Chemistry Class 12 NCERT Solutions Chapter 8 – d and f Block Elements – Important Questions

Introduction

Chemistry is a fundamental subject in the curriculum of Class 12, especially for students pursuing science streams. Chapter 8 of the Class 12 NCERT Chemistry textbook, titled "d and f Block Elements," covers essential concepts related to the transition metals and inner transition metals. This chapter is crucial for understanding various aspects of chemistry and is often the subject of examination questions. In this article, we'll explore important questions from Chapter 8, providing detailed solutions and explanations to help students prepare effectively for their exams.

Overview of Chapter 8 – d and f Block Elements

Chapter 8 of the NCERT Chemistry textbook introduces students to the d and f block elements, focusing on their properties, uses, and significance. Here are the key topics covered in this chapter:

1. d Block Elements: Transition metals, their general properties, and trends in their physical and chemical characteristics.
2. Electronic Configuration: The unique electronic configurations of d block elements and their implications.
3. Oxidation States: Various oxidation states exhibited by transition metals and their stability.
4. Color and Magnetism: The reasons behind the color of transition metal compounds and their magnetic properties.
5. Formation of Complex Compounds: The nature of complex compounds formed by transition metals and their applications.
6. f Block Elements: Lanthanides and actinides, their properties, and their role in various applications.

Important Questions and Solutions

1. Describe the general characteristics of d block elements.

Solution:

d block elements, also known as transition metals, exhibit several distinct characteristics:

• Electronic Configuration: d block elements have their valence electrons in the d orbitals. Their general electronic configuration is (n-1)d¹⁵ ns².
• Variable Oxidation States: They can exhibit multiple oxidation states. For example, iron (Fe) can exist in +2 and +3 oxidation states.
• Formation of Colored Compounds: Transition metal compounds often show vibrant colors due to d-d electron transitions.
• Magnetic Properties: They can be paramagnetic or diamagnetic depending on the presence of unpaired electrons.
• Catalytic Properties: Many d block elements act as catalysts in various chemical reactions due to their ability to change oxidation states.
• Complex Formation: They readily form complex compounds with ligands, which is due to their ability to accept electron pairs.

2. Explain the significance of the variable oxidation states of transition metals with examples.

Solution:

The variable oxidation states of transition metals are significant because they influence the reactivity and properties of these metals. Examples include:

• Iron (Fe): Iron exhibits oxidation states of +2 and +3. In ferrous compounds (Fe²⁺), iron shows a +2 oxidation state, while in ferric compounds (Fe³⁺), it shows a +3 oxidation state. This variability is crucial in biological systems, such as in hemoglobin, where iron’s oxidation state affects its ability to bind and release oxygen.
• Copper (Cu): Copper exhibits +1 and +2 oxidation states. Cuprous (Cu⁺) and cupric (Cu²⁺) ions have different chemical properties, such as reactivity and solubility, which are important in various industrial processes and applications.
• Chromium (Cr): Chromium can exhibit oxidation states ranging from +2 to +6. For example, in potassium dichromate (K₂Cr₂O₇), chromium is in the +6 oxidation state, which is important for its role as an oxidizing agent in chemical reactions.

3. Discuss the reasons behind the color of transition metal compounds.

Solution:

The color of transition metal compounds is primarily due to the d-d electron transitions. When light strikes a transition metal compound, it excites the d electrons from lower to higher energy d orbitals. The energy difference between these orbitals corresponds to specific wavelengths of visible light, which are absorbed. The color observed is the complement of the absorbed light.

For example:

• Copper Sulfate (CuSO₄): Copper sulfate is blue because it absorbs light in the red and green regions, reflecting the blue color.
• Chromium Chloride (CrCl₃): Chromium chloride is green due to the absorption of red and yellow wavelengths and reflection of green light.

The exact color depends on factors such as the ligand field strength, the nature of the ligands, and the oxidation state of the metal.

4. What are the common applications of transition metal complexes?

Solution:

Transition metal complexes have a wide range of applications in various fields:

• Catalysis: Many transition metal complexes act as catalysts in industrial processes. For example, the Haber process for ammonia synthesis uses an iron catalyst.
• Medicine: Some transition metal complexes are used in medicinal applications. For instance, cisplatin (a platinum complex) is used as a chemotherapy drug.
• Dyeing and Pigments: Complexes of transition metals like chromium and cobalt are used as dyes and pigments in textiles and ceramics.
• Electroplating: Transition metals such as nickel and chromium are used in electroplating to enhance the appearance and durability of metals.
• Sensors: Transition metal complexes are used in sensors for detecting gases and other substances.

