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Properties and uses of transition metals

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 1 Properties and uses of transition metals

Key unit competence: Explain the relationship between the electronic structure, properties and applications of transition metals.

Key ideas

  • A transition element forms at least one stable ion with a partly filled d sub-shell. First series: Sc to Zn, filling 3d. Exceptions: Cr [Ar] 3d5 4s1 and Cu [Ar] 3d10 4s1 (half-filled and filled d sub-shells are extra stable). On forming ions, 4s electrons are lost first: Fe2+ [Ar] 3d6, Fe3+ [Ar] 3d5.
  • Typical properties: variable oxidation states (Mn +2 to +7), coloured ions (d–d electron transitions in a split d sub-shell), complex ions, catalytic activity, paramagnetism (unpaired d electrons), high melting points and densities.
  • Sc and Zn are not typical: Sc3+ has 3d0 and Zn2+ has 3d10, so their compounds are colourless, diamagnetic and show one oxidation state.
  • A complex ion has a central metal ion bonded to ligands by dative (coordinate) bonds. Coordination number 6: octahedral; 4: tetrahedral or square planar; 2: linear. Ligands: monodentate (H2O, NH3, Cl−, CN−), bidentate (ethane-1,2-diamine 'en', ethanedioate), hexadentate (EDTA4−).
  • Naming: ligands in alphabetical order with prefixes (di-, tri-, tetra-, hexa-), then the metal and its oxidation number; an anionic complex ends in -ate: [Cu(NH3)4]2+ tetraamminecopper(II); [Fe(CN)6]3− hexacyanidoferrate(III). Isomerism: cis/trans (e.g. [Pt(NH3)2Cl2], cisplatin is the anticancer drug) and optical isomerism ([Co(en)3]3+).
  • Tests with NaOH(aq): Cu2+ blue ppt (dissolves in excess NH3 to a deep blue solution); Fe2+ green ppt turning brown in air; Fe3+ red-brown ppt; Zn2+ white ppt soluble in excess NaOH and excess NH3; Cr3+ grey-green ppt soluble in excess NaOH; Mn2+ off-white ppt darkening in air. Fe3+ with thiocyanate: blood-red colour.

Common misconceptions

  • "Zinc is a typical transition metal because it is in the d-block." No: Zn forms only Zn2+ with a full 3d10 sub-shell, so it is not a transition element by definition.
  • "4s electrons are lost last because they were added first." No: On forming ions the 4s electrons are lost FIRST: Fe [Ar]3d6 4s2 → Fe2+ [Ar]3d6.
  • "The colour of a complex is the light it absorbs." No: We see the complementary colour: light that is NOT absorbed is transmitted or reflected.

Worked examples

Example 1. Explain why [Cu(H2O)6]2+ is blue but [Zn(H2O)6]2+ is colourless.

Solution

In the complex, ligands split the five 3d orbitals into two energy levels.

Cu2+ is 3d9: an electron can jump from a lower to a higher d orbital by absorbing red-orange light; the transmitted light looks blue.

Zn2+ is 3d10: all d orbitals are full, so no d–d transition is possible and no visible light is absorbed.

So zinc compounds are white or colourless.

Example 2. Describe how you would show that a solution contains Fe2+ and not Fe3+.

Solution

Add NaOH(aq): Fe2+ gives a green precipitate of Fe(OH)2 that slowly turns brown at the surface; Fe3+ gives red-brown Fe(OH)3 at once.

Add potassium hexacyanidoferrate(III): Fe2+ gives a dark blue precipitate.

Add KSCN: Fe3+ gives blood red; Fe2+ gives no strong red colour.

Acidified KMnO4 is decolourised by Fe2+ but not by Fe3+.

Practice questions

  1. Define a transition element.

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    An element that forms at least one stable ion with a partly filled d sub-shell.

    Common mistake: Saying any d-block element.

  2. Write the electron configuration of Cu+.

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    [Ar] 3d10.

    Common mistake: Writing [Ar] 3d9 4s1.

  3. Give three typical properties of transition metals.

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    Variable oxidation states, coloured compounds, complex formation, catalytic activity, paramagnetism (any three).

    Common mistake: Listing 'low density'.

  4. Explain why scandium is not considered a typical transition element.

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    Its only ion Sc3+ has no d electrons (3d0), so its compounds are colourless.

    Common mistake: Saying scandium has no 3d electrons in the atom.

  5. What is meant by a bidentate ligand? Give an example.

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    A ligand that forms two dative bonds to the same metal ion, e.g. ethane-1,2-diamine or ethanedioate.

    Common mistake: Giving H2O as an example.

  6. Name [Fe(H2O)6]3+.

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    Hexaaquairon(III) ion.

    Common mistake: Calling it hexahydroxoiron(III).

  7. State the shape and bond angle of [Ni(CN)4]2− (square planar).

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    Square planar; 90°.

    Common mistake: Saying tetrahedral 109.5°.

  8. What is seen when excess NaOH is added to Cr3+(aq)?

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    Grey-green precipitate that dissolves in excess to a dark green solution.

    Common mistake: Saying the precipitate is blue.

  9. Give one catalytic use of vanadium(V) oxide.

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    Contact process: 2SO2 + O2 ⇌ 2SO3.

    Common mistake: Saying it is used in the Haber process.

  10. Why are transition metals paramagnetic?

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    They have unpaired d electrons.

    Common mistake: Saying they contain iron.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Cisplatin, used against cancer, is the cis isomer of:a. [Ar] 3d5 4s1
    2. The electron configuration of chromium is:b. [Pt(NH3)2Cl2]
    3. Which ion is colourless in aqueous solution?c. Zn2+
    4. Ethane-1,2-diamine (en) is a:d. bidentate ligand
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    1 → b ([Pt(NH3)2Cl2]) · 2 → a ([Ar] 3d5 4s1) · 3 → c (Zn2+) · 4 → d (bidentate ligand)

    • Square planar Pt(II) complex.
    • A half-filled 3d sub-shell is more stable.
    • 3d10: no d–d transitions possible.
    • It has two nitrogen donor atoms.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    +7  ·  6  ·  Fe3+  ·  +4
    1. The maximum oxidation state of manganese is .
    2. A blood-red colour with potassium thiocyanate shows the presence of .
    3. The coordination number of copper in [Cu(H2O)6]2+ is .
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    1: +7 · 2: Fe3+ · 3: 6

    • In MnO4− manganese is +7 (all 4s and 3d electrons used).
    • [Fe(SCN)(H2O)5]2+ forms.
    • Six water ligands.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The electron configuration of the Fe3+ ion is:a. linear
    2. EDTA4− forms how many dative bonds to one metal ion?b. [Ar] 3d5
    3. Which ion gives a white precipitate with NaOH that dissolves in excess NaOH?c. 6
    4. The shape of [Ag(NH3)2]+ is:d. Zn2+
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    1 → b ([Ar] 3d5) · 2 → c (6) · 3 → d (Zn2+) · 4 → a (linear)

    • Fe loses 4s2 then one 3d electron.
    • It is hexadentate.
    • Zinc hydroxide is amphoteric.
    • Coordination number 2.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    octahedral  ·  iron  ·  linear  ·  tetraamminecopper(II) ion
    1. The name of [Cu(NH3)4]2+ is .
    2. The catalyst used in the Haber process is .
    3. The shape of a complex with coordination number 6 is .
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    1: tetraamminecopper(II) ion · 2: iron · 3: octahedral

    • Cationic complex: the metal keeps its name.
    • Fe catalyses N2 + 3H2 ⇌ 2NH3.
    • Six ligands at 90° to each other.

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