CAIE A-Level Chemistry 28.1 First-Row Transition-Element Properties

Practise definitions, d-orbital shapes and explanations for variable oxidation states, catalysis, complexes and colour.

Exam points

apply the stable-ion incomplete-d definition rather than classifying every d-block element automatically
sketch 3dxy and 3dz² orbitals with correct orientation, lobes and the z² torus
explain variable states, catalysis and complex formation using 3d/4s energies and accessible vacant d orbitals

Covered syllabus points

28.1.1—Transition element 28.1.2—Sketch the shape of a 3dxy orbital and 3dz² 28.1.3—Transition elements have properties 28.1.4—Why transition elements have variable 28.1.5—Why transition elements behave as catalysts 28.1.6—Why transition elements form complex ions

[Maximum number: 3]

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)

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

[Maximum number: 1]

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

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

$reaction $12 \mathrm{Cu}^{2+}+4 \mathrm{I}^{-} \rightarrow 2 \mathrm{CuI}+\mathrm{I}_{2}$$

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

\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}^{-}

Copper(I) and copper(II) both contain electrons in all five 3d orbitals.

Sketch the shape of a $3 \mathrm{~d}_{\mathrm{xy}}$ orbital on the axes provided.

Figure for Question 28.1 Physical and chemical properties of first-row transition elements question 2 — CAIE A-Level Chemistry A2

$3 \mathrm{~d}_{\mathrm{xy}}$

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Covered syllabus points

28.1 Physical and chemical properties of first-row transition elements question 1

Question (a)

Question (b)

Question (c)

Question (i)

28.1 Physical and chemical properties of first-row transition elements question 2