5. Explain the role of f block elements in the periodic table and their general properties.

Solution:

The f block elements, consisting of the lanthanides and actinides, are positioned in two separate rows below the main body of the periodic table:

• Lanthanides (4f Block): These elements, from lanthanum (La) to lutetium (Lu), are known for their high magnetic susceptibility and similar chemical properties. They are often used in the production of strong magnets, phosphors for lighting, and catalysts in petroleum refining.
• Actinides (5f Block): These elements, from actinium (Ac) to lawrencium (Lr), include radioactive elements such as uranium and plutonium. They are significant in nuclear chemistry and energy production due to their radioactive properties and applications in nuclear reactors and weapons.

Both lanthanides and actinides show the filling of f orbitals and exhibit unique properties such as high density, high melting points, and reactivity.

6. How do the properties of transition metals differ from those of s block metals?

Solution:

Transition metals (d block) and s block metals have distinct properties due to their different electronic configurations:

• Electronic Configuration: Transition metals have partially filled d orbitals, whereas s block metals have their valence electrons in s orbitals.
• Oxidation States: Transition metals exhibit a range of oxidation states, while s block metals typically show only one or two oxidation states.
• Color: Transition metals often form colored compounds due to d-d transitions, while s block metals usually form colorless compounds.
• Magnetic Properties: Transition metals can be magnetic due to unpaired d electrons, whereas s block metals are generally not magnetic.
• Catalytic Activity: Transition metals are known for their catalytic properties, while s block metals are not typically used as catalysts.

7. Discuss the reasons for the formation of complex ions by transition metals.

Solution:

Transition metals form complex ions due to their ability to accept electron pairs from ligands. The reasons include:

• Empty d Orbitals: Transition metals have empty d orbitals that can accept electron pairs from ligands.
• Coordination Number: Transition metals can form complex ions with various coordination numbers (the number of ligand atoms bonded to the central metal ion). This flexibility allows for the formation of a variety of complex structures.
• Ligand Properties: Ligands are molecules or ions that donate electron pairs to the metal center. They can be monodentate (donating one pair of electrons) or polydentate (donating multiple pairs of electrons), influencing the formation of different types of complexes.
• Stability: The formation of complex ions often leads to increased stability due to chelation (when a ligand forms multiple bonds with the metal center) and favorable entropy changes.

8. What are the typical reactions of lanthanides and actinides with water?

Solution:

• Lanthanides: Lanthanides react with water to form hydroxides and release hydrogen gas. For example, lanthanum (La) reacts with water as follows:

  $$2 \text{La} + 6 \text{H}_2\text{O} \rightarrow 2 \text{La(OH)}_3 + 3 \text{H}_2$$

  These reactions are typically less vigorous compared to alkali metals but still produce significant amounts of hydrogen gas.

• Actinides: Actinides, such as uranium (U) and thorium (Th), also react with water, often more vigorously. For example, uranium reacts with water as follows:

  $$\text{U} + 2 \text{H}_2\text{O} \rightarrow \text{UO}_2\text{(OH)}_2 + 2 \text{H}_2$$

  The reactions of actinides with water can be influenced by their oxidation states and are often more complex due to their radioactive nature.

9. Explain the term 'Lanthanide Contraction' and its consequences.

Solution:

Lanthanide contraction refers to the gradual decrease in the atomic and ionic sizes of the lanthanide elements as the atomic number increases. This phenomenon occurs due to the poor shielding effect of the 4f electrons, which causes an increased effective nuclear charge.

Consequences of Lanthanide Contraction:

• Similar Properties: The lanthanide contraction leads to the similarity in properties of successive lanthanides, making their separation and identification challenging.
• Influence on Chemical Behavior: The contraction affects the chemical behavior of lanthanides and their compounds, influencing their reactivity and interaction with other elements.
• Impact on Transition Metals: The contraction affects the properties of the transition metals that follow the lanthanides, as they have similar sizes to the lanthanides. This influences their chemistry and physical properties.

10. Compare the reactivity of transition metals with that of alkali metals.

Solution:

Transition metals and alkali metals exhibit different reactivities due to their distinct electronic configurations and properties:

• Alkali Metals: Alkali metals (Group 1 elements) are highly reactive, especially with water, due to their single valence electron, which is easily lost to form a cation. Their reactivity increases down the group.

• Transition Metals: Transition metals are generally less reactive than alkali metals. They do not react as vigorously with water and often require specific conditions to undergo reactions. Their reactivity varies with oxidation states and ligands.

Conclusion

Chapter 8 of the Class 12 NCERT Chemistry textbook, focusing on d and f block elements, is integral for understanding the properties and applications of transition and inner transition metals. The important questions and solutions provided here offer a comprehensive overview of the key concepts, helping students prepare for their exams effectively. By mastering these topics, students will gain a solid foundation in chemistry that will be valuable in their academic and professional pursuits.