Chapter 8 - The d and f Block Elements Free PDF Download
FAQs on NCERT Books Free Download for Class 12 Chemistry Chapter 8 - The d and f Block Elements
1. What are the most important topics from 'The d and f-Block Elements' for the CBSE Class 12 Board Exam 2025-26?
For the Class 12 board exams, the highest-priority topics from this chapter are:
- Properties of Transition Elements: Focus on the trends in atomic radii, ionisation enthalpy, variable oxidation states, and magnetic properties.
- K₂Cr₂O₇ and KMnO₄: The structure, methods of preparation, and especially the oxidising properties in acidic, basic, and neutral media are frequently asked.
- Lanthanoid Contraction: You must understand the definition, cause, and its key consequences.
- Comparison of Lanthanoids and Actinoids: Be prepared to differentiate their properties.
- Electronic Configuration: Practise writing the configurations for d-block and f-block elements and their common ions.
2. Which questions on the preparation and properties of KMnO₄ and K₂Cr₂O₇ are frequently asked?
From this section, you can expect questions focusing on:
- The balanced chemical equations for the preparation of Potassium Permanganate from pyrolusite ore (MnO₂).
- The oxidising action of KMnO₄ in acidic, alkaline, and neutral mediums. Be prepared to write balanced ionic equations for reactions with Fe²⁺, oxalate ions (C₂O₄²⁻), and iodide ions (I⁻).
- The effect of pH on the interconversion of chromate (CrO₄²⁻) and dichromate (Cr₂O₇²⁻) ions, which is a key concept.
- Drawing the structures of the permanganate (MnO₄⁻) and dichromate (Cr₂O₇²⁻) ions.
3. What type of 1-mark or MCQ questions can be expected from the d and f-Block Elements chapter?
For objective-type questions, focus on concepts that can be answered quickly:
- Identifying elements with a specific electronic configuration.
- Calculating the spin-only magnetic moment or finding the number of unpaired electrons in an ion.
- Reasoning for the colour of transition metal ions (e.g., explaining why Zn²⁺ is colourless but Cu²⁺ is blue).
- Determining the correct oxidation state of the central metal atom in compounds like K₂Cr₂O₇ or KMnO₄.
- Identifying the correct products in specific redox reactions.
4. From which topics are 5-mark questions typically framed in this chapter?
A 5-mark question from this chapter is usually a composite question with several parts. The most probable structure involves:
- Part A: A set of 3-4 'give reason' statements based on trends like atomic size, ionisation enthalpy, catalytic behaviour, or variable oxidation states.
- Part B: A question on Lanthanoid Contraction, requiring a definition and an explanation of two of its major consequences.
- Part C: Completing and balancing a chemical reaction involving the oxidising action of KMnO₄ or K₂Cr₂O₇ in a specified medium.
5. Why is Lanthanoid Contraction considered an important concept for board exams, and what are its key consequences?
Lanthanoid Contraction is a crucial concept as it explains many chemical trends across the periodic table. It is the steady decrease in the size of lanthanoid atoms and ions with increasing atomic number. This occurs due to the poor shielding effect of 4f electrons. Its most important consequences, often tested in exams, are:
- Similarity in properties of 2nd and 3rd transition series: Elements in the same group, such as Zirconium (Zr) and Hafnium (Hf), have almost identical atomic radii and chemical properties, which makes their separation difficult.
- Basicity of lanthanoid hydroxides: The basic character of the hydroxides decreases steadily from La(OH)₃ to Lu(OH)₃ as the ionic size decreases and covalent character increases.
6. How can one predict the magnetic properties of transition metal ions? What makes this a common exam question?
Magnetic properties are predicted by examining the electronic configuration of the ion. The steps are:
- Write the electronic configuration of the neutral atom.
- Remove electrons to form the ion, starting from the outermost 's' orbital, then from the '(n-1)d' orbital.
- Count the number of unpaired electrons ('n') in the d-orbitals.
If the ion has one or more unpaired electrons, it is paramagnetic. If all electrons are paired, it is diamagnetic. This is a common exam question because it directly tests your understanding of both electronic configuration and the origin of magnetism in d-block elements. You may also be asked to calculate the spin-only magnetic moment (µ) using the formula µ = √n(n+2) Bohr Magnetons.
7. What is the most common mistake students make when answering questions about the oxidizing action of KMnO₄ in different media?
The most frequent error is writing the incorrect reduction product for the permanganate ion (MnO₄⁻) based on the reaction medium. Students often default to using the acidic medium product for all scenarios. For full marks, you must remember the specific products:
- In an acidic medium, MnO₄⁻ is reduced to the nearly colourless Mn²⁺ ion.
- In a neutral or weakly alkaline medium, MnO₄⁻ forms a brown precipitate of MnO₂.
- In a strongly alkaline medium, MnO₄⁻ is reduced to the green manganate ion, MnO₄²⁻.
Using the wrong product leads to an incorrect balanced equation and a significant loss of marks.
8. Compare the properties of Lanthanoids and Actinoids. Why is this comparison a frequently asked 3-mark question?
This comparison is a classic exam question because it assesses your ability to distinguish between the two f-block series. Key differences to mention for full marks are:
- Oxidation States: Lanthanoids predominantly show a +3 oxidation state. Actinoids exhibit a much wider range of oxidation states (up to +7) due to the small energy gap between 5f, 6d, and 7s orbitals.
- Radioactivity: All actinoids are radioactive, whereas among lanthanoids, only promethium is radioactive.
- Complex Formation: Actinoids have a greater tendency to form complexes than lanthanoids. This is due to their higher charge densities.
- Chemical Reactivity: Actinoids are generally more reactive than lanthanoids, especially the earlier members of the series.
