A LevelChemistryCIEExam Questions28. Chemistry of Transition Elements28.3 Colour of Complexes

Syllabus Edition

First teaching 2023

First exams 2025

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Colour of Complexes (CIE A Level Chemistry)

Exam Questions

44 mins4 questions
Easy
Medium
1a1 mark

The d-orbitals of a transition metal are involved in the formation of complexes.

In terms of energy, explain what is meant by degenerate orbitals. 

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1b3 marks

Fig. 1.1 shows the shape of the dxy orbital. 

 
dxy-orbital
 
Fig. 1.1
 

The dxz orbital has a similar shape but the lobes are between the x-axis and z-axis and the dyz orbital has a similar shape but the lobes are between the y-axis and z-axis.

 
i)
State the difference between the shape of a dxy orbital and the shape of a dx2–y2 orbital.
 
[1]
 
ii)
Complete the three-dimensional diagram in Fig. 1.2 to show the shape of a dz2 orbital. 
 
xyz-orbital-axes
 
Fig. 1.2 
 
[2] 

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1c1 mark

Suggest what causes d-orbitals in a transition metal ion to change from degenerate to non-degenerate.

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1d2 marks

The splitting of degenerate d-orbitals gives two sets of non-degenerate d-orbitals at different energy levels.

 

Complete Table 1.1 to show the different energy levels of the non-degenerate d-orbitals in octahedral and tetrahedral complexes.

 
Table 1.1 
 
  Higher energy Lower energy
Octahedral    
Tetrahedral    

 

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1a1 mark

Define a transition element.

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1b3 marks
i)
NH3 acts as a monodentate ligand. State what is meant by monodentate ligand.

[2]

ii)
Aqueous silver ions, Ag+ (aq), react with aqueous ammonia, NH3 (aq), to form a linear complex.

Suggest the formula of this complex, including its charge.

[1]

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1c5 marks

There are two isomeric complex ions with the formula [Cr(NH3)4Cl2]+. One is green and the other is violet.

i)
Suggest the type of isomerism shown by these two complex ions.

 [1]

ii)

Explain why these two complex ions are coloured and why they have different colours.

[4]

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1d2 marks

The ligand ethane-1,2-diamine, H2NCH2CH2NH2, is represented by en.
Nickel forms the complex ion [Ni(en)3]2+ in which it is surrounded octahedrally by six nitrogen atoms.

Draw three-dimensional diagrams to show the stereoisomers of [Ni(en)3]2+.

q-6d-9701-y22-sp-4-cie-ial-chem

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1e3 marks

Ethane-1,2-diamine is a useful reagent in organic chemistry.

i)
Explain how the amino groups in ethane-1,2-diamine allow the molecule to act as a Brønsted-Lowry base.

[2]

ii)
Write an equation for the reaction of ethane-1,2-diamine with an excess of hydrochloric acid.

[1]

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1f3 marks
i)
Under certain conditions ethane-1,2-diamine reacts with ethanedioic acid, HOOCCOOH, to form the polymer Z.
Draw the structure of polymer Z, showing two repeat units.

[2]

ii)
Name the type of reaction occurring during this polymerisation.
[1]

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2a2 marks

This question is about the geometry and colours of transition metal complexes. 

Explain why transition metals exhibit variable oxidation states compared to the elements in group 1.

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2b3 marks

Transition metal compounds and ions are often coloured. For example, [Cr(H2O)6]3+ is green. 

Explain why [Cr(H2O)6]3+ and other complex ions are coloured.

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2c3 marks

Zinc and cobalt ions react with water to form the complexes [Zn(H2O)4]2+ and [Co(H2O)6]2+.

i)
Explain how water acts as a ligand in forming these complexes.
 
[2]
 
ii)
Predict the shape of [Co(H2O)6]2+.
 
[1]

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2d4 marks

Explain why solutions containing [Co(H2O)6]2+ are coloured but solutions containing [Zn(H2O)4]2+ are not.

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3a3 marks

A characteristic property of transition elements, like chromium, is that they form coloured compounds.

Explain why Ni2+(aq) is green but Sc3+(aq) is colourless.

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3b3 marks

The colour intensity of solutions of complex ions is one method of determining the concentration of transition metal ions. Excess aqueous ammonia is sometimes added before measuring the absorption of copper(II) ions. 

Describe why the addition of excess ammonia to aqueous copper(II) ions causes the shade of the blue colour to change.

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3c2 marks
c)
Ferrocyanide salts,  [Fe(CN)6]4−, are used in the production of Prussian blue, which was the first modern synthetic pigment.

i)
Deduce the oxidation number of iron in [Fe(CN)6]4−

ii)
Draw the abbreviated orbital diagram for the iron ion in [Fe(CN)6]4– using the arrow-in-box notation to represent electrons.

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