CBSE Class 11 Chemistry Exemplar Chapter 9: Hydrogen - NCERT Solutions

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CBSE Class 11 Chemistry Chapter 9, 'Hydrogen,' NCERT Solutions offer a comprehensive exploration of this fundamental element. The solutions meticulously explain hydrogen's intriguing similarities with halogens, the elusive nature of the H+ ion, and the ionic characteristics of metal hydrides. They further categorize different types of hydrides, providing clarity on their formation and properties. A significant focus is placed on hydrogen's radioactive isotope, tritium, and the versatile nature of hydrogen peroxide. Students will find detailed explanations of how hydrogen peroxide acts as both an oxidizing and reducing agent, depending on the reaction conditions. These solutions are crafted to demystify complex topics, offering step-by-step guidance to help students build a strong foundation and excel in their examinations.

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BoardCBSE
ClassClass 11
SubjectChemistry Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 9

Chapter summary

This chapter focuses on the element Hydrogen, exploring its unique position in the periodic table and its chemical properties. The NCERT Exemplar solutions cover multiple-choice questions that test understanding of hydrogen's resemblance to halogens, the nature of the H+ ion, the trend in ionic character of metal hydrides, and the classification of hydrides (electron-precise). It also touches upon the radioactive isotope of hydrogen and the redox behavior of hydrogen peroxide. The solutions offer clear explanations and reasoning for each answer.

Learning outcomes

  • Understand the reasons for hydrogen's resemblance to halogens.
  • Explain why the H+ ion is always associated with other molecules.
  • Determine the trend in ionic character among alkali metal hydrides.
  • Identify different types of hydrides based on their electron count.
  • Recognize tritium as the radioactive isotope of hydrogen.
  • Differentiate between the oxidizing and reducing roles of hydrogen peroxide in reactions.

Topics covered

Paper topics

  • Position of Hydrogen in Periodic Table
  • Isotopes of Hydrogen
  • Physical and Chemical Properties of Hydrogen
  • Comparison with Alkali Metals and Halogens
  • Hydrides: Ionic, Covalent, Molecular
  • Classification of Hydrides (Electron-Precise, Deficient, Excess)
  • Hydrogen Peroxide: Properties and Reactions
  • Oxidizing and Reducing Properties of Hydrogen Peroxide
  • Radioactivity of Hydrogen Isotopes
  • H+ Ion Behavior

Important topics

  • Resemblance of Hydrogen to Halogens
  • Nature and Classification of Hydrides
  • Ionic Character Trend in Metal Hydrides
  • Redox Behavior of Hydrogen Peroxide
  • Radioactive Isotope of Hydrogen (Tritium)
  • Instability of H+ Ion

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Questions and Solutions

Question 1

Hydrogen resembles halogens in many respects for which several factors are responsible. Of the following factors, which one is the most important in this respect?

(a) Its tendency to lose an electron to form a cation

(b) Its tendency to gain a single electron in its valence shell to attain a stable electronic configuration

(c) Its low negative electron enthalpy value

(d) Its small size

Solution: The most important factor explaining hydrogen's resemblance to halogens is its tendency to gain a single electron. Like halogens, hydrogen needs just one electron to complete its valence shell and achieve the stable electron configuration of the nearest noble gas, Helium (1s²). This makes it readily accept an electron to form a hydride ion (H⁻), similar to how halogens form halide ions (X⁻).

Answer: (b)

Question 2

Why does the H⁺ ion always get associated with other atoms or molecules?

(a) Ionisation enthalpy of hydrogen resembles that of alkali metals

(b) Its reactivity is similar to halogens

(c) It resembles both alkali metals and halogens

(d) Loss of an electron from a hydrogen atom results in a nucleus of very small size as compared to other atoms or ions. Due to its small size, it cannot exist freely.

Solution: The H⁺ ion is essentially a bare proton, as it consists only of a nucleus with no electrons. This nucleus is extremely small compared to other atomic nuclei or ions. Due to this minuscule size and high charge density, it cannot exist independently and is highly reactive, readily associating with other atoms or molecules that possess lone pairs of electrons, such as water molecules, to form species like the hydronium ion (H₃O⁺).

Answer: (d)

Question 3

Metal hydrides can be ionic, covalent, or molecular in nature. Among LiH, NaH, KH, RbH, CsH, what is the correct order of increasing ionic character?

(a) LiH > NaH > CsH > KH > RbH

(b) LiH < NaH < KH < RbH < CsH

(c) RbH > CsH > NaH > KH > LiH

(d) NaH > CsH > RbH > LiH > KH

Solution: The ionic character of metal hydrides is influenced by the size and electronegativity of the metal. As the size of the metal cation increases down the group (from Li to Cs), its ability to polarize the hydride ion decreases, leading to an increase in ionic character. Conversely, smaller, more electronegative metals tend to form more covalent hydrides. Therefore, the ionic character increases down the group.

The correct order of increasing ionic character is: LiH < NaH < KH < RbH < CsH

Answer: (b)

Question 4

Which of the following hydrides is an electron-precise hydride?

(a) B_2 H_6

(b) NH_3

(c) H_2O

(d) CH_4

Solution: Electron-precise hydrides are those compounds where the central atom has the exact number of valence electrons required to form normal covalent bonds with the surrounding atoms. In methane (CH_4), carbon forms four single covalent bonds with four hydrogen atoms, utilizing all its valence electrons without any excess or deficiency. Ammonia (NH_3) and water (H_2O) also have sufficient electrons for bonding, but diborane (B_2H_6) is an example of an electron-deficient hydride due to its 'banana' bonds.

Answer: (d)

Question 5

Radioactive elements emit alpha (α), beta (β), and gamma (γ) rays and are characterized by their half-lives. What is the radioactive isotope of hydrogen?

