CIE A Level Chemistry

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A LevelChemistryCIETopic Questions6. Inorganic Chemistry (A Level Only)6.3 Transition Element Complexes: Isomers, Reactions & Stability (A Level Only)

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First teaching 2020

Last exams 2024

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6.3 Transition Element Complexes: Isomers, Reactions & Stability (A Level Only)

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1a2 marks

This question is about ligand exchange in copper(II) complexes.

Complete Table 1.1 to show whether the coordination number and shape of the complex changes in the given situations.

 
Table 1.1 
 
  Ligand exchange of similarly sized ligands Ligand exchange of differently sized ligands
Coordination number    
Shape    
 

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1b1 mark

When copper(II) sulfate dissolves in water, the conventional equation for this can be represented as

 
CuSO4 (s) + aq → Cu2+ (aq) + SO42– (aq) 
 

State the formula of the hexaaquacopper(II) complex represented by Cu2+ (aq).

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1c2 marks

Complete the three-dimensional diagram in Fig. 1.1 to show the hexaaquacopper(II) ion complex.

 
 
 
 
Cu
 
 
 
 
Fig. 1.1 

 

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1d2 marks

Hexaaquacopper(II) ions react with concentrated hydrochloric acid to form complex A.

 

Complete Table 1.2 with the colour and geometry of the hexaaquacopper(II) complex and the colour, geometry and formula of complex A.

 
Table 1.2 
 
  Hexaaquacopper(II) complex A
colour of complex    
geometry of complex    
formula of complex    
 

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1e3 marks

In the presence of concentrated ammonia, hexaaquacopper(II) initially reacts to form Cu(OH)2(H2O)4.

 
i)
Give the colour and state of Cu(OH)2(H2O)4.
 
colour ..................................................
 
state ..................................................
 
[1]
 
ii)
In the presence of excess concentrated ammonia, the Cu(OH)2(H2O)4 further reacts to form [Cu(NH3)4(H2O)2]2+.
 
Give the colour and state of [Cu(NH3)4(H2O)2]2+.
 
colour ..................................................
 
state ..................................................
 
[1]
 
iii)
The [Cu(NH3)4(H2O)2]2+ formed is a mixture of two geometrically isomeric octahedral complexes.
 
Fig. 1.2 shows the three-dimensional diagram of the trans isomer. Complete Fig. 1.2 to show the three-dimensional diagram of the cis isomer.
 
cuii-conc-ammonia-cis-trans-q
 
Fig. 1.2 
 
[1]

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2a2 marks

The feasibility of redox reactions can be determined using standard electrode potential, Eθ, values.

The following electrochemical cell is set up.

 
Pb4+ (aq) + 2Mn2+ (aq) → 2Mn3+ (aq) + Pb2+ (aq) 
 
Table 2.1
 
Electrode reaction Eθ / V
Mn2+ + 2e– ⇌ Mn – 1.18
Mn3+ + e– ⇌ Mn2+ + 1.49
Pb2+ + 2e– ⇌ Pb – 0.13 
Pb4+ + 2e– ⇌ Pb2+ + 1.69

 
Use Table 2.1 to determine the half-equations for reduction and oxidation. Your half-equations should be written in the conventional reduction format. 

Reduction half-equation ...................................................................... 

Oxidation half-equation ...................................................................... 

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2b
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Use Table 2.1 and your answer to part (a) to calculate the standard cell potential, Eθcell, for the reaction.

 
Eθcell = ...................... V

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2c1 mark

Use your answer to part (b) to suggest whether the reaction is feasible. Explain your answer.

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3a2 marks

Potassium manganate(VII) can be used to titrate a sample containing an unknown percentage of iron(II).

i)
Complete the half-equation for the reduction of manganate ions, MnO4–, to manganese ions, Mn2+.
 
MnO4– (aq) + ........H+ (aq) + ........e– → Mn2+ (aq) + ........H2O (aq)
 
[1]
 
ii)
Explain whether this reaction is oxidation or reduction.
 
[1]

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3b2 marks
i)
Write the half-equation for the oxidation of Fe2+ to Fe3+. Your answer should include state symbols. 
 
[1]
 
ii)
Explain whether this reaction is oxidation or reduction.
 
[1]

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3c2 marks

Using your answers to parts (a) and (b), write the full equation for the reaction between Fe2+ ions and MnO4– ions.

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1a2 marks

Carboplatin and oxaliplatin are both platinum(II) diamines, similar to cisplatin, which are widely used in chemotherapy to treat a wide variety of cancers. However, their use is often plagued with severe toxicity and the development of resistance, which leads to disease progression. Recently, oxoplatin, iproplatin, ormaplatin and satraplatin are Pt complexes that have been used clinically or in clinical trials. 

