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Important Questions: CBSE Class 12 Chemistry Chapter 4 The d- and f-Block Elements

The d- and f-Block Elements is a conceptual and numerical powerhouse in CBSE Class 12 Chemistry, blending periodic trends, coordination chemistry, and industrial applications. Questions range from one-mark MCQs on electronic configuration to five-mark case studies on catalytic processes. This page organizes 18 meticulously framed questions by marks weightage, complete with model answers that mirror CBSE marking schemes and NCERT language, ensuring you walk into the Board exam fully prepared.

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Key takeaways

  • ✓Chapter 4 consistently contributes 4–6 marks in the CBSE Class 12 Chemistry paper, split between MCQs and long-answer questions.
  • ✓Transition metals, lanthanoid contraction, and oxidation-state variability are high-frequency topics in Board exams.
  • ✓CBSE often asks property comparisons between 3d, 4d, and 5d series transition elements or lanthanoids versus actinoids.
  • ✓Case-based questions on industrial catalysts (like Fe, Ni, V₂O₅) and interstitial compounds appear regularly since 2023.
  • ✓Numerical problems on magnetic moments using spin-only formula are straightforward scoring opportunities.
  • ✓Common mistakes include confusing electronic configurations of Cr and Cu and misapplying the lanthanoid contraction concept.
  • ✓CBSETUTOR.ai offers 24×7 doubt-solving for tricky d-block numericals via photo upload at ₹999/month with a 3-day free trial.

Chapter Overview and Marks Weightage in CBSE Class 12 Exam

Chapter 4 — The d- and f-Block Elements — falls under the Inorganic Chemistry unit, which collectively accounts for roughly 15 marks in the CBSE Class 12 Chemistry paper. This chapter alone typically delivers 4 to 6 marks across multiple question types: one or two MCQs (1 mark each), one VSA or SA-I (2 marks), and occasionally a 3-mark or 5-mark long-answer or case-based question. The 2024 and 2025 Board papers saw increased emphasis on lanthanoid contraction applications, catalytic roles of transition metals, and magnetic-moment calculations. Understanding the periodic trends within the 3d, 4d, and 5d series, the unique chemistry of lanthanoids and actinoids, and industrially significant compounds like K₂Cr₂O₇ and KMnO₄ is non-negotiable. NCERT exemplar problems and previous-year questions reveal that CBSE favours property-comparison tables, electronic-configuration exceptions (Cr, Cu, Mo, Ag), and spin-only magnetic-moment numericals. Mastering these 18 curated questions will cover every angle the Board can test.
  • Inorganic Chemistry unit weightage: ~15 marks; Chapter 4 contributes 4–6 marks.
  • Question mix: 1–2 MCQs, 1 VSA (2 marks), 1 SA or LA (3–5 marks).
  • High-frequency topics: lanthanoid contraction, variable oxidation states, catalytic properties, interstitial compounds.
  • Numerical problems appear on magnetic moment (spin-only formula) and ionic radii trends.
  • Case-based questions (5 marks) often link d-block metals to real catalysts (Haber, Contact, or Ostwald processes).

1-Mark Questions: MCQ and Very Short Answer (VSA)

One-mark questions test quick recall of definitions, electronic configurations, and simple property trends. CBSE typically includes two to three such questions in Section A (MCQs) of the Chemistry paper. Focus on NCERT in-text questions and the summary tables at the end of Chapter 4. Ensure you know the exceptions in 3d electronic configurations (Cr: [Ar]3d⁵4s¹, Cu: [Ar]3d¹⁰4s¹) and can identify lanthanoids versus actinoids by their f-orbital occupancy. These are scoring questions where careless reading or half-learned facts cost marks.

2-Mark Questions: Short Answer Type-I

Two-mark questions demand concise explanations, property comparisons, or reasoning backed by electronic structure. CBSE expects two distinct points or a definition plus one application. Common themes include why transition metals are good catalysts, the cause and consequence of lanthanoid contraction, and the color origin in d-block compounds. Write in clear, bulleted statements during the exam to avoid verbose answers that waste time. Remember to use NCERT language: say 'incomplete d subshell' rather than 'partially filled d orbitals' unless both are acceptable. Each question below models the two-point structure CBSE markers look for.

