JEE Advanced 2026 Paper 2 — Full Mock Test (54 Questions, 180 Marks)
Mathematics · Physics · Chemistry
This MCQ module is based on: Stability Applications
TOPIC 22 OF 23
Stability Applications
🎓 Class 12
Chemistry
CBSE
Theory
Ch 5 – Coordination Compounds
⏱ ~14 min
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Stability Applications
5.7 Bonding in Metal Carbonyls — The Synergic Effect
Most transition metals form homoleptic carbonyls in which CO is the only ligand. They have well-defined geometries:
| Carbonyl | Metal oxidation state | Geometry |
|---|---|---|
| [Ni(CO)4] | 0 | Tetrahedral |
| [Fe(CO)5] | 0 | Trigonal bipyramidal |
| [Cr(CO)6] | 0 | Octahedral |
| [Mn2(CO)10] | 0 | Two square pyramids joined by Mn–Mn bond |
| [Co2(CO)8] | 0 | Co–Co bond bridged by two CO groups |
Synergic σ–π bonding
The metal-carbon bond in carbonyls has dual character:
- σ-bond: the lone pair on the carbonyl carbon donates into a vacant metal hybrid orbital (CO → M).
- π-bond: a filled metal d-orbital donates back into the empty antibonding π* orbital of CO (M → CO).
This synergic effect reinforces both bonds — the σ donation increases the d-electron density on the metal, making π-back-donation easier, and the π-back-donation removes some of that electron density, making the next σ donation stronger. The result is a particularly strong, short M–C bond and weakened C–O bond (longer than in free CO).
5.8 Stability of Coordination Compounds
The thermodynamic stability of a complex in solution is described by its formation constant (β) — the equilibrium constant for the overall replacement of solvent (water) by ligand:
\[ M^{n+} + nL \rightleftharpoons [ML_n]^{n+}, \qquad \beta_n = \frac{[ML_n^{n+}]}{[M^{n+}][L]^n} \]Larger β means a more stable complex. Typical trends (in order of increasing β for a given metal): F− < Cl− < Br− < I− for "soft" metal ions; chelates always > comparable monodentate complexes (chelate effect).
| Equilibrium | log β | Comment |
|---|---|---|
| Cu²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄]²⁺ | ≈ 11.6 | Deep-blue colour test for Cu²⁺. |
| Cu²⁺ + 4CN⁻ ⇌ [Cu(CN)₄]³⁻ | ≈ 30 (very high) | So stable that H₂S no longer precipitates CuS. |
| Ag⁺ + 2CN⁻ ⇌ [Ag(CN)₂]⁻ | ≈ 21 | Used in electroplating & gold extraction. |
| Fe³⁺ + 3C₂O₄²⁻ ⇌ [Fe(C₂O₄)₃]³⁻ | ≈ 20 | Chelated → very stable (chelate effect). |
| Ca²⁺ + EDTA⁴⁻ ⇌ [Ca(EDTA)]²⁻ | ≈ 10.7 | Hardness titration of water. |
The chelate effect
Replacing several monodentate ligands by one polydentate (chelating) ligand always increases stability — even when the donor atoms are chemically identical. The driving force is mostly entropic: one large polydentate ligand displaces several small monodentate ligands, increasing the number of free particles in solution and \(\Delta S^\circ\).
