24- Transition Elements

Question 1

What’s the difference between a d-block element and a transition element?

Answer 1

d-block: Elements Sc–Zn (Period 4) where d-orbitals are being filled.
Transition element: A d-block element that forms at least one ion with a partially filled d-subshell.
→ Sc³⁺ (empty d) and Zn²⁺ (full d¹⁰) are not transition elements.

Question 2

Why do Cr and Cu have unexpected electron configurations?

Answer 2

Cr: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) → half-filled d-subshell = extra stability
Cu: [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²) → fully filled d-subshell = extra stability

Question 3

When forming ions, which electrons are lost first: 4s or 3d?

Answer 3

4s electrons are lost before 3d.
E.g., Fe: [Ar] 3d⁶ 4s² → Fe²⁺: [Ar] 3d⁶ → Fe³⁺: [Ar] 3d⁵

Question 4

Name three properties that distinguish transition metals from other metals.

Answer 4

Variable oxidation states (e.g., Fe²⁺/Fe³⁺)
Formation of coloured compounds/ions
Act as catalysts (e.g., Fe in Haber process)

Question 5

Why are transition metal compounds coloured?

Answer 5

Partially filled d-orbitals allow d–d electron transitions when visible light is absorbed. The complementary colour is observed.

Question 6

What are a ligand and a complex ion?

Answer 6

Ligand: Molecule/ion that donates a lone pair to form a coordinate (dative covalent) bond with a metal ion.
Complex ion: Central metal ion bonded to ligands via coordinate bonds (e.g., [Cu(H₂O)₆]²⁺).

Question 7

Give examples of monodentate and bidentate ligands.

Answer 7

Monodentate: H₂O, NH₃, Cl⁻, CN⁻ (donate 1 lone pair)
Bidentate: 1,2-diaminoethane (en), ethanedioate (oxalate) (donate 2 lone pairs)

Question 8

What shapes do 4- and 6-coordinate complexes adopt?

Answer 8

Coordination number 6: Octahedral (90° bond angles)
Coordination number 4: Tetrahedral (109.5°) or square planar (90°)
→ Square planar common for d⁸ ions (e.g., Pt²⁺, Ni²⁺)

Question 9

Describe cis-trans isomerism in octahedral [Co(NH₃)₄Cl₂]⁺.

Answer 9

Cis: Cl ligands adjacent (90° apart) → violet
Trans: Cl ligands opposite (180° apart) → green

Question 10

When does optical isomerism occur in transition metal complexes?

Answer 10

In octahedral complexes with bidentate ligands that lack a plane of symmetry (e.g., cis-[Co(en)₂Cl₂]⁺).
→ Non-superimposable mirror images (enantiomers)

Question 11

How does cis-platin work as an anti-cancer drug?

Answer 11

Cis isomer binds to DNA → prevents replication → triggers apoptosis
Trans-platin is ineffective → geometry matters

Question 12

What happens when excess NH₃ is added to [Cu(H₂O)₆]²⁺?

Answer 12

Pale blue ppt of Cu(OH)₂ (with few drops)
Ppt dissolves in excess NH₃ → deep blue [Cu(NH₃)₄(H₂O)₂]²⁺

Question 13

What happens when conc. HCl is added to [Cu(H₂O)₆]²⁺?

Answer 13

Pale blue → green (mix) → yellow [CuCl₄]²⁻ (tetrahedral).
Reversible with water.

Question 14

How does CO poisoning occur via ligand substitution?

Answer 14

CO replaces O₂ in haemoglobin → forms carboxyhaemoglobin.
CO binds more strongly than O₂ → irreversible → blocks O₂ transport.

Question 15

Give the precipitate colours for Cu²⁺, Fe²⁺, Fe³⁺, Cr³⁺ with NaOH.

Answer 15

Cu²⁺: blue ppt (insoluble in excess)
Fe²⁺: green ppt → brown on standing
Fe³⁺: orange-brown ppt
Cr³⁺: green ppt → dissolves in excess → dark green [Cr(OH)₄]⁻

Question 16

Write equations for:
a) Oxidation of Fe²⁺ by MnO₄⁻
b) Reduction of Fe³⁺ by I⁻

Answer 16

a) MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
b) 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

Question 17

What happens when Cu₂O reacts with dilute H₂SO₄?

Answer 17

Cu⁺ disproportionates:

Cu₂O + H₂SO₄ → Cu(s) + CuSO₄(aq) + H₂O

→ Cu⁺ → Cu⁰ (reduction) and Cu²⁺ (oxidation)

Question 18

What is the oxidation state of Cr in Cr₂O₇²⁻?

Answer 18

2x + 7(–2) = –2 → 2x = 12 → x = +6

Question 19

Why are complexes with bidentate ligands more stable?

Answer 19

Chelate effect: Multidentate ligands form ring structures → greater entropy → more stable complex.

Question 20

Why do transition elements form complex compounds?

Answer 20

It forms complexes due to the presence of vacant d orbitals.

Question 21

Why are transition elements used as catalysts?

Answer 21

Transition metals and their compounds function as catalysts either because of their ability to change oxidation state or, in the case of the metals, to adsorb other substances on to their surface and activate them in the process.

Question 22

What determines the shape of a transition metal complex ion, and what are the common geometries?

Answer 22

The coordination number (number of coordinate bonds to the central metal ion) determines the shape:

Coordination number 6 → Octahedral
Bond angle: 90°
Example: [Cu(H₂O)₆]²⁺, [Co(NH₃)₆]³⁺

Coordination number 4 → Two possible shapes:

Tetrahedral (most common): bond angle 109.5°
Example: [CoCl₄]²⁻, [CuCl₄]²⁻

Square planar: bond angle 90°
Common for d⁸ ions (e.g., Ni²⁺, Pt²⁺, Pd²⁺)
Example: [Pt(NH₃)₄]²⁺

Ligand size and electronic configuration also influence geometry.

Question 23

Explain Ligand Substitution in Haemoglobin?

Answer 23

Haemoglobin is one of nature’s complexes using a transition metal ion

The haem molecule is a complex with iron(II) at its centre

Oxygen atoms form a dative covalent bond with the Fe(II) which enables oxygen molecules to be transported around the body in the blood

Oxygen molecules are not very good ligands and bond weakly to the iron(II)

The weak bonds allows them to break off easily and be transported into cells