3-Mark Questions: Short Answer Type-II and Reasoning

Three-mark questions require structured explanations, often asking 'why' or 'compare and contrast.' A model answer should contain three distinct points or a two-point explanation plus a relevant example or equation. CBSE values clarity: label points (i), (ii), (iii) if helpful. Popular 3-mark themes include explaining lanthanoid contraction consequences, distinguishing lanthanoids from actinoids, describing the preparation and properties of KMnO₄ or K₂Cr₂O₇, and calculating magnetic moments. Show all steps in numerical problems even if the arithmetic is trivial; partial marks are awarded for method. The six questions below mirror the difficulty and phrasing seen in 2023–2025 papers.

5-Mark Questions: Long Answer and Case-Based

Five-mark questions are the crown jewels of Chapter 4, often appearing as case studies that test understanding across multiple subtopics. A typical case-based question presents a passage on an industrial process (Haber, Contact, manufacture of steel) or a series property trend, followed by four or five sub-questions totaling 5 marks. Answer each sub-question separately, using headings if space permits. CBSE awards marks cumulatively: even if you miss one sub-part, the others still earn credit. Structure your answer with clear chemical equations, oxidation-state assignments, and references to NCERT concepts like variable valency or catalytic activity. The three questions below are representative of 2024–2025 Board patterns.

How CBSE Frames Questions from The d- and f-Block Elements

CBSE question setters mine Chapter 4 for conceptual depth and numerical application. Expect assertion-reason MCQs that pair a statement ('Transition elements are good catalysts') with a reason ('They have variable oxidation states and large surface area'). The Board loves property-trend questions: 'Arrange Sc, Ti, V in increasing first ionization enthalpy' or 'Why do ionic radii decrease from La³⁺ to Lu³⁺?' Case-based questions tie theory to industry — Haber process (Fe catalyst), Contact process (V₂O₅), or extraction of metals — and probe oxidation states, reaction mechanisms, and environmental impacts. Numerical problems stick to the spin-only magnetic-moment formula and simple stoichiometry in redox titrations. Since 2020, assertion-reason and case-based formats dominate, so practicing these question styles is critical. Always quote NCERT definitions verbatim when a term like 'lanthanoid contraction' or 'interstitial compound' appears; examiners cross-check against the textbook glossary.
  • Assertion-Reason MCQs test conceptual links (e.g., colour origin and incomplete d subshell).
  • Property-comparison questions require tabular or point-wise answers (3d vs 4d series, lanthanoids vs actinoids).
  • Case studies integrate industrial chemistry: catalysts, alloy composition, redox applications.
  • Numerical problems focus on magnetic moment (μ = √[n(n+2)] BM) and redox stoichiometry.
  • CBSE values NCERT language: use 'imperfect shielding' for lanthanoid contraction, 'variable oxidation states' for catalytic activity.
  • Diagrams are rarely asked, but labeling oxidation states in reactions earns method marks.

Additional 3-Mark and 5-Mark Practice Questions with Answers

To deepen your preparation, here are six more questions mirroring recent Board trends. Work through each under timed conditions, then compare your answer structure against the model. Pay attention to keywords: 'account for,' 'compare,' 'calculate,' 'explain with an example.' Each verb demands a different response format. For instance, 'account for' means list reasons; 'compare' requires a two-column or point-by-point contrast; 'calculate' needs the formula, substitution, and final answer with units. These nuances are where careful students gain extra marks.

Common Mistakes Students Make in The d- and f-Block Elements

Even strong students stumble on Chapter 4 due to conceptual overlaps and memory-intensive data. The most frequent error is writing the electronic configuration of Cr as [Ar]3d⁴4s² instead of the correct [Ar]3d⁵4s¹. Similarly, Cu is often mis-stated as [Ar]3d⁹4s² rather than [Ar]3d¹⁰4s¹. These exceptions must be memorised. Another pitfall is confusing lanthanoid contraction with actinoid contraction; the former is responsible for the nearly identical radii of 4d and 5d series elements (Zr–Hf, Nb–Ta pairs), while actinoid contraction affects post-actinide elements. Students also mix up oxidation states: writing Mn⁷⁺ in MnO₄⁻ is correct, but forgetting that Mn²⁺ is pale pink, not colourless, costs marks in qualitative analysis questions. In numericals, many apply the spin-only formula without first determining whether the complex is high-spin or low-spin; always check ligand field strength. When writing redox equations, balance atoms and charge separately, then combine; skipping the half-reaction method leads to incorrect coefficients. Finally, students often cite 'large size' as the reason for catalytic activity, whereas the NCERT-approved reason is 'variable oxidation states and ability to adsorb reactants due to partially filled d orbitals.' Precision in language earns marks.
  • Electronic configuration errors: Cr is [Ar]3d⁵4s¹, Cu is [Ar]3d¹⁰4s¹; memorise exceptions.
  • Confusing lanthanoid contraction (La to Lu) with actinoid contraction; know the consequence (Zr ≈ Hf in size).
  • Incorrectly stating Mn²⁺ as colourless; it is very pale pink (d⁵, high-spin weak colour).
  • Applying spin-only formula without checking ligand field: strong-field ligands cause pairing.
  • Redox equation balancing: use half-reactions; don't guess coefficients, especially for Cr₂O₇²⁻ and MnO₄⁻ equations.
  • Vague reasons: say 'variable oxidation states and surface adsorption' for catalytic activity, not just 'transition metals are reactive.'
  • Omitting units in magnetic moment (BM) and not showing substitution steps in numericals.

