CBSE Class 11 Chemistry Exemplar Chapter 10: The s-Block Elements NCERT Solutions

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NCERT Solutions PDF Class 11 PDF

CBSE Class 11 Chemistry Chapter 10, The s-Block Elements, NCERT Solutions, offers a detailed exploration of Group 1 (alkali metals) and Group 2 (alkaline earth metals). This resource covers essential concepts like the physical and chemical properties of these elements, including their melting points, reactivity with water, and reducing power. It also delves into the thermal stability of their carbonates and the basicity of their hydroxides, providing clear explanations for various questions. The solutions are structured to enhance understanding of the trends and principles governing s-block elements, making them an invaluable tool for students preparing for their examinations. By offering step-by-step guidance, this guide aims to solidify students' grasp of inorganic chemistry concepts related to the s-block.

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BoardCBSE
ClassClass 11
SubjectChemistry Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 10

Chapter summary

Chapter 10 of the CBSE Class 11 Chemistry Exemplar focuses on the s-Block Elements, specifically Group 1 (Alkali Metals) and Group 2 (Alkaline Earth Metals). The NCERT Solutions provided here address key concepts like their low melting points, vigorous reactions with water, varying reducing strengths, thermal stability of their carbonates, and the basicity of their hydroxides. The solutions offer detailed explanations for MCQs, clarifying the trends and factors influencing these properties.

Learning outcomes

  • Understand the trends in physical properties of alkali and alkaline earth metals.
  • Explain the reactivity of alkali metals with water and the factors influencing it.
  • Analyze the reducing power of alkali metals, particularly lithium in aqueous solution.
  • Compare the thermal stability of alkaline earth metal carbonates.
  • Determine the basicity of alkaline earth metal hydroxides.

Topics covered

Paper topics

  • Alkali Metals
  • Alkaline Earth Metals
  • Reactivity with Water
  • Reducing Power
  • Thermal Stability of Carbonates
  • Basicity of Hydroxides
  • Trends in Group 1
  • Trends in Group 2
  • Lithium Anomaly
  • Beryllium Anomaly
  • Physical Properties of s-Block Elements
  • Chemical Properties of s-Block Elements

Important topics

  • Reactivity and Reducing Power of Alkali Metals
  • Thermal Stability of Alkaline Earth Metal Carbonates
  • Basicity of Alkaline Earth Metal Hydroxides
  • Anomalous behavior of Lithium and Beryllium
  • Trends in Physical Properties of s-Block Elements

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Questions and Solutions

Question 1

The alkali metals are low melting. Which of the following alkali metal is expected to melt if the room temperature rises to 30°C?
  1. Na (b) K (c) Rb (d) Cs
Solution: The alkali metals possess low melting and boiling points. This is attributed to the weak metallic bonding resulting from their large atomic radii and the presence of only one valence electron per atom, which leads to low cohesive forces within the crystal lattice. As we move down the group from Lithium (Li) to Caesium (Cs), the atomic size increases, and consequently, the cohesive forces decrease. This trend is reflected in their melting points, which steadily decrease down the group. Caesium (Cs) has a melting point of approximately 302 K, which is equivalent to 29°C. Therefore, if the room temperature rises to 30°C, Caesium is the alkali metal most likely to melt.

Question 2

Alkali metals react with water vigorously to form hydroxides and dihydrogen. Which of the following alkali metals reacts with water least vigorously?
  1. Li (b) Na (c) K (d) Cs
Solution: While the reaction of alkali metals with water is thermodynamically favorable, the observed vigor depends on kinetic factors as well. Lithium (Li) has the most negative standard reduction potential, indicating a strong tendency to lose electrons. However, the reaction of Li with water is surprisingly gentle compared to Na and K. This is because the process of Li reacting with water involves significant energy input for its sublimation, vaporization, and ionization. Although the reaction with Li evolves the maximum energy, the energy consumed in these initial steps slows down the overall reaction rate. In contrast, Na and K have lower melting points; when they react with water, they melt and spread over the surface, increasing the contact area and leading to a more vigorous, explosive reaction.

Question 3

The reducing power of a metal depends on various factors. Suggest the factor which makes Li, the strongest reducing agent in aqueous solution.
  1. Sublimation enthalpy (b) Ionisation enthalpy
  2. Hydration enthalpy (d) Electron-gain enthalpy
Solution: The reducing power of an element in aqueous solution is determined by its standard reduction potential (E_{RP}^{\circ}). A more negative E_{RP}^{\circ} indicates a greater tendency to lose electrons and thus stronger reducing power. The E_{RP}^{\circ} value is influenced by three main energy changes: enthalpy of sublimation, ionization enthalpy, and enthalpy of hydration. For Lithium (Li), while its ionization enthalpy is high, its extremely small ionic radius leads to a very high (highly exothermic) enthalpy of hydration for Li+(aq). This highly exothermic hydration step significantly contributes to Li having the most negative E_{RP}^{\circ} value among alkali metals (–3.04 V), making it the strongest reducing agent in aqueous solution. The overall process can be represented as:

Li(s) \longrightarrow Li^+(g) + e^-

This involves:

  1. Li(s) \longrightarrow Li(g) (Enthalpy of sublimation, \Delta H_s)
  2. Li(g) \longrightarrow Li^+(g) + e^- (Ionisation enthalpy, IE_1)
  3. Li^+(g) \longrightarrow Li^+(aq) (Enthalpy of hydration, \Delta H_h)
The highly exothermic nature of the hydration step for the small Li+ ion is the key factor.