5.9 Importance and Applications of Coordination Compounds
1. Analytical chemistry
EDTA, dimethylglyoxime (DMG, brick-red Ni complex), α-nitroso-β-naphthol and cupron form characteristic coloured complexes used in qualitative tests and instrumental analysis.2. Hardness of water
Ca2+ and Mg2+ form stable EDTA chelates with different formation constants, allowing selective complexometric titration of total hardness using Na2EDTA.3. Extraction of metals
Gold: 4Au + 8CN− + O2 + 2H2O → 4[Au(CN)2]− + 4OH−; gold is then displaced by zinc. Silver is extracted similarly.4. Purification of nickel — Mond's process
Impure Ni + CO at 50–60 °C → [Ni(CO)4] (volatile). Heating the carbonyl to ~230 °C decomposes it back to pure Ni and CO (recycled).5. Catalysis
Wilkinson's catalyst [(PPh3)3RhCl] selectively hydrogenates alkenes. Many industrial processes (Wacker, hydroformylation) rely on coordination chemistry.6. Electroplating
Plating with silver and gold from [Ag(CN)2]− and [Au(CN)2]− baths gives much smoother, evener deposits than from solutions of free metal ions.7. Photography
In black-and-white photography, the developed film is "fixed" with hypo (Na2S2O3) which dissolves undecomposed AgBr by forming [Ag(S2O3)2]3−.8. Medicine — chelation therapy
EDTA — for lead poisoning. D-penicillamine and desferrioxime B — for excess Cu and Fe (Wilson's disease, β-thalassaemia). cis-platin and related Pt(II) complexes — anti-cancer drugs.5.9.1 Coordination Compounds in Biology
| Bio-molecule | Metal centre | Function |
|---|---|---|
| Haemoglobin / Myoglobin | Fe2+ in porphyrin ring | O2 transport (RBC) / O2 storage (muscle) |
| Chlorophyll | Mg2+ in porphyrin ring | Light absorption for photosynthesis |
| Vitamin B12 (cyanocobalamin) | Co3+ in corrin ring | Anti-pernicious-anaemia factor; methyl-group transfer |
| Carboxypeptidase A, carbonic anhydrase | Zn2+ | Enzymes catalysing peptide hydrolysis / CO2 hydration |
🧪 Activity 5.4 — The Disappearing Cu²⁺ (Predict → Observe → Explain)
Setup: Aqueous CuSO4 (blue) is divided into three test tubes. To tube 1 add aqueous KF; to tube 2 add aqueous KCl; to tube 3 add excess aqueous KCN, then bubble H2S through the resulting solution.
Predict: What colour change occurs in each tube? Will tube 3 give a black CuS precipitate when H2S is added?
Tube 1 (KF): Cu2+ + 4F− → [CuF4]2−; the solution turns green-pale (a green precipitate of CuF2 can also form initially).
Tube 2 (KCl): Cu2+ + 4Cl− → [CuCl4]2−; bright green solution (Cl− gives a different absorption).
Tube 3 (KCN): 2Cu2+ + 10CN− → 2[Cu(CN)4]3− + (CN)2 (CN− reduces Cu(II) to Cu(I)). The very high formation constant (~1030) means [Cu+] is so low that Ksp of CuS is not exceeded — no CuS precipitate on bubbling H2S.
This shows how a high-stability complex effectively removes a metal ion from "free-ion" chemistry.
🔧 Interactive: Application Explorer
Pick an application area; the simulation shows the relevant coordination compound, the chemistry behind it, and a real-life example.
Pick an application above.
Worked Example 5.10 — Coordination compound formed by excess CN⁻ on CuSO₄
What is the entity formed when excess KCN is added to aqueous CuSO4? Why does H2S no longer precipitate CuS from this solution?
The very stable [Cu(CN)4]3− ion is formed (Cu(I), as CN− reduces Cu(II) to Cu(I) with release of cyanogen). Its formation constant is enormous (~1030). The free [Cu+] in solution is so vanishingly small that the ionic product [Cu+]2[S2−] never reaches Ksp(Cu2S), so no precipitate forms.
Worked Example 5.11 — Why do Fe²⁺ tests work on Mohr's salt but not on K₄[Fe(CN)₆]?
Mohr's salt FeSO4·(NH4)2SO4·6H2O is a double salt that fully dissociates in water to give free Fe2+. Standard tests (e.g. K3[Fe(CN)6] gives the deep-blue Turnbull's blue) work as expected.
K4[Fe(CN)6] is a complex; the [Fe(CN)6]4− ion has β ≈ 1035. Free [Fe2+] is too low to be detected by standard tests. Similarly, with CuSO4 + 4 NH3 the deep-blue [Cu(NH3)4]2+ forms and no Cu2+ spot test works.
K4[Fe(CN)6] is a complex; the [Fe(CN)6]4− ion has β ≈ 1035. Free [Fe2+] is too low to be detected by standard tests. Similarly, with CuSO4 + 4 NH3 the deep-blue [Cu(NH3)4]2+ forms and no Cu2+ spot test works.
🎯 Competency-Based Questions
Q1. The metal ion at the centre of haemoglobin is: L1 Remember
Answer: (b) Fe²⁺ in a porphyrin ring (haem). Mg²⁺ is in chlorophyll, Co³⁺ in vitamin B₁₂.
Q2. Which complex has the highest stability? L3 Apply
Answer: (c) [Fe(C₂O₄)₃]³⁻. Oxalate is didentate, so this is a chelate complex — much more stable than the corresponding monodentate complexes (chelate effect).
Q3. In the Mond process for nickel refining, the role of the coordination compound is: L3 Apply
Impure nickel reacts with CO at ~50 °C to form volatile Ni(CO)₄. The carbonyl is separated by distillation/passage through a hot zone (~230 °C), where it decomposes back to pure Ni and CO. The CO is recycled. Coordination chemistry separates Ni from impurities that cannot form a volatile carbonyl.
