Chapter 5: Coordination Compounds Application Questions | chemistry Class 12 CBSE

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Chapter 5: Coordination Compounds Application Questions | chemistry Class 12 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A patient with lead poisoning is treated with a chelating agent. With reference to coordination chemistry, explain how EDTA works as an antidote for lead poisoning.

EDTA4- (ethylenediaminetetraacetate) is a hexadentate chelate ligand that can bind through two nitrogen and four oxygen donor atoms simultaneously to a single metal ion like Pb2+ in the bloodstream.
EDTA forms a highly stable coordination complex with Pb2+ by the chelate effect — the simultaneous multi-point binding creates an extremely stable, soluble complex that can be excreted renally.
The stability of EDTA-metal complexes is used selectively: the high stability constant of [Pb(EDTA)]2- ensures effective sequestration of Pb2+ from tissues; similarly, EDTA complexes with Ca2+ and Mg2+ are used in water hardness estimation.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

With reference to the spectrochemical series, explain why anhydrous CuSO4 is white while CuSO4.5H2O is blue in colour.

Anhydrous CuSO4 lacks water ligands; there are no ligands to create an asymmetric crystal field around Cu2+, so the d orbitals remain degenerate. Without crystal field splitting, no d-d transition is possible and the compound appears colourless/white.
In CuSO4.5H2O, four water molecules coordinate to Cu2+ to form [Cu(H2O)4]2+; H2O creates crystal field splitting of the d orbitals (Do). The d-d transition absorbs red light (around 600 nm), and the complementary colour blue is observed.
This confirms the CFT principle that colour in coordination compounds arises due to d-d electronic transitions, which only occur when ligands create a sufficiently asymmetric crystal field to split d orbital energies.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

A gold ore is treated with sodium cyanide solution in the presence of oxygen and water. Explain the coordination chemistry involved in gold extraction and its subsequent recovery.

Gold combines with cyanide ions (CN-) in the presence of oxygen and water to form the soluble coordination entity [Au(CN)2]- (dicyanidoaurate(I)) in aqueous solution; CN- acts as a strong field ligand forming a stable linear complex with Au+.
The stability of [Au(CN)2]- allows gold to be selectively leached from the ore into aqueous solution, leaving other minerals behind; this is a hydrometallurgical process exploiting complex formation.
Gold is recovered from the solution by the addition of zinc metal, which displaces gold: Zn reduces Au+ from [Au(CN)2]- back to metallic gold (Au0), while zinc forms its own cyanide complex; similarly, impure nickel is purified via [Ni(CO)4] formation and decomposition.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

With reference to coordination chemistry, explain why [Fe(CN)6]3- does not give the test for Fe3+ ion, while FeSO4.(NH4)2SO4.6H2O does give the test for Fe2+ ion.

[Fe(CN)6]3- is a complex ion (not a double salt); the Fe3+ ion is tightly bound within the coordination sphere by six CN- ligands and does not dissociate freely in solution. Therefore, reagents specific to free Fe3+ ions do not react with the complex.
FeSO4.(NH4)2SO4.6H2O (Mohr's salt) is a double salt that completely dissociates into its constituent ions (Fe2+, NH4+, SO42-, H2O) when dissolved in water; free Fe2+ ions are available to react with the test reagents.
This fundamental difference — complex ions do not dissociate into free metal ions while double salts do — distinguishes complexes from double salts and illustrates the concept of secondary (coordinate) vs ionic bonding.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 5

Applied Concept

In a black and white photography lab, the film is 'fixed' using sodium thiosulphate (hypo). Explain the coordination chemistry involved in this process.

After development, the film contains unreacted silver bromide (AgBr) crystals; if not removed, they would darken on exposure to light and ruin the image. 'Fixing' removes this unreacted AgBr.
Sodium thiosulphate (Na2S2O3, hypo) acts as a ligand source: S2O32- ions coordinate with Ag+ from AgBr to form the soluble, stable complex ion [Ag(S2O3)2]3- (dithiosulpatoargentate(I)).
This coordination complex is water-soluble and washes away from the film, removing all unreacted AgBr; the chemical formula of the complex demonstrates how thiosulphate (a ligand) stabilises Ag+ as a coordination entity, preventing further photochemical reaction.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 6

Applied Concept

A sample of hard water has high concentrations of Ca2+ and Mg2+ ions. With reference to coordination chemistry, explain how its hardness is estimated using Na2EDTA.

Ca2+ and Mg2+ ions in hard water form stable coordination complexes with EDTA4-; Na2EDTA is titrated against the hard water sample to determine total hardness.
EDTA4- is a hexadentate ligand that bonds through two nitrogen and four oxygen donor atoms to envelop Ca2+ or Mg2+ completely; the high stability constants of these complexes ensure quantitative reaction.
Selective estimation of Ca2+ and Mg2+ is possible because their EDTA stability constants differ; by adjusting pH and using appropriate indicators, Ca2+ can be titrated separately; the chelate effect of EDTA ensures precise and stable complex formation, making it ideal for volumetric analysis.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

With reference to the Valence Bond Theory, predict whether [MnBr4]2- is tetrahedral or square planar, given its magnetic moment is 5.9 BM.