Question 4

Metal carbonates decompose on heating to give metal oxide and carbon dioxide. Which of the metal carbonates is most stable thermally?
  1. MgCO3
  2. CaCO3
  3. SrCO3
  4. BaCO3
Solution: Alkaline earth metals form carbonates with the general formula MCO3, which decompose upon heating according to the equation:

MCO_3(s) \rightleftharpoons MO(s) + CO_2(g)

The thermal stability of these carbonates increases as we move down the group from Beryllium (Be) to Barium (Ba). This trend is represented as:

BeCO_3 < MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3

Barium carbonate (BaCO3) is the most thermally stable. This increased stability is due to the large size of the Ba2+ cation. A larger cation exerts a weaker polarizing effect on the carbonate ion (CO32-). Additionally, the formation of the metal oxide (MO) is favored when the cation is large enough to effectively stabilize the oxide ion (O2-) through lattice energy, which is the case for BaO compared to the oxides of lighter alkaline earth metals. Therefore, BaCO3 requires higher temperatures to decompose.

Question 5

Which of the carbonates given below is unstable in air and is kept in CO2 atmosphere to avoid decomposition?
  1. BeCO3
  2. MgCO3
  3. CaCO3
  4. BaCO3
Solution: Beryllium carbonate (BeCO3) is the least stable among the alkaline earth metal carbonates and is unstable even in the presence of atmospheric air. It readily decomposes, and to prevent this decomposition, it must be stored in an atmosphere rich in carbon dioxide (CO2). The instability arises from the strong polarizing power of the small Be2+ ion on the large, polarizable carbonate ion. This strong polarization weakens the carbonate structure. Furthermore, the resulting beryllium oxide (BeO) is exceptionally stable due to the high lattice energy formed by the small Be2+ cation and the small O2- ion, which favors the decomposition reaction:

BeCO_3(s) \rightleftharpoons BeO(s) + CO_2(g)

This decomposition is reversible, and maintaining a high concentration of CO2 shifts the equilibrium to the left, stabilizing BeCO3.

Question 6

Metals form basic hydroxides. Which of the following metal hydroxide is the least basic?
  1. Mg(OH)2
  2. Ca(OH)2
  3. Sr(OH)2
  4. Ba(OH)2
Solution: All alkaline earth metals form hydroxides of the general formula M(OH)2. The basicity of these hydroxides increases significantly as we move down the group from Beryllium (Be) to Barium (Ba). This is because the solubility of these hydroxides increases down the group, and increased solubility leads to greater dissociation into ions, resulting in a higher concentration of hydroxide ions (OH-) in solution, which is characteristic of a stronger base. Beryllium hydroxide (Be(OH)2) and Magnesium hydroxide (Mg(OH)2) are sparingly soluble, while the hydroxides of Ca, Sr, and Ba are increasingly soluble and thus more basic. Therefore, Magnesium hydroxide (Mg(OH)2) is the least basic among the given options because the Mg2+ ion is smaller than Ca2+, Sr2+, and Ba2+, leading to a stronger attraction between Mg2+ and OH-, which reduces its solubility and dissociation into ions.

Common mistakes

  • Confusing thermodynamic and kinetic factors in reaction rates.
  • Overlooking the role of hydration enthalpy in determining reducing power.
  • Incorrectly predicting the thermal stability of carbonates based solely on cation size.
  • Assuming solubility trends for hydroxides are the same as for carbonates.

Revision tips

  • Focus on the trends down the group for both alkali and alkaline earth metals.
  • Pay close attention to the specific reasons behind anomalous behavior (e.g., Li).
  • Memorize the order of stability for carbonates and basicity for hydroxides.
  • Relate physical properties (like melting point) to atomic structure and bonding.

Practice MCQs

Q1. Which alkali metal is expected to melt if the room temperature rises to 30°C?

Q2. Which of the following alkali metals reacts with water least vigorously?

Q3. What is the primary factor that makes Lithium (Li) the strongest reducing agent in aqueous solution?

Q4. Which of the following metal carbonates is the most thermally stable?

Q5. Which metal carbonate is unstable in air and must be kept in a CO2 atmosphere to prevent decomposition?

Q6. Which of the following metal hydroxides is the least basic?

Frequently asked questions

What are the s-Block Elements?

The s-Block Elements are those in which the last electron enters the s-orbital. This group includes Group 1 (Alkali Metals) and Group 2 (Alkaline Earth Metals) of the periodic table.

Why do alkali metals have low melting points?

Alkali metals have low melting points due to their large atomic radii and only one valence electron per atom, which results in weak metallic bonding and low cohesive forces in their crystal lattices.

Why is Lithium the strongest reducing agent in aqueous solution despite having a high ionization enthalpy?

Lithium's strong reducing power in aqueous solution is primarily due to its very high hydration enthalpy, which releases a significant amount of energy when the small Li+ ion is hydrated, overcoming the energy required for sublimation and ionization.

How does the thermal stability of alkaline earth metal carbonates change down the group?

The thermal stability of alkaline earth metal carbonates increases down the group from BeCO3 to BaCO3. This is because larger cations (like Ba2+) have a weaker polarizing effect on the larger carbonate ion, making the carbonate more stable.

Which is the least basic hydroxide among the alkaline earth metals?

Magnesium hydroxide (Mg(OH)2) is the least basic among the alkaline earth metal hydroxides. Basicity increases down the group as the size of the metal cation increases, leading to weaker attraction to the hydroxide ion and greater dissociation.

How can these NCERT solutions help in exam preparation?

These solutions provide clear, step-by-step explanations for MCQs, reinforcing understanding of the concepts, trends, and reasons behind the properties of s-block elements, which is crucial for exam revision and problem-solving.

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