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CAIE A-Level Chemistry A2 28.2 Chemical Properties of First Row Transition Elements Questions

Practise ligand definitions, complex geometry, ligand exchange, precipitation and redox reactions of first-row ions.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • identify ligands, denticity and coordination number from the number of donated lone pairs and bonds
  • write ligand-exchange or hydroxide-deprotonation equations and predict formulae and colours
  • distinguish precipitation, ligand exchange and redox when interpreting Cu2+ or Co2+ observations

Covered syllabus points

28.2.1—Ligand reactions forming complexes28.2.2—Terms: ligand as a species that contains28.2.3—Terms28.2.4—Complex ions28.2.5—Shapes of transition metal complexes28.2.6—Coordination number and complex formula28.2.7—Ligand exchange reactions28.2.8—Predict, using E28.2.9—Transition metal redox calculations28.2.10—Calculations: other redox systems

Question 1

[Maximum number: 4]

Titanium is a transition element in Period 4. It is commonly found as TiO2\mathrm{TiO}_{2} in minerals.

Question (a)

(a)

Some reactions of TiO2\mathrm{TiO}_{2} are shown in Fig. 3.1.

The anion, acac −^{-}, is a bidentate ligand.

Fig. 3.1

Fig. 3.1

[ 4 ]

Question (i)

(i)

The titanium ions in TiF62−\mathrm{TiF}_{6}{ }^{2-} and Ti(acac)2Cl2\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{Cl}_{2} have a coordination number of 6 . State what is meant by coordination number.

[ 1 ]

Question (ii)

(ii)

Write an equation for the formation of TiF62−\mathrm{TiF}_{6}{ }^{2-} from TiO2\mathrm{TiO}_{2}.

[ 1 ]

Question (iii)

(iii)

State what is meant by bidentate ligand.

[ 2 ]

Question 2

[Maximum number: 4]

Ruthenium and osmium are transition metals below iron in Group 8 of the Periodic Table.

Question (a)

(a)

Two different complex ions, X and Y, can form when anhydrous RuCl3\mathrm{RuCl}_{3} reacts with water under certain conditions.
X and Y have octahedral geometry.
Aqueous samples of X and Y react separately with an excess of AgNO3(aq)\mathrm{AgNO}_{3}(\mathrm{aq}). Different amounts of AgCl are precipitated:
- 1 mole of complex ion X produces 2 moles of AgCl
- 1 mole of complex ion Y produces 1 mole of AgCl.

[ 2 ]

Question (i)

(i)

Complete Table 4.1 to suggest formulae for X and Y.

Table 4.1

Table 4.1

[ 2 ]

Question (b)

(b)

Fig. 4.1 shows another ruthenium complex.

Fig. 4.1

Fig. 4.1

This complex contains the neutral ligand pyrazine.

Figure for Question (b) — CAIE A-Level Chemistry A2
[ 1 ]

Question (i)

(i)

Suggest how pyrazine is able to bond to two separate ruthenium ions.

[ 1 ]

Question (c)

(c)

Osmium tetroxide, OsO4\mathrm{OsO}_{4}, reacts with alkenes in a similar manner to cold dilute acidified MnO4−\mathrm{MnO}_{4}{ }^{-}.

Fig. 4.2 shows a proposed synthesis of a condensation polymer G

Fig. 4.2 shows a proposed synthesis of a condensation polymer G

[ 1 ]

Question (i)

(i)

Suggest a reagent for step 1.

[ 1 ]

Question 3

[Maximum number: 4]

Question (a)

(a)

Aqueous solutions of iron(II) salts contain the complex ion [Fe(H2O)6]2+\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}.

Define complex ion.

[ 1 ]

Question (b)

(b)

[Fe(H2O)6]2+\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+} can be converted into [Fe(H2O)4(OH)2]\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right].

[ 3 ]

Question (i)

(i)

[Fe(H2O)4(OH)2]\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] is a green precipitate that turns brown on standing in air.

Table 2.1 shows electrode potentials for some electrode reactions.

Table 2.1

Table 2.1

Use the information in Table 2.1 to explain why [Fe(H2O)4(OH)2]\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] turns brown on standing in air.
Include an equation for this reaction.

[ 3 ]

Question 4

[Maximum number: 1]

Titanium is a transition element in Period 4. It is commonly found as TiO2\mathrm{TiO}_{2} in minerals.

Some reactions of TiO2\mathrm{TiO}_{2} are shown in Fig. 3.1.

The anion, acac −^{-}, is a bidentate ligand.

Fig. 3.1

Fig. 3.1

Write an equation for the formation of TiF62−\mathrm{TiF}_{6}{ }^{2-} from TiO2\mathrm{TiO}_{2}.

Question 5

[Maximum number: 1]

Ruthenium and osmium are transition metals below iron in Group 8 of the Periodic Table.

Fig. 4.1 shows another ruthenium complex.

Fig. 4.1

Fig. 4.1

This complex contains the neutral ligand pyrazine.

Figure for Question 5 — CAIE A-Level Chemistry A2

Suggest how pyrazine is able to bond to two separate ruthenium ions.

Question 6

[Maximum number: 2]

Titanium is a transition element in Period 4. It is commonly found as TiO2\mathrm{TiO}_{2} in minerals.

Some reactions of TiO2\mathrm{TiO}_{2} are shown in Fig. 3.1.

