Key Fact
Catalytic behavior
Variable oxidation states + ability to form complexes → catalysis. V₂O₅ (contact process), Fe (Haber), Ni (hydrogenation), TiCl₃ (Ziegler-Natta).
-- NCERT Class 12 Chemistry, Ch. 4, p. 14Catalytic Behaviour
8 MCQs9-step worked example
Source: NCERT The d and f Block ElementsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
Transition metals act as catalysts in many industrial and biological reactions — and NEET asks about the why, not just the what. The core fact is straightforward: d-block elements exhibit catalytic behaviour because of (a) variable oxidation states allowing intermediate formation with reactants, and (b) large surface area in finely divided form (NCERT Class 12 Chemistry Chapter 4, page 14).
The mechanism link: A catalyst provides an alternative reaction pathway of lower activation energy. Transition metals achieve this by forming short-lived intermediates — cycling between oxidation states during the reaction and returning to the original state at the end. Example: Fe³⁺/Fe²⁺ in the Contact process; V₂O₅ cycling between V⁵⁺ and V⁴⁺ in sulfuric acid manufacture.
What NEET actually tests:
- Why transition metals (and not s-block metals) make effective catalysts — the answer hinges on variable oxidation states and partially filled d-orbitals enabling bonding with reactant molecules.
- Specific examples: Fe in Haber process (N₂ + 3H₂ → 2NH₃), V₂O₅ in Contact process (2SO₂ + O₂ → 2SO₃), Ni in hydrogenation, MnO₂ in KClO₃ decomposition.
- Finely divided form increases surface area → more active sites → better catalytic activity.
Watch-out: Do not confuse "variable oxidation states" with "high oxidation states." Zinc (+2 only) and scandium (+3 only) are poor catalysts precisely because they lack multiple accessible oxidation states — yet both are d-block elements. NEET uses this distinction as a distractor.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which property of transition metals is primarily responsible for their catalytic behaviour?
MCQ 2Easy RecallPractice
V₂O₅ is used as a catalyst in the Contact process. During catalysis, vanadium cycles between which oxidation states?
MCQ 3Direct ApplicationPractice
Which of the following d-block elements is a poor catalyst due to lack of variable oxidation states?
MCQ 4Direct ApplicationPractice
Finely divided iron is used in the Haber process rather than a large iron block because:
MCQ 5Direct ApplicationPractice
Which of the following correctly explains why scandium (Sc) is not commonly used as a catalyst?
MCQ 6Easy RecallPractice
In a catalysed reaction, the transition metal catalyst:
MCQ 7Easy RecallPractice
Among MnO₂, ZnO, Fe₂O₃, and CuO, which is commonly used as a catalyst for decomposition of KClO₃?
MCQ 8Concept TrapPractice
A student claims: "All d-block elements are good catalysts." Which pair of elements best disproves this claim?
Worked Example
- 1
Given
- V₂O₅: vanadium in +5 oxidation state - ZnO: zinc in +2 oxidation state - Reaction catalysed: 2SO₂ + O₂ → 2SO₃ (Contact process)
- 2
Required
Identify the property enabling V₂O₅ to act as a catalyst and explain why ZnO fails.
- 3
Concept
Catalytic behaviour in transition metals requires variable oxidation states to form intermediates with reactants, creating an alternative low-energy pathway.
- 4
Formula
No numerical formula required — this is a conceptual application of the variable-oxidation-state criterion.
- 5
Substitution / Application
- Vanadium: accessible oxidation states include +5, +4, +3, +2. In the Contact process, V⁵⁺ is reduced to V⁴⁺ by SO₂ (forming the intermediate), then reoxidised by O₂ back to V⁵⁺. - Zinc: only stable oxidation state is +2 (d¹⁰ configuration). Cannot be reduced or oxidised under Contact process conditions to form an intermediate.
- 6
Analysis
V₂O₅ cycles: V⁵⁺ + SO₂ → V⁴⁺ + SO₃, then V⁴⁺ + ½O₂ → V⁵⁺. The catalyst is regenerated. ZnO cannot undergo this cycling because Zn²⁺ has no accessible lower or higher oxidation state under these conditions.
- 7
Final answer
V₂O₅ is effective because vanadium has multiple accessible oxidation states (+4/+5 cycling), enabling intermediate formation and catalyst regeneration. ZnO fails because Zn²⁺ (d¹⁰) has only one stable oxidation state (+2) — the fundamental requirement for catalytic activity (variable oxidation states) is absent.
- 8
Common trap
Students may argue "ZnO is a d-block oxide, so it should catalyse reactions." The trap is equating "d-block membership" with "catalytic ability." The criterion is not membership in the d-block but the availability of multiple oxidation states for intermediate cycling.
