A LevelChemistryCambridge (CIE)Exam Questions28. Chemistry of Transition ElementsStability Constants, Kstab

Stability Constants, Kstab (Cambridge (CIE) A Level Chemistry): Exam Questions

Exam code: 9701

52 mins5 questions
Medium
Hard
1a
1 mark

Sketch the shape of a 3dxy orbital.

Blank 3D Cartesian axes (x, y, z) for sketching the d_xy orbital shape

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1b
3 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 1.1 by placing one tick (✓) in each row to indicate the type of catalysis in each reaction.

Table 1.1

 

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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1c
2 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 2.2 lists electrode potentials for some electrode reactions of these ions.

Table 2.2

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) Construct an equation for this reaction.

[1]

ii) Calculate Eɵcell for this reaction.

[1]

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1d
5 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

violet ⇌ colourless

Kstab = 2.0 × 105 mol–1 dm3

equilibrium 2:

[Fe(H2O)6]3+ + SCN– ⇌ [Fe(H2O)5SCN]2+ + H2O

violet ⇌ deep-red

Kstab = 1.0 × 103 mol–1 dm3

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

[4]

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

[1]

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1e
2 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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2a
2 marks

Define the term ligand.

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2b
1 mark

[Cu(H2O)6]2+ reacts with ammonia to form a new complex. Write an expression for the overall stability constant, Kstab, for the formation of [Cu(NH3)4(H2O)2]2+

Kstab =

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2c
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3 marks

Table 2.1 shows the stability constants for each stage in the replacement of four water molecules for the [Cu(H2O)6]2+ complex

Table 2.1

Kn

value / dm3 mol-1

K1

1.78 x 104

K2

4.07 x 103

K3

9.55 x 102

K4

1.74 x 102

i) Using Table 2.1, calculate the stability constant, Kstab, and state the units. Show your working.

[2]

ii) Deduce what this value indicates about the relative stabilities of the two complexes.

[1]

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3a
2 marks

Define the term stability constant, Kstab

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3b
4 marks

Silver forms different complexes. The stability constant, Kstab, values at 298 K are shown in Table 3.1

Table 3.1

complex

stability constant, Kstab

[Ag(CN)2]–

5.3 x 1018

[Ag(NH3)]+

1.7 x 107

[Ag(S2O3)2]3–

2.9 x 1013

The following equilibrium exists between two complex ions of silver in the +1 oxidation state.

Ag+ + NH3 ⇌ [Ag(NH3)]+

i) Write the expression for the stability constant, Kstab, for this equilibrium

Kstab =

[1]

ii) When a 0.500 mol dm–3 solution of AgNO3 (aq) is mixed with 0.500 mol dm–3 NH3 (aq) the equilibrium mixture of [Ag(NH3)]+ (aq) was found to be 0.4998 mol dm–3

Calculate Kstab and state the units. Show your working.

[3]

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3c
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1 mark

Using Table 3.1, deduce the order of stability of the silver complexes

most stable

least stable

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4a
3 marks

Nickel(II) ions, Ni2+ (aq), react with a variety of ligands to form octahedral complexes.

i) Define the term stability constant, Kstab

[2]

Nickel(II) ions react with an excess of aqueous ammonia to form the complex ion [Ni(NH3)6]2+.

ii) Write an expression for the stability constant, Kstab, for this complex.

[1]

Kstab =

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4b
4 marks

Ethane-1,2-diamine (represented as en) is a bidentate ligand.

Nickel(II) forms the complex ion [Ni(en)3]2+. This complex exists as two stereoisomers.

i) Define the term bidentate ligand.

[2]

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

[2]

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4c
3 marks

The complex [Ni(EDTA)]2– is significantly more stable than the complex [Ni(NH3)6]2+.

By considering the ligand exchange reaction between [Ni(NH3)6]2+ and EDTA4–, explain this difference in stability in terms of entropy changes.

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1a
5 marks

When chromium(III) sulfate dissolves in water, a violet 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.

[4]

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1b
5 marks

Ethylenediaminetetraacetate, EDTA4−, shown in Fig. 1.1, is a polydentate ligand.

Structural formula of EDTA showing two nitrogen atoms each connected to two CH2COO- groups and linked by an ethylene bridge

Fig. 1.1

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 [Cr(EDTA)]– in this reaction.

[1]

iii) The numerical value of the Kstab of [Cr(EDTA)]– is 2.51 × 1023 in this reaction.

Suggest what this indicates about the position and entropy of the equilibrium.

[3]

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1c
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3 marks

Chromium(III) picolinate, shown in Fig. 1.2, is a neutral complex that can be prepared from the weak acid, picolinic acid.

Structural formula of chromium(III) picolinate showing a Cr3+ ion coordinated to three picolinate ligands in an octahedral arrangement

Fig. 1.2

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. Show your working.

[2]

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1d
3 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.

i) Deduce the formula of complex ion M.

[1]

ii) Draw three-dimensional diagrams to show the two stereoisomers of complex ion M.

[2]

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