The anion, acac −^{-}, is a bidentate ligand.

Fig. 3.1

Fig. 3.1

State what is meant by bidentate ligand.

Question 7

[Maximum number: 1]

Aqueous solutions of iron(II) salts contain the complex ion [Fe(H2O)6]2+\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}.

Define complex ion.

Question 8

[Maximum number: 1]

Copper is a transition element and has atomic number 29.

The following equilibrium exists between two complex ions of copper in the +2 oxidation state.

[Cu(H2O)6]2++4Cl−⇌[CuCl4]2−+6H2O\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons\left[\mathrm{CuCl}_{4}\right]^{2-}+6 \mathrm{H}_{2} \mathrm{O}

State the shape of the [CuCl4]2−\left[\mathrm{CuCl}_{4}\right]^{2-} ion.

Question 9

[Maximum number: 1]

Titanium is a transition element in Period 4. It is commonly found as TiO2\mathrm{TiO}_{2} in minerals.

Some reactions of TiO2\mathrm{TiO}_{2} are shown in Fig. 3.1.

The anion, acac −^{-}, is a bidentate ligand.

Fig. 3.1

Fig. 3.1

The titanium ions in TiF62−\mathrm{TiF}_{6}{ }^{2-} and Ti(acac)2Cl2\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{Cl}_{2} have a coordination number of 6 . State what is meant by coordination number.

Question 10

[Maximum number: 1]

Copper is a transition element and has atomic number 29.

The following equilibrium exists between two complex ions of copper in the +2 oxidation state.

[Cu(H2O)6]2++4Cl−⇌[CuCl4]2−+6H2O\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons\left[\mathrm{CuCl}_{4}\right]^{2-}+6 \mathrm{H}_{2} \mathrm{O}

Name the type of reaction occurring here.

Question 11

[Maximum number: 3]

[Fe(H2O)6]2+\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+} can be converted into [Fe(H2O)4(OH)2]\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right].

[Fe(H2O)4(OH)2]\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] is a green precipitate that turns brown on standing in air.

Table 2.1 shows electrode potentials for some electrode reactions.

Table 2.1

Table 2.1

Use the information in Table 2.1 to explain why [Fe(H2O)4(OH)2]\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] turns brown on standing in air.
Include an equation for this reaction.

Question 12

[Maximum number: 2]

Iodine is found naturally in compounds in many different oxidation states.

The concentration of Cu2+(aq)\mathrm{Cu}^{2+}(\mathrm{aq}) in a solution can be determined by the reaction of Cu2+\mathrm{Cu}^{2+} ions with I−\mathrm{I}^{-} ions.

reaction 12Cu2++4I−→2CuI+I212 \mathrm{Cu}^{2+}+4 \mathrm{I}^{-} \rightarrow 2 \mathrm{CuI}+\mathrm{I}_{2}

The I2\mathrm{I}_{2} produced in reaction 1 is titrated against a solution containing thiosulfate ions, S2O32−\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}, using a suitable indicator.

 reaction 22 S2O32−+I2→ S4O62−+2I−\text { reaction } 2 \quad 2 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}+\mathrm{I}_{2} \rightarrow \mathrm{~S}_{4} \mathrm{O}_{6}^{2-}+2 \mathrm{I}^{-}

A 25.0 cm325.0 \mathrm{~cm}^{3} portion of a Cu2+(aq)\mathrm{Cu}^{2+}(\mathrm{aq}) solution reacts with an excess of I−(aq)\mathrm{I}^{-}(\mathrm{aq}).

The end-point of the titration occurs when 22.30 cm322.30 \mathrm{~cm}^{3} of 0.150 moldm−3 S2O32−(aq)0.150 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{~S}_{2} \mathrm{O}_{3}{ }^{2-}(\mathrm{aq}) is added.
Calculate the concentration of Cu2+(aq)\mathrm{Cu}^{2+}(\mathrm{aq}) in the original solution.

Question 13

[Maximum number: 3]

An impure sample of a vanadium(V) compound of mass 0.250 g is dissolved in aqueous acid. This solution contains VO3−\mathrm{VO}_{3}{ }^{-} ions.

An excess of zinc is added to this solution. All the VO3−\mathrm{VO}_{3}{ }^{-} ions are reduced to V2+\mathrm{V}^{2+} ions and Zn atoms are oxidised to Zn2+\mathrm{Zn}^{2+} ions.

The unreacted zinc is removed and the resulting solution is titrated with acidified MnO4−\mathrm{MnO}_{4}^{-}.
The end-point is reached when 22.5 cm322.5 \mathrm{~cm}^{3} of 0.0750 moldm−3MnO4−0.0750 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{MnO}_{4}^{-} is added.
A redox reaction takes place and all the V2+\mathrm{V}^{2+} reacts forming VO3−\mathrm{VO}_{3}{ }^{-}.

3MnO4−+5 V2++3H2O→3Mn2++5VO3−+6H+3 \mathrm{MnO}_{4}^{-}+5 \mathrm{~V}^{2+}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow 3 \mathrm{Mn}^{2+}+5 \mathrm{VO}_{3}^{-}+6 \mathrm{H}^{+}

Calculate the percentage by mass of vanadium in the 0.250 g of impure sample.

Assume the impurities do not contain any vanadium ions.
Show your working.
percentage of vanadium =

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