- 9
Similar NEET-style question
"Among TiO₂, V₂O₅, ZnO, and CuO, which oxide(s) can act as effective catalysts for redox reactions and why?" ---
Before solving, remember these
Formulas
Spin-only magnetic moment
Magnetic moment from n unpaired electrons. 1 unpaired: 1.73 BM; 5: 5.92 BM.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
| mu | magnetic moment | Bohr magneton |
Valid when
- Spin-only contribution (no orbital contribution)
- Octahedral or tetrahedral complex
Spin-only magnetic moment for transition metal
Predicts paramagnetic moment of d-block ion. n unpaired electrons in d-orbitals.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
| mu | magnetic moment | BM |
Valid when
- Spin-only contribution
- Octahedral or tetrahedral complex
- First-row d-block
Common oxidation states (first-row TM)
Catalogues common stable oxidation states across first-row transition metals.
| Symbol | Quantity | SI Unit |
|---|---|---|
| OS | oxidation state | - |
Valid when
- First-row d-block
- Common (not exotic) compounds
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student assumes Mn²⁺ is the product regardless of medium. Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).
When it triggers
Question gives KMnO4 oxidation in unspecified or specific medium.
How to avoid
Always check medium. In acidic: Mn(+7) → Mn(+2). In neutral: → Mn(+4) (MnO₂). In basic: → Mn(+6) (manganate). The number of electrons (n) in Nernst calculations depends accordingly.
Root cause: concept gap
Correction
Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).
Root cause: concept gap
Correction
Lanthanoid contraction (imperfect 4f shielding) makes 5d elements similar in size to 4d. Zr/Hf, Nb/Ta, Mo/W have nearly identical chemistry.
Past Year Questions
10 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
NEET 2025
The correct order of decreasing basic strength of the given amines is:
1benzenamine > ethanamine > N-methylaniline > N-ethylethanamine
2N-methylaniline > benzenamine > ethanamine > N-ethylethanamine
3N-ethylethanamine > ethanamine > benzenamine > N-methylaniline
4N-ethylethanamine > ethanamine > N-methylaniline > benzenamine
NTA Answer: Option 4(final)
NEET 2024Revised key
1B and D only
2A and E only
3B and C only
4A and D only
NTA Answer: Option 1(revised_final)
NEET 2024Revised key
1d5 to d4 configuration
2d5 to d2 configuration
3d4 to d5 configuration
4d3 to d5 configuration
NTA Answer: Option 3(revised_final)
NEET 2024Revised key
The pair of lanthanoid ions which are diamagnetic is
1Ce4+ and Yb2+
2Ce3+ and Eu2+
3Gd3+ and Eu3+
4Pm3+ and Sm3+
NTA Answer: Option 1(revised_final)
NEET 2023
Which one of the following statements is correct?
1All enzymes that utilise ATP in phosphate transfer require Ca as the cofactor
2The bone in human body is an inert and unchanging substance
3Mg plays roles in neuromuscular function and interneuronal transmission
4The daily requirement of Mg and Ca in the human body is estimated to be 0.2-0.3 g
NTA Answer: Option 4(final)
NEET 2023
1B and D only
2C and D only
3B and C only
4A and E only
NTA Answer: Option 2(final)
NEET 2022
12.57
25.57
33.57
44.57
NTA Answer: Option 2(final)
NEET 2022
Gadolinium has a low value of third ionisation enthalpy because of
1high basic character
2small size
3high exchange enthalpy
4high electronegativity
NTA Answer: Option 3(final)
NEET 2022
1+6 to +5
2+7 to +4
3+6 to +4
4+7 to +3
NTA Answer: Option 2(final)
NEET 2021
Tritium, a radioactive isotope of hydrogen, emits which of the following particles? (β–)
1Neutron (n)
2Beta (α) (γ)
3Alpha
4Gamma
NTA Answer: Option 2(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Use KMnO4 or K2Cr2O7 in acidic/neutral/basic medium for oxidation. Different products by medium.
InterpretationMedium
Common distractors
uses acidic formula in basic medium
Assumes Mn²⁺ regardless of medium
Effect of lanthanoid contraction on properties of 4d/5d transition metals. Closer atomic/ionic radii, similar properties (Zr/Hf, Nb/Ta).
RecallEasy
Common distractors
predicts large difference due to period shift
Expects 5d to be much larger than 4d
Predict highest stable oxidation state for transition metal. Mn: +7 in MnO4-. Fe: +3 stable. Cu: +2.
InterpretationMedium
Common distractors
misses d0 d10 stability
Doesn't account for closed-shell stability
Sources
NCERT refs: Class 12 Chemistry Chapter 4, p.14
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