH₂) and give H⁻. Covalent hydrides form with non-metals (CH₄, NH₃, H₂O, HCl) by sharing electrons. Metallic hydrides form with transition metals and are non-stoichiometric and hydrogen-absorbing.

  • Ionic hydrides are high-melting solids that react with water to release H₂.
  • Covalent hydrides are molecular: water, ammonia, methane.
  • Electron-precise hydrides (CH₄), electron-rich (NH₃, with lone pair) and electron-deficient (B₂H₆) are sub-types of covalent hydrides.

Heavy Water D₂O

Heavy water

D2O=2H2OD_2O = {}^2H_2O

Heavy water is water with deuterium. It is obtained by prolonged electrolysis or fractional distillation of ordinary water because D₂O concentrates in the residue. It is used as a moderator in nuclear reactors and as a tracer.

  • Higher boiling point, density and viscosity than H₂O but similar ionic chemistry.
  • Physical constants differ measurably for the isotopes.
  • Used in NMR and tracer studies to follow reaction paths.

Hydrogen Peroxide H₂O₂

Hydrogen peroxide

2H2O2→2H2O+O22H_2O_2 \to 2H_2O + O_2

A pale blue liquid, miscible with water, that decomposes slowly into water and oxygen; it is a powerful oxidising agent that is also reduced by stronger oxidants. It can act as both oxidant and reductant.

  • Stored in dark bottles with a stabiliser; catalase speeds decomposition in biological tissue.
  • Oxidising: H₂O₂ + 2e⁻ → 2OH⁻; reducing: H₂O₂ → O₂ + 2H⁺ + 2e⁻.
  • Antiseptic (dilute), bleaching agent for whitening, and rocket propellant oxidiser.
  • Volumes strength: '10 volume' H₂O₂ releases 10 L of O₂ per litre.

Solved Examples

Example: Why is hydrogen not placed with the halogens permanently?

Solution: Hydrogen shares one electron with halogens (forms H⁻ like halides) but one valence electron with alkali metals (forms H⁺). Neither analogy is complete, so periodic tables place it above Group 1 in its own position.

Example: What is the oxidation state of hydrogen in NaH versus HCl?

Solution: In NaH (an ionic hydride), hydrogen is the more electronegative partner, so it is −1. In HCl, chlorine is more electronegative, so hydrogen is +1.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

Isotopes

1H,2D,3T{}^1H, {}^2D, {}^3T

Lab preparation

Zn+2H+→Zn2++H2Zn + 2H^+ \to Zn^{2+} + H_2

Water-gas shift

C+H2O→CO+H2C + H_2O \to CO + H_2

H₂O₂ decomposition

2H2O2→2H2O+O22H_2O_2 \to 2H_2O + O_2

H₂O₂ as oxidant

H2O2+2e−→2OH−H_2O_2 + 2e^- \to 2OH^-

H₂O₂ as reductant

H2O2→O2+2H++2e−H_2O_2 \to O_2 + 2H^+ + 2e^-

Ionic hydride hydrolysis

NaH+H2O→NaOH+H2NaH + H_2O \to NaOH + H_2

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

  • Isotopes: protium ¹H, deuterium ²D, tritium ³T — only tritium is radioactive.
  • Position: hydrogen sits above Group 1, resembling both alkali metals and halogens.
  • Hydrides: ionic (NaH, H⁻), covalent (CH₄, NH₃), metallic (TiH₂).
  • H is −1 in ionic hydrides, +1 elsewhere.
  • Heavy water D₂O is a nuclear moderator.
  • H₂O₂ is both an oxidising and a reducing agent.
  • Dihydrogen burns to water — clean fuel.

FAQ

Common questions

What are the three isotopes of hydrogen?

Protium (¹H, no neutron), deuterium (²H or D, one neutron) and tritium (³H, two neutrons, radioactive). They react the same chemically but differ in mass and physical properties.

Why is hydrogen placed in its own position in the periodic table?

Hydrogen has one electron like alkali metals but needs one more like halogens, and it forms both H⁺ and H⁻. Since neither analogy is exact, it gets a unique position above Group 1.

What are the main types of hydrides?

Ionic (saline) hydrides of s-block metals such as NaH; covalent hydrides of non-metals such as CH₄ and NH₃; and metallic hydrides of transition metals such as TiH₂.

Why is hydrogen peroxide a good bleaching agent?

H₂O₂ releases oxygen that oxidises coloured pigments, and its products are only water and oxygen, so it leaves no residue. It is used as an antiseptic, bleach and oxidiser.

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