AQA A-Level Chemistry: Transition metals (7405) – Revision Notes

These notes condense section 3.2.5, Transition metals, of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams from June 2016 onwards. Every part of 3.2.5 (3.2.5.1 to 3.2.5.6) is marked “A-level only”, so none of it is on the AS papers. At A-level, inorganic chemistry is listed under Paper 1, and Paper 3 can assess any content.

For full explanations and worked examples, use the study guide. Then try the practice questions. Related notes: oxidation, reduction and redox equations and electrode potentials. The course hub, printable checklist and free diagnostics help you plan.

3.2.5.1 Definitions to learn exactly

• Transition metal characteristics (Ti–Cu) arise from an incomplete d sub-level in atoms or ions.
• Four properties: complex formation, coloured ions, variable oxidation state, catalytic activity.
• Ligand: a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.
• Complex: a central metal atom or ion surrounded by ligands.
• Co-ordination number: the number of co-ordinate bonds to the central metal atom or ion.

3.2.5.2 Ligands and substitution

Ligand	Type	Bonds formed
H₂O, NH₃, Cl⁻	monodentate	1
H₂NCH₂CH₂NH₂ (en)	bidentate	2
C₂O₄²⁻ (ethanedioate)	bidentate	2
EDTA⁴⁻	multidentate	6

Method in steps: predicting a ligand swap

1. Is the new ligand NH₃ replacing H₂O? Similar size, both uncharged, so co-ordination number unchanged (stays 6, octahedral).
2. Is it Cl⁻ replacing H₂O? Cl⁻ is larger, so co-ordination number can fall from 6 to 4 (tetrahedral).
3. Is a bidentate or multidentate ligand replacing monodentate ones? The chelate effect makes it favourable.
4. Balance: count ligands out, ligands in, and recompute the complex charge.

Key equations:

[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O   (incomplete; CN stays 6)
[Co(H₂O)₆]²⁺ + 4Cl⁻ → [CoCl₄]²⁻ + 6H₂O            (CN 6 → 4)
[Fe(H₂O)₆]³⁺ + 4Cl⁻ → [FeCl₄]⁻ + 6H₂O             (CN 6 → 4)

Chelate effect in one line: ΔH ≈ small (similar co-ordinate bonds broken and made), ΔS positive (more particles), so ΔG = ΔH − TΔS is negative.

Haem: iron(II) complex with a multidentate ligand. O₂ forms a co-ordinate bond to Fe(II) for transport in blood. CO is toxic because it replaces the O₂ co-ordinately bonded to Fe(II).

3.2.5.3 Shapes and isomers

Shape	Typical ligands	Angle	Isomerism
Octahedral	small: H₂O, NH₃	90°	cis–trans (monodentate); optical (bidentate)
Tetrahedral	larger: Cl⁻	109.5°	—
Square planar	e.g. [Pt(NH₃)₂Cl₂]	90°	cis–trans; cisplatin = cis
Linear	[Ag(NH₃)₂]⁺	180°	—

• Cis–trans isomerism here is a special case of E–Z isomerism.
• Optical isomers of [M(en)₃]ⁿ⁺ are non-superimposable mirror images. Draw both with wedges, as mirror images.

Must-know distinction: co-ordination number counts bonds, not ligands. [Fe(C₂O₄)₃]³⁻ has three ligands but co-ordination number 6.

3.2.5.4 Colour

• Some visible wavelengths are absorbed; the rest are transmitted or reflected and give the colour you see.
• Absorbed light moves d electrons from the ground state to an excited state.
• ΔE = hν = hc/λ. Changes in oxidation state, co-ordination number or ligand change ΔE and so change the colour.
• Colorimetry: filter for the colour absorbed → zero with a blank → calibration graph from standards (absorbance vs concentration) → read the unknown.

Quantity	Unit to use
λ	m (convert nm × 10⁻⁹)
ν	s⁻¹ (Hz)
ΔE per ion	J
ΔE per mole	J mol⁻¹, then ÷ 1000 for kJ mol⁻¹

3.2.5.5 Variable oxidation states

Zinc in acid reduces vanadate(V): VO₂⁺ (+5, yellow) → VO²⁺ (+4, blue) → V³⁺ (+3, green) → V²⁺ (+2, violet).

• The redox potential for a higher → lower oxidation state change depends on pH and on the ligand.
• Tollens’ reagent [Ag(NH₃)₂]⁺ is reduced to silver by aldehydes, not ketones.

Titration ratios

Reaction	Mole ratio MnO₄⁻ : other
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺	1 : 5
2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂	2 : 5
2MnO₄⁻ + 6H⁺ + 5H₂O₂ → 2Mn²⁺ + 8H₂O + 5O₂	2 : 5

Method in steps: any MnO₄⁻ titration

1. n(MnO₄⁻) = c × V/1000.
2. Multiply by the mole ratio to get the other species in the titrated portion.
3. Scale up to the whole solution (for example × 250/25.0).
4. Convert: mass = n × M, or Mr = mass ÷ n, or % = mass found ÷ sample mass × 100.

Self-indicating: end point is the first permanent pale pink. Use dilute sulfuric acid.