Mn2+ has the electronic configuration 3d5; a magnetic moment of 5.9 BM corresponds to 5 unpaired electrons (spin-only formula: sqrt(n(n+2)) BM; for n=5, sqrt(35) = 5.92 BM).
Square planar geometry requires dsp2 hybridisation, which needs one 3d orbital to be vacated; this would require pairing of 3d electrons, giving fewer unpaired electrons (incompatible with 5 unpaired).
Tetrahedral geometry uses sp3 hybridisation using only 4s and 4p orbitals; the 3d electrons remain unpaired (5 unpaired electrons), consistent with the observed magnetic moment of 5.9 BM. Therefore [MnBr4]2- is tetrahedral.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 8

Applied Concept

Explain how cis-platin functions as an anticancer drug. What structural feature makes cis-platin effective while its trans isomer (transplatin) is not?

Cis-platin (cis-[PtCl2(NH3)2]) is a coordination compound where the two Cl ligands are adjacent (cis) and the two NH3 ligands are also adjacent; in the body, Cl ligands are displaced by water and then the complex binds to the nitrogen atoms of adjacent guanine bases in DNA.
The cis geometry allows both Pt-binding sites to simultaneously coordinate to the same DNA strand (intra-strand crosslinks), distorting the double helix and inhibiting DNA replication and transcription, thereby preventing cancer cell division.
The trans isomer (transplatin) has the two Cl ligands on opposite sides; this geometry does not allow simultaneous binding to adjacent DNA bases on the same strand; instead it forms less effective inter-strand crosslinks. This geometric difference explains why only cis-platin is clinically effective — geometric isomerism directly affects biological activity.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

The Wilkinson catalyst [(Ph3P)3RhCl] is used in industrial hydrogenation of alkenes. With reference to coordination chemistry, explain why this is significant.

[(Ph3P)3RhCl] is a coordination compound of rhodium(I) with three triphenylphosphine (PPh3) ligands and one chlorido ligand; it is a homogeneous catalyst for alkene hydrogenation at mild conditions.
Triphenylphosphine is a bulky, strong field phosphine ligand that creates a sterically and electronically tuned coordination environment around Rh; the complex can reversibly bind H2 and alkene at vacant coordination sites during the catalytic cycle.
The use of coordination compounds as industrial catalysts is important because they allow reactions to be carried out at lower temperatures and pressures than heterogeneous metal catalysts; the tunability of ligands around the metal centre allows optimization of selectivity and efficiency.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 10

Applied Concept

With reference to structural isomerism, explain how [Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl can be distinguished experimentally.

These two compounds are ionisation isomers: in [Co(NH3)5Cl]SO4, Cl- is the ligand and SO42- is the counter ion; in [Co(NH3)5(SO4)]Cl, SO42- is the ligand and Cl- is the counter ion.
Adding excess BaCl2 solution: [Co(NH3)5Cl]SO4 gives a white precipitate of BaSO4 (free SO42- present), while [Co(NH3)5(SO4)]Cl gives no precipitate (SO42- is coordinated and not free).
Adding excess AgNO3 solution: [Co(NH3)5Cl]SO4 gives no precipitate (Cl- is coordinated), while [Co(NH3)5(SO4)]Cl gives a white precipitate of AgCl (free Cl- present); these contrasting results confirm their identity as ionisation isomers.

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Chapter 5: Coordination Compounds | Application Questions

A patient with lead poisoning is treated with a chelating agent. With reference to coordination chemistry, explain how EDTA works as an antidote for lead poisoning.

With reference to the spectrochemical series, explain why anhydrous CuSO4 is white while CuSO4.5H2O is blue in colour.

A gold ore is treated with sodium cyanide solution in the presence of oxygen and water. Explain the coordination chemistry involved in gold extraction and its subsequent recovery.

With reference to coordination chemistry, explain why [Fe(CN)6]3- does not give the test for Fe3+ ion, while FeSO4.(NH4)2SO4.6H2O does give the test for Fe2+ ion.

In a black and white photography lab, the film is 'fixed' using sodium thiosulphate (hypo). Explain the coordination chemistry involved in this process.

A sample of hard water has high concentrations of Ca2+ and Mg2+ ions. With reference to coordination chemistry, explain how its hardness is estimated using Na2EDTA.

With reference to the Valence Bond Theory, predict whether [MnBr4]2- is tetrahedral or square planar, given its magnetic moment is 5.9 BM.

Explain how cis-platin functions as an anticancer drug. What structural feature makes cis-platin effective while its trans isomer (transplatin) is not?

The Wilkinson catalyst [(Ph3P)3RhCl] is used in industrial hydrogenation of alkenes. With reference to coordination chemistry, explain why this is significant.

With reference to structural isomerism, explain how [Co(NH3)5Cl]SO4 and [Co(NH3)5(SO4)]Cl can be distinguished experimentally.

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