Questions on transition metal characteristics and complex formation

Question 1

Explain, with reference to the structure of the haem group, how haemoglobin carries oxygen round the body and why carbon monoxide is toxic.

Accepted Answer

Indicative content

haem contains an Fe²⁺ ion at the centre of a flat porphyrin ring, which bonds to the iron through four nitrogen atoms arranged in a square
a nitrogen atom from the protein chain bonds from below, so five of the six octahedral positions are filled and the sixth, above the ring, is free
in the lungs an oxygen molecule forms a coordinate bond to the Fe²⁺ at that sixth position, replacing a water molecule by ligand substitution
the bond to oxygen is weak and reversible, so the oxygen is released in respiring tissues where its concentration is low
carbon monoxide bonds to the same position through its carbon atom, and its coordinate bond to Fe²⁺ is much stronger than the bond to oxygen
carbon monoxide replaces the oxygen and is released only very slowly, so that haem cannot carry oxygen and less oxygen reaches the cells

Levels

Level 3 (5–6 marks) both parts are fully answered: the haem structure is described with the four nitrogen atoms of the ring, the protein nitrogen below and the free sixth position, the reversible coordinate bond to oxygen is explained with its release in the tissues, and the toxicity of carbon monoxide is explained through its much stronger bond at the same position, released only very slowly
Level 2 (3–4 marks) the Fe²⁺ in the ring and the coordinate bond to oxygen are described and carbon monoxide is said to bind more strongly; the free sixth position, the reversibility of the oxygen bond, or the ligand substitution is not mentioned
Level 1 (1–2 marks) iron is said to bond to oxygen; carbon monoxide is often said to react with the oxygen in the blood, and the structure of haem is largely absent

Question 2

Name the four characteristic properties of the transition metals, and explain why the transition metals show variable oxidation states.

Accepted Answer

B1 they form complexes and they form coloured ions; allow coloured compounds B1 they show variable oxidation states, and they and their compounds act as catalysts; ignore high density or melting point B1 the 3d and 4s electrons are similar in energy B1 so the successive ionisation energies rise gradually, with no large jump after the 4s electrons are removed B1 the energy needed to form a more highly charged ion can be repaid by its larger lattice or hydration enthalpy, so several ions are stable

Question 3

State what is meant by the term transition metal. Ti⁴⁺ has no d electrons; use the electron configurations of Ti²⁺, Ti³⁺ and Ti⁴⁺ to explain why titanium is still a transition metal.

Accepted Answer

B1 an element that forms at least one stable ion with an incomplete (partly filled) d subshell; ignore any reference to the d block B1 Ti⁴⁺ is [Ar] 3d⁰: the two 4s and the two 3d electrons have all been removed, so it has no d electrons B1 Ti³⁺ is [Ar] 3d¹ and Ti²⁺ is [Ar] 3d², each with an incomplete d subshell B1 the definition needs only one such stable ion, so titanium qualifies; a definition in terms of the atom is not accepted

Question 4

Give the electron configurations of a chromium atom and of the Cr³⁺ ion, using [Ar] for the argon core, and explain why the ion has no electrons left in its 4s subshell.

Accepted Answer

B1 Cr is [Ar] 3d⁵ 4s¹; reject [Ar] 3d⁴ 4s² B1 Cr³⁺ is [Ar] 3d³; allow the full form 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³ B1 once the 3d orbitals are occupied, the 3d subshell is lower in energy than the 4s subshell B1 so the 4s electron is the outermost and is removed first, followed by two 3d electrons

Question 5

Explain how the water molecules are bonded to the copper ion in [Cu(H₂O)₆]²⁺, using the terms ligand and coordination number.

Accepted Answer

B1 the oxygen atom of each water molecule has a lone pair of electrons B1 the lone pair is donated to the Cu²⁺ ion, forming a coordinate (dative covalent) bond in which both electrons come from the oxygen B1 water is therefore a ligand, a molecule that donates a lone pair of electrons to the metal ion B1 the coordination number is 6, the number of coordinate bonds to the copper; accept 'six coordinate bonds'

Question 6

Deduce the oxidation state of the metal and the coordination number in each of the two complexes in the table, which gives the ligands and the overall charge of each.

Accepted Answer

B1 in [Co(NH₃)₅Cl]²⁺ cobalt is +3, since x − 1 = +2; allow III B1 the coordination number of [Co(NH₃)₅Cl]²⁺ is 6, from five ammonia ligands and one chloride ligand B1 in [Cr(C₂O₄)₃]³⁻ chromium is +3, since x − 6 = −3; allow III B1 the coordination number of [Cr(C₂O₄)₃]³⁻ is 6, because each of the three ethanedioate ligands forms two coordinate bonds; reject 3

Question 7

Compare water, ethane-1,2-diamine and EDTA⁴⁻ as ligands for a Cu²⁺ ion.