The central platinum ions in the four complexes shown in Fig. 1.1 all have the same shape and oxidation number.

 

platins

Fig. 1.1


Give the following information for the complexes.

oxidation number of the platinum ion ..................................................

shape of the complexes ..................................................

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1b4 marks

One isomer of the [Pt(NH3)2Cl2(OH)2] complex oxoplatin is shown in part (a) Fig. 1.1.

Complete three-dimensional diagrams to show the shape of the other oxoplatin stereoisomers and indicate the chiral one(s).

screenshot-2023-06-01-130550

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1c1 mark

Platinum complexes can be considered prodrugs that are intracellularly activated to kill cancer cells.

Cis-platin can be produced by the reduction of the prodrug cis, trans, cis-[PtCl2(OCOCH3)2(NH3)2], which has a similar structure to satraplatin.

Draw the structure of cis, trans, cis-[PtCl2(OCOCH3)2(NH3)2].

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2a4 marks

When chromium(III) sulfate dissolves in water, a green solution containing the [Cr(H2O)6]3+ ion forms.

i)
State the bond angles found in this complex ion.
 
[1]
 
ii)
Explain why the chromium(III) complex ion is coloured.
 
[3]

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2b5 marks

Ethylenediaminetetraacetate, EDTA4−, shown in Fig. 2.1. is a polydentate ligand.

edta-ligand

Fig. 2.1

When a solution of EDTA4− is added to a solution of [Cr(H2O)6]3+ ions, a new complex ion is formed.

[Cr(H2O)6]3+ + EDTA4– ⇌ [Cr(EDTA)]– + 6H2O

i)
Name the type of reaction occurring here.
 
[1]
 
ii)
Write an expression for the stability constant, Kstab, of [CrEDTA]– in this reaction.
 
[1]
 
iii)
The numerical value of the Kstab of [CrEDTA]– is 2.51 × 1023 in this reaction. 
 
Suggest what this indicates about the position and entropy of the equilibrium.
 
[3]

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2c3 marks

Compound L is a complex with the empirical formula CrN4H12Cl3 

The formula of compound L contains one chloride ion and a complex ion M, which has two stereoisomers.

Complete three-dimensional diagrams to show the shape of the stereoisomers of complex ion M.

 
 
 
 
 
Cr 
 
 
 
 
 
 
 
 
 
Cr 
 
 
 
 
 

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2d
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Chromium (III) picolinate, shown in Fig. 2.2, is a neutral complex that can be prepared from the weak acid, picolinic acid.2-5

Fig. 2.2

Chromium(III) picolinate is used in tablets as a nutritional supplement for chromium. 

i)
Draw the structure of the ligand in chromium(III) picolinate.
 
[1]
 
ii)
A typical tablet of chromium(III) picolinate contains 200 μg of chromium.
 
Calculate the mass, in g, of chromium (III) picolinate in a typical tablet. Give your answer to three significant figures.
 
[2]

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3a
Sme Calculator4 marks

Iron(II) gluconate, C12H22FeO14, is the active ingredient in some brands of iron supplements.

A student carries out an experiment to determine the mass of iron(II) gluconate in one tablet of an iron supplement, using the method below.

  1. The student crushes two tablets and dissolves the powdered tablets in dilute sulfuric acid
  2. The student makes up the solution from step 1 to 250.0 cm3 in a volumetric flask.
  3. The student then titrated 25.0 cm3 portions of the solution obtained in step 2 with 0.00200 mol dm−3 potassium manganate(VII).

The student obtains a mean titre of 13.50 cm3.

In this titration, 1 mole of manganate(VII) ions reacts with 5 moles of iron(II) ions.

Determine the mass, in mg, of iron(II) gluconate in one tablet. Give your answer to 3 significant figures.

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3b
Sme Calculator2 marks

The typical concentration of a potassium manganate(VII) solution used in redox titrations is 0.0200 mol dm-3.

Use the information in part (a) to explain, quantitatively, why the student replaced this with a 0.00200 mol dm-3 potassium manganate(VII) solution for this titration.

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3c
Sme Calculator1 mark

Some iron supplements contain iron(II) sulfate or iron(II) fumarate.

Table 3.1 shows the information taken from the labels of two iron supplements, A and B.

Table 3.1

Iron
supplement

Iron
compound

Mass of iron compound
in one tablet / mg

A

Iron(II) sulfate, FeSO4

180

B

Iron(III) fumarate, C4H2FeO4

210

 

State which iron supplement, A or B, would provide the greater mass of iron per tablet. Show your workings.

