Stability Constants, Kstab — Structured Questions (Cambridge International A-Level Chemistry): Exam Questions

Sketch the shape of a 3d_xy orbital.

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

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

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

E^ɵ 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.

ii) Calculate E^ɵ_cell for this reaction.

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

K_stab = 2.0 × 10⁵ mol^–1 dm³

equilibrium 2:

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

violet ⇌ deep-red

K_stab = 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.

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

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)

K_a = 8.9 × 10^–4 mol dm^–3

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

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

Define the term ligand.

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

K_stab =

Extract:

[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+^

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

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

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

Define the term stability constant, K_stab

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

Table 3.1

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, K_stab, for this equilibrium

K_stab =

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 K_stab and state the units. Show your working.

Extract:

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

Table 3.1

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

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

i) Define the term stability constant, K_stab

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, K_stab, for this complex.

K_stab =

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.

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

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 EDTA^4–, explain this difference in stability in terms of entropy changes.

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.

ii) Explain why the chromium(III) complex ion is coloured.

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

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⁺ + EDTA^4– ⇌ [Cr(EDTA)]^– + 6H₂O

i) Name the type of reaction occurring here.

ii) Write an expression for the stability constant, K_stab, of [Cr(EDTA)]^– in this reaction.

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

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

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

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.

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.

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.

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