3.2.5.6 Catalysts

	Heterogeneous	Homogeneous
Phase	different from reactants	same as reactants
How it works	active sites on the surface	through an intermediate species
Examples	V₂O₅ (Contact process), Fe (Haber process)	Fe²⁺ in S₂O₈²⁻ + I⁻; Mn²⁺ (autocatalyst) in C₂O₄²⁻ + MnO₄⁻

• Support medium: maximises surface area and minimises the cost of catalyst.
• Poisoning: impurities block active sites, so efficiency falls; this costs money.
• Variable oxidation states let the catalyst accept and donate electrons, then be regenerated.

Contact:   SO₂ + V₂O₅ → SO₃ + V₂O₄;   V₂O₄ + ½O₂ → V₂O₅
Fe²⁺:      S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺;   2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
Mn²⁺:      4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O;   2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂

Small worked reminder: why Fe²⁺ can catalyse S₂O₈²⁻ + I⁻. Iron must be able to do both steps. Fe²⁺ is oxidised by S₂O₈²⁻ (step 1), and the Fe³⁺ formed oxidises I⁻ (step 2). Both steps are feasible because the Fe³⁺/Fe²⁺ electrode potential (+0.77 V) lies between those of I₂/I⁻ (+0.54 V) and S₂O₈²⁻/SO₄²⁻ (+2.01 V). Use the values a question gives you.

Must-know distinctions

• Monodentate vs bidentate vs multidentate: one, two or several co-ordinate bonds per ligand. Count bonds for co-ordination number.
• Complete vs incomplete substitution: [Co(NH₃)₆]²⁺ has all six water ligands replaced; [Cu(NH₃)₄(H₂O)₂]²⁺ has four of six replaced.
• Cis vs trans: cis means the two identical ligands are at 90°; trans means 180° (octahedral) or opposite corners (square planar).
• Absorbed vs seen: the colour seen is the light transmitted or reflected, not the light absorbed.
• Heterogeneous vs homogeneous: surface active sites vs an intermediate species in the same phase.
• Catalyst vs autocatalyst: an autocatalyst is a product of the reaction it speeds up (Mn²⁺).
• Fe²⁺ vs C₂O₄²⁻ titrations: 1 : 5 for iron(II); 2 : 5 for ethanedioate, and the ethanedioate mixture is warmed.

Quick self-test

1. State the co-ordination number and oxidation state of iron in [Fe(C₂O₄)₃]³⁻.
2. Why can Cl⁻ change the co-ordination number of [Cu(H₂O)₆]²⁺ but NH₃ cannot?
3. State the shape and bond angle of [CuCl₄]²⁻.
4. How many particles are on each side of [Co(H₂O)₆]²⁺ + EDTA⁴⁻ → [Co(EDTA)]²⁻ + 6H₂O?
5. A ligand swap has ΔH = +2.0 kJ mol⁻¹ and ΔS = +160 J K⁻¹ mol⁻¹. Find ΔG at 298 K.
6. A complex absorbs at 450 nm. Find ΔE per ion (h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹).
7. Give the formula and colour of vanadium in the +4 state.
8. What amount of Fe²⁺ reacts with 20.0 cm³ of 0.0100 mol dm⁻³ MnO₄⁻? What mass of iron is that (Ar Fe = 55.8)?
9. What amount of H₂O₂ reacts with 15.0 cm³ of 0.0200 mol dm⁻³ MnO₄⁻?
10. Why is the uncatalysed reaction between S₂O₈²⁻ and I⁻ slow?
11. Name the isomer of [Pt(NH₃)₂Cl₂] that is cisplatin, and give the Cl–Pt–Cl angle.

Answers

1. 6 (three bidentate ligands); Fe + 3(−2) = −3, so +3.
2. Cl⁻ is larger, so only four fit; NH₃ and H₂O are similar in size and uncharged.
3. Tetrahedral, 109.5°.
4. 2 on the left, 7 on the right, so ΔS is positive.
5. ΔG = 2.0 − 298 × 0.160 = −45.7 kJ mol⁻¹. Feasible even though ΔH is positive.
6. ΔE = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(450 × 10⁻⁹) = 4.42 × 10⁻¹⁹ J.
7. VO²⁺, blue.
8. 2.00 × 10⁻⁴ × 5 = 1.00 × 10⁻³ mol; mass = 0.0558 g.
9. 3.00 × 10⁻⁴ × 5/2 = 7.50 × 10⁻⁴ mol.
10. Both reactants are negative ions, which repel, so the activation energy is high.
11. The cis isomer; 90°.

Where marks are usually lost

• Writing “a ligand donates electrons” without “a pair of electrons” and “co-ordinate bond”.
• Stating the chelate effect as “stronger bonds” with no mention of the increase in the number of particles.
• Forgetting H⁺ in manganate(VII) equations, so charges do not balance.
• Not scaling up from the titrated portion to the whole volumetric flask.
• Using nm instead of m in hc/λ, giving an answer 10⁹ times too small.
• Describing colour as the light absorbed rather than the light transmitted or reflected.
• Writing one Contact process equation only; the oxidation of V₂O₄ back to V₂O₅ shows regeneration.
• Leaving out the oxidation states when asked how variable oxidation states help catalysis.

Official syllabus

AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards (AQA). Section 3.2.5 Transition metals (A-level only).