Accepted Answer

B1 water is monodentate, forming one coordinate bond through a lone pair on its oxygen atom B1 ethane-1,2-diamine is bidentate, forming two coordinate bonds through a lone pair on each nitrogen atom B1 EDTA⁴⁻ is multidentate, forming six coordinate bonds through two nitrogen and four oxygen atoms B1 so six water molecules, three ethane-1,2-diamine molecules or one EDTA⁴⁻ ion give a coordination number of 6

Question 8

Explain why zinc is a d-block element but is not a transition metal.

Accepted Answer

B1 zinc's highest-energy electrons are in the 3d subshell, so it is in the d block B1 zinc forms only one ion, Zn²⁺, with the configuration [Ar] 3d¹⁰ B1 the 3d subshell of Zn²⁺ is full, so zinc forms no stable ion with an incomplete d subshell

Question 9

Describe how EDTA⁴⁻ bonds to a Cu²⁺ ion, and give the formula of the complex formed.

Accepted Answer

B1 EDTA⁴⁻ is a multidentate (hexadentate) ligand with six donor atoms B1 it forms coordinate bonds through two nitrogen atoms and four oxygen atoms, filling all six positions round the ion B1 one EDTA⁴⁻ binds one Cu²⁺, giving the complex [Cu(EDTA)]²⁻

Question 10

Calculate the concentration of Ca²⁺ ions in a sample of tap water, given that 25.0 cm³ of the water needs 12.40 cm³ of 0.0100 mol dm⁻³ EDTA⁴⁻ solution to complex all of its Ca²⁺ ions. EDTA⁴⁻ and Ca²⁺ react in a 1 : 1 ratio.

Accepted Answer

M1 moles of EDTA⁴⁻ = 0.0100 × 12.40 ÷ 1000 = 1.24 × 10⁻⁴ mol, equal to the moles of Ca²⁺ M1 concentration = moles ÷ volume in dm³ = 1.24 × 10⁻⁴ ÷ 0.0250; allow error carried forward from M1 A1 4.96 × 10⁻³ mol dm⁻³, to three significant figures

Question 11

Calculate the percentage by mass of copper in hydrated copper(II) sulfate, CuSO₄·5H₂O. Ar: Cu 63.5, S 32.1, O 16.0, H 1.0.

Accepted Answer

M1 Mr = 63.5 + 32.1 + 4(16.0) + 5(18.0) = 249.6 M1 percentage = 63.5 ÷ 249.6 × 100 A1 25.4%, to three significant figures

Question 12

Suggest why the disodium salt of EDTA is added to some mayonnaise, given that traces of Fe³⁺ and Cu²⁺ ions catalyse the oxidation of fats, which makes food rancid.

Accepted Answer

B1 EDTA⁴⁻ forms complexes with the Fe³⁺ and Cu²⁺ ions B1 the complexes are very stable (chelate effect), so the metal ions are held and cannot take part in the oxidation as catalysts B1 the fats are oxidised more slowly, so the mayonnaise keeps for longer

Question 13

Explain why excess silver nitrate precipitates three moles of silver chloride from one mole of [Co(NH₃)₆]Cl₃ but only two moles from one mole of [Co(NH₃)₅Cl]Cl₂.

Accepted Answer

B1 in [Co(NH₃)₆]Cl₃ all three chloride ions are outside the complex, free in solution, and all react with Ag⁺ B1 in [Co(NH₃)₅Cl]Cl₂ one chloride is a ligand, held to the cobalt by a coordinate bond B1 the ligand chloride is not free to react with Ag⁺, so only the two chloride ions outside the brackets precipitate

Question 14

Suggest the structure of a green hydrate with the formula CrCl₃·6H₂O, given that one mole of it gives one mole of silver chloride with excess silver nitrate and that chromium(III) has a coordination number of 6.

Accepted Answer

B1 only one chloride ion is free, so two of the three chloride ions are ligands B1 four water ligands complete the coordination number of 6, giving the complex ion [Cr(H₂O)₄Cl₂]⁺ B1 the formula is [Cr(H₂O)₄Cl₂]Cl·2H₂O, with two water molecules as water of crystallisation

Question 15

Justify the statement that copper is a transition metal although its Cu⁺ ion has a completely filled 3d subshell.

Accepted Answer

B1 a transition metal forms at least one stable ion with a partly filled d subshell; the definition needs one such ion, not every ion B1 Cu²⁺ is [Ar] 3d⁹, with one d orbital partly filled, and copper(II) compounds are common and stable, so copper qualifies B1 Cu⁺, [Ar] 3d¹⁰, has no partly filled d orbital, which is why copper(I) compounds such as CuCl are white while copper(II) compounds are coloured; allow zinc, whose only ion is Zn²⁺ 3d¹⁰, as the contrast

Question 16

Describe the shapes of the 3dxy and 3dz² orbitals, relative to the x, y and z axes.

Accepted Answer

B1 3dxy has four lobes in the plane of the x and y axes, pointing between the axes B1 3dz² has two lobes along the z axis with a ring of electron density round the middle in the xy plane

Complex	Ligands	Overall charge
[Co(NH₃)₅Cl]²⁺	five NH₃ and one Cl⁻	2+
[Cr(C₂O₄)₃]³⁻	three C₂O₄²⁻	3−