Exam Strategy and Time Management for Chapter 4 Questions

Allocate your time according to marks: roughly 1 minute per mark as a rule of thumb. For a 5-mark case-based question, spend 5–6 minutes reading the passage, underlining keywords, and drafting sub-answers. Start with 1-mark MCQs to build confidence and bank easy marks. In Section B (VSA/SA), tackle 2-mark and 3-mark questions from Chapter 4 before moving to organic or physical chemistry if you find inorganic more comfortable. Use the first reading time (15 minutes in CBSE) to scan all d-block questions and tick those you can solve immediately. For numerical problems, write the formula first, then substitute values, and box the final answer; this makes it easy for the examiner to award method marks even if arithmetic goes wrong. In long answers, underline or bold the key terms (e.g., 'lanthanoid contraction,' 'interstitial compound') so they stand out. Avoid lengthy prose; bullet points and chemical equations score better. If a question asks for three points, write exactly three distinct points; extra points rarely fetch bonus marks and waste time. Finally, reserve 5 minutes at the end to review Chapter 4 answers: check equation balancing, verify oxidation states, and ensure all sub-parts are attempted. One missed sub-question in a case study can cost 2 marks.
  • Allocate ~1 minute per mark; 5-mark case study = 5–6 minutes including reading time.
  • Attempt 1-mark MCQs first to secure quick wins and build momentum.
  • In 15-minute reading time, identify all d-block questions and prioritise by confidence.
  • Write formula → substitution → boxed answer for numericals; method marks are awarded even if final answer is wrong.
  • Use bullet points and underline keywords (lanthanoid contraction, +6 oxidation state) for examiner clarity.
  • Answer exactly the number of points asked; three reasons means three bullets, not a paragraph.
  • Reserve final 5 minutes to cross-check equation balancing, oxidation states, and units (BM for magnetic moment).

Leveraging CBSETUTOR.ai for Mastery of d- and f-Block Elements

The d- and f-Block Elements chapter rewards deep understanding and pattern recognition, both of which come from solving diverse problems and clarifying doubts instantly. CBSETUTOR.ai provides exactly that: a 24×7 AI tutor trained on NCERT, CBSE marking schemes, and past Board papers, available at ₹999 per month for all classes (6–12). Simply snap a photo of any tricky question — whether it is calculating the spin-only magnetic moment of a complex ion, balancing a redox equation involving Cr₂O₇²⁻, or comparing lanthanoid and actinoid properties — and receive a step-by-step solution within seconds. The platform also generates unlimited practice MCQs and case-based questions tailored to Chapter 4, helping you internalise the question patterns CBSE loves. Parents appreciate the flat pricing and transparency: one subscription covers every subject and chapter, with a 3-day free trial so students can test the AI before committing. For Class 12 Chemistry, where d-block numericals and conceptual reasoning intertwine, having an on-demand tutor ensures no doubt lingers unresolved, keeping your preparation sharp and exam-ready.
  • 24×7 AI tutor for instant doubt-solving: upload photos of d-block numericals, redox equations, or case studies.
  • Step-by-step solutions aligned to CBSE marking schemes and NCERT terminology.
  • Unlimited practice MCQs and assertion-reason questions for Chapter 4.
  • Flat ₹999/month for Classes 6–12, all subjects; no hidden fees or per-question charges.
  • 3-day free trial lets students experience AI-powered learning risk-free.
  • Ideal for revising lanthanoid contraction, magnetic moments, and catalytic-property questions before Board exams.