(a) Protium

(b) Deuterium

(c) Tritium

(d) Hydronium

Solution: Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H). Among these, Tritium is radioactive. It has one proton and two neutrons. Nuclides with a neutron-to-proton ratio greater than approximately 1.5 are generally radioactive. For Tritium, the ratio is 2/1 = 2, indicating its radioactive nature.

Answer: (c)

Question 6

Consider the following reactions: (i) H_2O_2 + 2HI \longrightarrow I_2 + 2H_2O (ii) HOCl + H_2O_2 \longrightarrow H_3O^+ + Cl^- + O_2 Which of the following statements is correct about H_2O_2 with reference to these reactions? Hydrogen peroxide is .........

(a) an oxidising agent in both (i) and (ii)

(b) an oxidising agent in (i) and a reducing agent in (ii)

(c) a reducing agent in (i) and an oxidising agent in (ii)

(d) a reducing agent in both (i) and (ii)

Solution: To determine the role of H_2O_2, we need to examine the changes in oxidation states:

In reaction (i): H_2O_2 + 2HI \longrightarrow I_2 + 2H_2O

The oxidation state of oxygen in H_2O_2 is -1. The oxidation state of iodine in HI is -1, and in I_2 it is 0. Since H_2O_2 causes the oxidation of I^- to I_2 (oxidation state increases from -1 to 0) and is itself reduced (oxygen's oxidation state changes from -1 to -2 in H_2O), H_2O_2 acts as an oxidizing agent.

In reaction (ii): HOCl + H_2O_2 \longrightarrow H_3O^+ + Cl^- + O_2

The oxidation state of oxygen in H_2O_2 is -1. The oxidation state of chlorine in HOCl is +1, and in Cl^- it is -1. Since H_2O_2 causes the reduction of Cl from +1 to -1 and is itself oxidized (oxygen's oxidation state changes from -1 to 0 in O_2), H_2O_2 acts as a reducing agent.

Answer: (b)

Common mistakes

  • Confusing the primary reason for hydrogen's resemblance to halogens.
  • Not understanding the reason for H+ ion's instability.
  • Incorrectly ordering the ionic character of metal hydrides.
  • Misidentifying the type of hydride (electron-precise, deficient, or excess).
  • Confusing the oxidizing and reducing actions of hydrogen peroxide.

Revision tips

  • Focus on the unique electronic configuration of hydrogen and its implications.
  • Memorize the trend of ionic character in metal hydrides and the underlying principle (Fajan's rule/polarization).
  • Understand the concept of electron-precise, deficient, and excess hydrides with examples.
  • Pay close attention to the oxidation states in redox reactions involving hydrogen peroxide.
  • Review the reasons for H+ ion's instability and its association with other species.

Practice MCQs

Q1. Hydrogen shares several similarities with halogens. Which of the following factors is the most significant reason for this resemblance?

Q2. Why does the H+ ion invariably associate with other atoms or molecules instead of existing freely?

Q3. For the alkali metal hydrides (LiH, NaH, KH, RbH, CsH), what is the correct order of increasing ionic character?

Q4. Which of the following compounds is classified as an electron-precise hydride?

Q5. Identify the radioactive isotope of hydrogen among the given options.

Q6. In the reaction H₂O₂ + 2HI → I₂ + 2H₂O, hydrogen peroxide acts as:

Q7. In the reaction HOCl + H₂O₂ → H₃O⁺ + Cl⁻ + O₂, hydrogen peroxide acts as:

Frequently asked questions

What are the main reasons for hydrogen's resemblance to halogens?

Hydrogen resembles halogens primarily due to its tendency to gain one electron to achieve a stable noble gas configuration (like Helium), forming a hydride ion (H⁻). It also shares similarities in forming diatomic molecules and exhibiting similar electronegativity values in certain compounds.

Why is the H+ ion unstable and always found associated with other molecules?

The H+ ion is essentially a bare proton, as it loses its only electron. This results in an extremely small size and a very high charge density, making it highly reactive and unstable. It readily associates with molecules that have lone pairs of electrons, such as water, to form species like the hydronium ion (H₃O⁺).

How does the ionic character of alkali metal hydrides change down the group?

The ionic character of alkali metal hydrides increases as you move down the group from Lithium to Cesium (LiH < NaH < KH < RbH < CsH). This is because the size of the alkali metal cation increases down the group, leading to less polarization of the hydride ion and a more ionic bond.

What is the difference between electron-precise, electron-deficient, and electron-excess hydrides?

Electron-precise hydrides (like CH₄, NH₃, H₂O) have the exact number of electrons to form normal covalent bonds. Electron-deficient hydrides (like diborane, B₂H₆) have fewer electrons than required for conventional 3-center-2-electron bonds. Electron-excess hydrides (formed by transition metals) have an excess of metal atoms and can exhibit non-stoichiometric compositions.

Which isotope of hydrogen is radioactive?

Tritium (³H) is the radioactive isotope of hydrogen. It has a half-life of about 12.3 years and decays by emitting beta particles.

Can hydrogen peroxide act as both an oxidizing and reducing agent?

Yes, hydrogen peroxide (H₂O₂) can act as both an oxidizing and reducing agent. It acts as an oxidizing agent when it oxidizes species with lower oxidation states (e.g., HI to I₂). It acts as a reducing agent when it reduces species with higher oxidation states (e.g., HOCl to Cl⁻), getting oxidized to O₂ itself.

How can these NCERT solutions help in exam preparation?

These solutions provide clear, step-by-step explanations for each question, reinforcing understanding of key concepts related to hydrogen. They help in identifying common mistakes and practicing different types of questions, which is crucial for effective exam revision and scoring well.

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