Q4. A patient with chronic lead poisoning is treated with EDTA. Explain the chemistry. L4 Analyse
Answer: EDTA4− is hexadentate (2N + 4O donor atoms) and forms an extremely stable [Pb(EDTA)]2− chelate. The chelate is water-soluble and excreted in urine, removing toxic Pb2+ from tissues. The chelate effect makes the rate of complex formation high and the stability great enough to outcompete biological binding.
Q5. HOT (Create): Design a school-lab demonstration to estimate Ca²⁺ + Mg²⁺ "total hardness" of tap water in mol L⁻¹. List apparatus, indicator, and one observation. L6 Create
Use complexometric titration with disodium-EDTA. Apparatus: 25 mL pipette, burette, conical flask. Add 25 mL water sample + 5 mL pH-10 ammonia/ammonium-chloride buffer + 2 drops Eriochromeblack-T indicator → wine-red colour. Titrate with standard 0.01 M Na2EDTA until colour changes sharply to blue (end point: all Ca²⁺/Mg²⁺ chelated by EDTA). Calculation: total hardness (mol L⁻¹) = (VEDTA × CEDTA) ÷ Vsample.
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Gold can be extracted from its ore by treating with NaCN solution and air.
R: Gold forms a soluble complex [Au(CN)2]− from which it can later be displaced by zinc.
Answer: (A). Both true; R correctly explains A. This is the cyanide leaching (MacArthur-Forrest) process.
A: Cisplatin is used as an anti-cancer drug.
R: Only the cis isomer of [Pt(NH3)2Cl2] can bind two adjacent guanine bases on DNA.
Answer: (A). Both true; R correctly explains A. The trans isomer is geometrically unable to make the same DNA cross-link and is biologically inactive.
A: Chelate complexes are more stable than complexes containing the same number of monodentate ligands of the same donor atom.
R: Formation of a chelate increases the entropy of the system because several free monodentate ligands are released when a single polydentate ligand binds.
Answer: (A). Both true; R correctly explains A. The chelate effect is largely entropic.
Frequently Asked Questions - Stability Applications
What is the main concept covered in Stability Applications?
In NCERT Class 12 Chemistry Chapter 5 (Coordination Compounds), "Stability Applications" covers the core chemistry principles and reactions students need for board exam success. The MyAiSchool lesson explains the topic with definitions, structural diagrams, reaction mechanisms, worked examples, and interactive simulations. Key reactions, IUPAC names, and chemical reasoning are highlighted throughout aligned with CBSE 2025-26 syllabus.
How is Stability Applications useful in real-life or applied chemistry?
Real-life applications of "Stability Applications" from NCERT Class 12 Chemistry Chapter 5 include drug design, polymer industry, food chemistry, electrochemical cells, fuel cells, dyes/pigments, agrochemicals, and biochemistry. The MyAiSchool lesson links every concept to a tangible industrial or biological example so students see chemistry as a problem-solving framework for the molecular world.
What are the key reactions students should memorize for Stability Applications?
Key reactions in "Stability Applications" (NCERT Class 12 Chemistry Chapter 5 Coordination Compounds) are tabulated in the MyAiSchool reaction map. Students should memorize each reaction with its reagent, conditions, mechanism class (SN1/SN2/E1/E2/electrophilic addition/etc), product, and stereochemistry. The Summary section provides a quick-reference reaction chart for last-minute revision.
How does this part connect to other parts of Chapter 5?
NCERT Class 12 Chemistry Chapter 5 (Coordination Compounds) is structured so each part builds chemical understanding sequentially. "Stability Applications" connects to neighbouring parts via shared functional groups, reaction mechanisms, and structural concepts. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Stability Applications?
CBSE board questions from "Stability Applications" typically include: (1) 1-mark MCQs on definitions and IUPAC naming, (2) 2-mark short-answer reactions/products, (3) 3-mark mechanism questions, (4) 5-mark long-answer combining mechanism + product + stereochemistry + application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Stability Applications" lesson allows students to explore reaction outcomes, predict products, or compare reaction conditions, with live visual feedback. To use it effectively: (1) try every option/configuration, (2) compare with the analytical reasoning, (3) check IUPAC names and structural correctness, (4) test edge cases from worked examples. The simulation reinforces conceptual intuition that pure mechanism memorisation cannot provide.
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JEE Advanced 2026 Paper 2 — Full Mock Test (54 Questions, 180 Marks)
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