Frequently asked questions

How many marks does Chapter 4 The d- and f-Block Elements carry in CBSE Class 12 Chemistry?+
Chapter 4 typically contributes 4 to 6 marks in the Board exam, split between 1-mark MCQs, 2-mark VSAs, and a 3-mark or 5-mark long-answer or case-based question. The exact distribution varies yearly, but inorganic chemistry as a unit accounts for roughly 15 marks total.
What are the most important topics in The d- and f-Block Elements for Board exams?+
High-frequency topics include electronic configurations (especially Cr and Cu exceptions), lanthanoid contraction and its consequences, variable oxidation states of transition metals, calculation of magnetic moments using the spin-only formula, properties and preparation of KMnO₄ and K₂Cr₂O₇, interstitial compounds, and comparison of lanthanoids versus actinoids.
How do I remember the electronic configuration exceptions for Cr and Cu?+
Chromium adopts [Ar]3d⁵4s¹ (not 3d⁴4s²) for extra stability from a half-filled d subshell. Copper takes [Ar]3d¹⁰4s¹ (not 3d⁹4s²) for a completely filled d subshell. A mnemonic: 'Cr loves half, Cu loves full.' Revise these daily until automatic.
What is lanthanoid contraction and why is it important?+
Lanthanoid contraction is the steady decrease in atomic and ionic radii from La³⁺ to Lu³⁺, caused by imperfect shielding of 4f electrons, which increases effective nuclear charge. It explains why 4d and 5d series elements (e.g., Zr and Hf, Nb and Ta) have nearly identical radii, affecting their chemistry and separation difficulty.
How do I approach case-based questions on d-block elements?+
Read the passage carefully, underline keywords (catalyst name, oxidation state, colour change), and note what each sub-question asks. Answer each sub-part separately with a clear heading. Show all working for numericals, write balanced equations for redox questions, and use NCERT terminology. Practice 5–6 case studies from previous years to build familiarity.
Why are transition metals good catalysts?+
Transition metals exhibit variable oxidation states and have partially filled d orbitals that can adsorb reactant molecules, lowering activation energy. They also provide a large surface area. Examples include Fe in the Haber process, V₂O₅ in the Contact process, and Ni in hydrogenation. Always cite these reasons in exams.
How do I calculate spin-only magnetic moment for a complex ion?+
First, determine the metal ion's oxidation state and electronic configuration. Identify the ligand field strength: strong-field ligands (CN⁻, CO) cause pairing (low-spin), weak-field ligands (F⁻, H₂O) do not (high-spin). Count unpaired electrons (n), then apply μ = √[n(n+2)] BM. Always show substitution and box the final answer with units.
What is the difference between lanthanoids and actinoids?+
Lanthanoids ([Xe]4f¹⁻¹⁴5d⁰⁻¹6s²) predominantly show +3 oxidation state, have well-shielded 4f electrons, and are less radioactive. Actinoids ([Rn]5f¹⁻¹⁴6d⁰⁻¹7s²) exhibit multiple oxidation states (+3 to +7), have poorly shielded 5f electrons that participate in bonding, and are mostly radioactive. This comparison is a frequent 3-mark or 5-mark question.
Why is potassium dichromate preferred over potassium permanganate in volumetric analysis?+
K₂Cr₂O₇ is a primary standard — it is available in high purity, stable, non-hygroscopic, and its solutions do not decompose. KMnO₄ is not a primary standard; it is hygroscopic, decomposes in light and acidic media, and requires standardisation against a primary standard before use.
Can CBSETUTOR.ai help with Chapter 4 important questions?+
Yes. CBSETUTOR.ai offers instant, step-by-step solutions to d-block numericals, redox equations, and conceptual questions via photo upload. It generates unlimited practice MCQs and case-based questions, all aligned to CBSE patterns. At ₹999/month for all subjects (Classes 6–12) with a 3-day free trial, it is an affordable 24×7 tutor for mastering Chapter 4.
What are interstitial compounds and why are they hard?+
Interstitial compounds form when small non-metal atoms (H, C, N) occupy voids in the metal lattice without disturbing the metallic structure. They are hard because the small atoms lock the metal atoms in place, reducing slip planes. Examples include TiH₁.₇ and Fe₃C (steel). They retain metallic conductivity and have high melting points.
How should I write redox equations involving Cr₂O₇²⁻ or MnO₄⁻ in exams?+
Use the half-reaction method: write the reduction half (Cr₂O₇²⁻ → Cr³⁺ or MnO₄⁻ → Mn²⁺) and oxidation half separately, balance atoms and charge by adding H⁺, H₂O, and electrons, multiply to equalise electrons, then add and simplify. Always verify final equation for atom and charge balance. Show all steps for method marks.

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