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4a1 mark

Write an equation for the reaction between ethanedioic acid, H2C2O4, and sodium hydroxide, NaOH.

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4b3 marks

15.00 cm3 of H2C2O4 (aq) requires 10.30 cm3 of a 0.25 mol dm-3 solution of NaOH for complete neutralisation using a phenolphthalein indicator for the first permanent colour change. 

15.00 cm3 of the same H2C2O4 (aq) required 12.35 cm3 of potassium permanganate, KMnO4, solution for complete oxidation to carbon dioxide and water in the presence of dilute sulfuric acid to further acidify the H2C2O4 (aq) for the first permanent
colour change. 

Calculate the moles of H2C2O4 (aq) therefore the concentration of the solution.

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4c3 marks

Write the full redox equation, including state symbols, for this redox titration.

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4d2 marks

Calculate the concentration of the potassium permanganate, KMnO4, solution.

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1a1 mark

Sketch the shape of a 3dxy orbital.

q7a-9701-y22-sp-4-cie-ial-chem

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1b3 marks
i)
Some transition elements and their compounds behave as catalysts. Explain why transition elements behave as catalysts.

[2]

ii)
Catalysis can be classified as heterogeneous or homogeneous.
Complete Table 7.1 by placing one tick (✓) in each row to indicate the type of catalysis in each reaction.

Table 7.1

  type of catalysis
reaction heterogeneous homogeneous
Fe in the Haber process    
Fe2+ in the I– / S2O82– reaction    
NO2 in the oxidation of SO2    

[1]

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1c2 marks

A solution containing a mixture of Sn2+(aq) and Sn4+(aq) is added to a solution containing a mixture of Fe2+(aq) and Fe3+(aq).

Table 7.2 lists electrode potentials for some electrode reactions of these ions.

Table 7.2

electrode reaction Eɵ / V
Fe2+ + 2e– ⇌ Fe –0.44
Fe3+ + 3e– ⇌ Fe –0.04
Fe3+ + e– ⇌ Fe2+ +0.77
Sn2+ + 2e– ⇌ Sn –0.14
Sn4+ + 2e– ⇌ Sn2+ +0.15

Eɵ data from the table can be used to predict the reaction that takes place when the two solutions are mixed.

i)
Write an equation for this reaction.

 [1]

ii)
Calculate E subscript cell superscript straight ɵ for this reaction.

[1]

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1d5 marks

Hexaaquairon(III) ions are pale violet. They form a colourless complex with fluoride ions, F–, as shown in equilibrium 1, and a deep-red complex with thiocyanate ions, SCN–, as shown in equilibrium 2.

equilibrium 1 [Fe(H2O)6]3+ + F– ⇌ [Fe(H2O)5F]2+ + H2O Kstab = 2.0 × 105 mol–1 dm3   

                          violet                          colourless

equilibrium 2 [Fe(H2O)6]3+ + SCN– ⇌ [Fe(H2O)5SCN]2+ + H2O Kstab = 1.0 × 103 mol–1 dm3

                           violet                                deep-red

The following two experiments are carried out.

Experiment 1: A few drops of KSCN(aq) are added to 5 cm3 of Fe3+(aq), followed by a few drops of KF(aq).

Experiment 2: A few drops of KF(aq) are added to 5 cm3 of Fe3+(aq), followed by a few drops of KSCN(aq).

i)

Predict and explain the sequence of colour changes you would observe in each of Experiment 1 and Experiment 2.

Experiment 1 ...................................................

Experiment 2 ....................................................

   [4]

ii)
Name the type of reaction occurring during the experiments in (d)(i).

[1]

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1e2 marks

Solutions of iron(III) salts are acidic due to the equilibrium shown.

 
[Fe(H2O)6]3+ (aq) ⇌ [Fe(H2O)5(OH)]2+ (aq) + H+ (aq)   Ka = 8.9 × 10–4 mol dm–3
 

Calculate the pH of a 0.25 mol dm–3 FeCl3 solution.
Show your working.

pH = ..........................................................

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2a1 mark

This question is about transition metal complexes.

An aqueous solution of copper(II) contains the [Cu(H2O)6]2+ complex ion.

 

Define the term complex ion.

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2b6 marks

A sample of [Cu(H2O)6]2+ is reacted with aqueous sodium hydroxide and a separate sample of [Co(H2O)6]2+ with an excess of aqueous ammonia.

 

Give the following information about these reactions.

  • the reaction of [Cu(H2O)6]2+ with aqueous sodium hydroxide
     
    colour and state of the copper-containing species

    .......................................................................................................
     
    ionic equation ................................................................................
     
    type of reaction ..............................................................................

