CAIE A-Level Chemistry 28 Chemistry of Transition Elements

Practise d-block definitions, complex-ion structures, ligand exchange, redox chemistry and catalytic behaviour.

Covered syllabus points

28.1 Physical and chemical properties of first-row transition elements 28.2 Chemical properties of first-row transition elements 28.3 Colour of complexes 28.4 Stereoisomerism in transition element complexes 28.5 Stability constants, K stab

[Maximum number: 13]

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

(a)

Define transition element.

[ 1 ]

(b)

Identify two typical properties of transition elements.

1

2

[ 1 ]

(c)

The $\mathrm{TiO}^{2+}$ ion forms when $\mathrm{TiO}_{2}$ reacts with an excess of sulfuric acid.
$\mathrm{TiO}^{2+}$ can be reduced by zinc metal in acidic conditions to form a purple solution containing $\mathrm{Ti}^{3+}(\mathrm{aq})$ .

[ 2 ]

(i)

$\mathrm{TiO}^{2+}(\mathrm{aq})$ is a colourless ion.

Suggest why.

[ 2 ]

Ti is in +4 oxidation state so no d electrons $/ \mathrm{d}^{0} \mathbf{OR ~ T i}$ in $\mathrm{TiO}^{2+}$ has no d electrons $/ \mathrm{d}^{0}$ [1]
cannot absorb photons / light in visible spectrum OR no wavelength / frequency absorbed in visible spectrum [1]

(d)

Acidified $\mathrm{Ti}^{3+}(\mathrm{aq})$ reacts with oxygen dissolved in water as shown.

4 \mathrm{Ti}^{3+}+\mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{TiO}^{2+}+4 \mathrm{H}^{+} \quad \Delta G^{\ominus}=-436.1 \mathrm{~kJ} \mathrm{~mol}^{-1}

The standard reduction potential, $E^{\ominus}$ , of $\mathrm{O}_{2}+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}$ is +1.23 V .

[ 1 ]

(i)

When aqueous citrate ions, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}{ }^{3-}$ , are added to $\mathrm{Ti}^{3+}(\mathrm{aq})$ , the $\left[\mathrm{Ti}\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}\right)_{2}\right]^{3-}(\mathrm{aq})$ complex forms.

Explain, in terms of d-orbitals, why $\mathrm{Ti}^{3+}$ is able to form complex ions.

[ 1 ]

$\mathrm{Ti}^{3+}$ empty / vacant d orbitals can form dative bonds / accept a lone pair from a ligand
OR $\mathrm{Ti}^{3+}$ has vacant d-orbitals which are energetically accessible

(e)

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

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

Fig. 3.1

[ 8 ]

(i)

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

[ 1 ]

(ii)

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

[ 1 ]

$\begin{aligned} \mathrm{TiO}_{2}+6 \mathrm{HF} \rightarrow \mathrm{TiF}_{6}{ }^{2-}+2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{H}^{+} \mathrm{OR} \mathrm{TiO}_{2}+6 \mathrm{HF} \rightarrow \mathrm{TiF}_{6}{ }^{2-}+2 \mathrm{H}_{3} \mathrm{O}^{+} \end{aligned}$

(iii)

State what is meant by bidentate ligand.

[ 2 ]

(iv)

$\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{C} l_{2}$ shows both optical and geometrical (cis/trans) isomerism.
$\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{C} l_{2}$ exists as three stereoisomers.
The structure of one stereoisomer of $\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{Cl}_{2}$ is shown in Fig. 3.2.

Fig. 3.2

Complete the structures of the other two stereoisomers of $\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{C} l_{2}$ .

Figure for Question (iv) — CAIE A-Level Chemistry A2

[ 2 ]

(v)

The acac- anion is symmetrical.

Deduce which, if any, of stereoisomers 1, 2 and 3 in (d)(iv) are polar. Explain your answer.

[ 2 ]

[Maximum number: 15]

(a)

Define a transition element.

[ 1 ]

(b)

The 3d orbitals in an isolated gaseous $\mathrm{Cu}^{2+}$ ion are degenerate.

[ 1 ]

(i)

Define the term degenerate.

[ 1 ]

(c)

State the colours of the aqueous solutions for the two copper(II) complex ions shown.
- $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}(\mathrm{aq})$
- $\left[\mathrm{CuCl}_{4}\right]^{2-}(\mathrm{aq})$

[ 1 ]

$\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}(\mathrm{aq})$ deep / dark / royal blue

AND

$\left[\mathrm{CuCl}_{4}\right]^{2-}(\mathrm{aq})$ yellow

(d)

Explain why aqueous complex ions of transition elements are usually coloured.

[ 3 ]

(e)

When an excess of $\mathrm{NH}_{3}(\mathrm{aq})$ is added to a solution of $\left[\mathrm{CuCl}_{4}\right]^{2-}(\mathrm{aq})$ , $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}(\mathrm{aq})$ is formed.

State the type of reaction.
Complete the equation for this reaction. State symbols are not required.
type of reaction equation

$\left[\mathrm{CuCl}_{4}\right]^{2-}+\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \rightarrow\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}+$

[ 2 ]

M1: ligand exchange / replacement / substitution / displacement

M2: $\left[\mathrm{CuCl}_{4}\right]^{2-}+4 \mathrm{NH}_{3}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}+4 \mathrm{Cl}^{-}$

(f)

The $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ complex ion shows stereoisomerism.

Complete the three-dimensional diagrams in Fig. 1.1 to show the two different stereoisomers of $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$ .

Fig. 1.1

[ 2 ]

Figure for Answer to Question (f) — CAIE A-Level Chemistry A2

M1: one correct 3D structure of $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}$

M2: correct 3D stereoisomer structure of isomer 1

(g)

Deduce which stereoisomer in (d)(ii) is polar.

Explain your answer.
polar isomer
explanation

[ 1 ]

(h)

The dianion P can act as a tridentate ligand.

Figure for Question (h) — CAIE A-Level Chemistry A2

[ 2 ]

(i)

Suggest how P can form three dative covalent bonds.

[ 1 ]

(ii)

2 moles of dianion $\mathbf{P}, \mathrm{C}_{4} \mathrm{H}_{5} \mathrm{NO}_{4}{ }^{2-}$ , react with 1 mole of aqueous cobalt(III) ions, $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ to form 1 mole of complex ion Q.

Deduce the formula and charge of Q.

[ 1 ]

$\left[\mathrm{Co}\left(\mathrm{C}_{4} \mathrm{H}_{5} \mathrm{NO}_{4}\right)_{2}\right]^{-}$

(i)

（f）Table 1.1 shows values for the stability constants， $K_{\text {stab }}$ ，of some silver（I）complexes．

Table 1.1

[ 2 ]

(i)

（i）Define the stability constant of a complex．

[ 1 ]

(ii)

（ii）Use the information in Table 1.1 to identify the most stable silver（I）complex．
Explain your answer．
most stable
explanation

[ 1 ]

[Ag(CN)2]-(aq) AND largest (value of) $K_{\text {stab }}$

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Covered syllabus points

28. Chemistry of transition elements question 1

Question (a)

Question (b)

Question (c)

Question (i)

Question (d)

Question (i)

Question (e)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (v)

28. Chemistry of transition elements question 2

Question (a)

Question (b)

Question (i)

Question (c)

Question (d)

Question (e)

Question (f)

Question (g)

Question (h)

Question (i)

Question (ii)

Question (i)

Question (i)

Question (ii)