  • the reaction of [Co(H2O)6]2+ with an excess of aqueous ammonia
     
    colour and state of the cobalt-containing species

    .......................................................................................................
     
    ionic equation ................................................................................
     
    type of reaction ..............................................................................

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2c3 marks

The [Fe(NH3)2CN4]– complex shows stereoisomerism.

 

Complete the three-dimensional diagrams to show the two isomers of  [Fe(NH3)2CN4]–.

 
6-3-2c-m-fe-txn-isomers-b
 

Suggest the type of stereoisomers.

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2d2 marks

Compound A, C4H13N3, is a tridentate ligand. 

 
6-3-2d-m-tridentate-ligand-example
 
i)
Suggest why one molecule of A can form three dative bonds.
 
[1]
 
ii)
C4H13N3 reacts with aqueous chromium(III) ions, [Cr(H2O)6]3+, in a 2:1 ratio to form a new complex ion.
 
Construct an equation for this reaction.
 
[1]

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3a2 marks

Cobalt is a transition element with atomic number 27.

Complete the electronic configurations of a Co atom and a Co2+ ion.

 
Co atom   1s2 2s2 2p6  ..................................
 
Co2+ ion   1s2 2s2 2p6  ..................................

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3b2 marks

Co2+ ions form a linear complex with Cl– ions, which are monodentate ligands.

 

Draw the structure of this complex and include its overall charge.

structure




charge ......................................

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3c3 marks

Co2+ ions exist as [Co(H2O)6]2+ complex ions in an aqueous solution.

 

Complete a three-dimensional diagram to show the shape of this complex.

 

Name its shape.

 

Label and state the value of two bond angles.

 
6-3-3c-m-blank-octahedral-co-a
 
name of shape ..................................................

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3d4 marks

When NH3 (aq) is added to Co2+ (aq), dropwise at first and then in excess, two chemical reactions occur as shown.

 
[Co(H2O)6]2+ rightwards arrow with 1. space dropwise space NH subscript 3 space open parentheses aq close parentheses on top A rightwards arrow with 2. space excess space NH subscript 3 space stretchy left parenthesis aq stretchy right parenthesis on top B
 

For each reaction, describe what you would see and write an equation.

 
reaction 1
 
   observation ................................................................................
 
   equation ................................................................................
 
reaction 2
 
   observation ................................................................................
 
   equation ................................................................................ 

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3e5 marks

EDTA4– is a hexadentate ligand. When a solution of EDTA4– is added to a solution containing [Co(H2O)6]2+ a new complex is formed. The formula of this complex is [CoEDTA]2–.

 
i)
Explain what is meant by the term hexadentate ligand.
 
[2]
 
ii)
Name the type of reaction occurring here.
 
[1]
 
iii)
Write an expression for the stability constant, Kstab, of [CoEDTA]2– in this reaction.
 
[1]
 
iv)
The numerical value of the Kstab of [CoEDTA]2– is 7.7 x 104 mol–6 dm18 at 298K.
 
State what this tells us about the [CoEDTA]2– complex ion.
 
[1] 

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4a4 marks
The chloride ion is a monodentate ligand. When concentrated hydrochloric acid is added to [Co(H2O)6]2+ (aq) ions the water ligands are replaced.

i)
Suggest an equation for the reaction of [Co(H2O)6]2+ (aq) with concentrated hydrochloric acid.
 
[2]
 
iii)
Draw the three-dimensional diagram to show the complex formed in the reaction between [Co(H2O)6]2+ (aq) and HCl.
 
[2]

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4b5 marks

[Co(NH3)4Cl2]+ is an example of an octahedral complex which exhibits cis-trans isomerism.

i)
Deduce the oxidation state of cobalt in this complex. Explain your answer.
 
[2]
 
ii)
Draw structures of the two [Co(NH3)4Cl2]+ isomers clearly identifying each isomer.
 
[3]

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4c3 marks

Explain the origin of colour in a transition element complex such as [Co(NH3)4Cl2]+.

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4d5 marks

Bidentate ligands form two coordinate bonds to a central metal atom or ion in a complex. 1,2-diaminoethane is an example of a compound which is a bidentate ligand.  When added to [Cu(H2O)6]2+ all of the other ligands are replaced and two optical isomers are formed.

i)
Suggest an equation for the reaction of [Cu(H2O)6]2+ with 1,2-diaminoethane.
 
[2]
 
ii)
Draw the two isomeric forms of the product of the reaction of [Cu(H2O)6]2+ with 1,2-diaminoethane.
 
[3]

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