Transition Elements — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

Section 1 of 25Definition of transition elements

1Transition element

Transition element a d-block element that can form at least one stable ion with an incomplete d subshell

• Scandium and zinc are d-block elements but do not fit this definition.
• Scandium forms only Sc³⁺ ([Ar] 3d⁰) and zinc only Zn²⁺ ([Ar] 3d¹⁰).

2First transition series

• First transition series titanium to copper

Section 2 of 25Electron configurations

Element	Configuration
Ti	[Ar] 3d2 4s2
V	[Ar] 3d3 4s2
Cr	[Ar] 3d5 4s1
Mn	[Ar] 3d5 4s2
Fe	[Ar] 3d6 4s2
Co	[Ar] 3d7 4s2
Ni	[Ar] 3d8 4s2
Cu	[Ar] 3d10 4s1

Filling order

• Filling order the 4s overlaps with 3d and fills first
• [Ar] represents 1s to 3p, the argon core.

Exceptions

• Cr and Cu promote a 4s electron to 3d to give a half-full or full d subshell, which is more stable

Ions

• 4s electrons are lost first
• Fe
  • [Ar] 3d⁶ 4s²;
    • Fe²⁺: [Ar] 3d⁶;
    • Fe³⁺: [Ar] 3d⁵.

Section 3 of 25General properties

• Distinctive properties of transition metals
  • Variable oxidation states.
  • They form complex ions.
  • They form coloured compounds.
  • They behave as catalysts.

Section 4 of 25Variable oxidation states

• Variable oxidation state
  • unlike other metals, transition elements form more than one positive ion
  • Roman numerals show the oxidation state of the metal ion.
  • Sodium forms only Na⁺, so it needs no numeral.
  • Iron forms Fe²⁺, Fe(II), and Fe³⁺, Fe(III).

Section 5 of 25Complex ions and colour

Complex ion

Complex ion a central metal atom or ion surrounded by molecules or ions called ligands

• Different oxidation states allow different numbers and kinds of ligands.
• Chromium(III) forms [Cr(NH₃)₆]³⁺, [Cr(OH)₆]³⁻ and [Cr(H₂O)₆]³⁺.

Coloured compounds

• Coloured compounds the colour depends on the ligand even at the same oxidation state
• [Cr(OH)₆]³⁻ is dark green.
• [Cr(NH₃)₆]³⁺ is purple.

Section 6 of 25Transition elements as catalysts

Why good catalysts

• Why good catalysts variable oxidation states let the element gain or donate electrons during catalysis
• Substances can also adsorb onto their surface and be activated.

Heterogeneous catalyst

• Heterogeneous catalyst in a different phase from the reactants, with the reaction at active sites on the surface
• Iron in the Haber processN₂(g) + 3H₂(g) ⇌ 2NH₃(g).

Homogeneous catalyst

• Homogeneous catalyst in the same phase as the reactants
• The decomposition of hydrogen peroxide, 2H₂O₂ → 2H₂O + O₂, is a common kinetics example using manganese(IV) oxide as catalyst.

Section 7 of 25Ligands and coordination number

Ligand

Ligand a molecule or ion that forms a coordinate (dative) bond to a transition metal by donating a pair of electrons

• It is a Lewis base (electron pair donor), with a lone pair or negative charge, like a nucleophile.

Coordination number

Coordination number the number of coordinate bonds to the central metal atom or ion

Monodentate ligands

• Monodentate ligands form one dative bond, e.g. H₂O, NH₃, Cl⁻, CN⁻, OH⁻

Bidentate ligands

• Bidentate ligands form two dative bonds because each contains two atoms with lone pairs
• 1,2-diaminoethane, H₂NCH₂CH₂NH₂ (en).
• The ethanedioate ion, C₂O₄²⁻ (ox).

Multidentate

• Multidentate some ligands form several dative bonds

Section 8 of 25Shapes of complex ions

Coordination number	Shape	Bond angle	Details
6	Octahedral	90°	Six small monodentate ligands (water, ammonia, hydroxide, thiocyanate) or three bidentate ligands (en, ethanedioate)
4	Tetrahedral	109.5°	Often four large chloride ligands, which is all that will fit
4	Square planar	90°	Sometimes instead of tetrahedral, e.g. cisplatin

Section 9 of 25Cis–trans isomerism in complexes

Occurrence

• Occurrence complexes have no double bond but can still show geometrical isomerism
• Square planar and octahedral complexes with two pairs of different ligands (or two ligands different from the rest) show cis–trans isomerism.

Cis and trans

• Cisthe two different ligands are next to each other.
• Transthe two different ligands are opposite each other.

Examples

• Cisplatin, a square planar anti-cancer drug, binds to DNA in cancer cells, whereas trans-platin cannot be used in treatment.
• [Co(NH₃)₄(H₂O)₂]²⁺ has the two water ligands adjacent in cis and opposite in trans.
• [Ni(en)₂(H₂O)₂]²⁺ also shows cis–trans isomerism.

Section 10 of 25Optical isomerism in complexes

Occurrence

• Occurrence octahedral complexes with bidentate ligands have optical isomers
• The two forms are non-superimposable mirror images, with no plane of symmetry.
• They differ only in rotating the plane of polarised light in opposite directions.

Examples

• [Ni(en)₃]²⁺ and the cis form of [Ni(en)₂(H₂O)₂]²⁺

Section 11 of 25Cisplatin as an anti-cancer drug

Discovery and use

• Discovery and use cisplatin was discovered in the 1960s and treats cancers such as testicular, ovarian, cervical, breast, lung and brain cancer

Action

• Action it binds to nitrogen atoms on the bases in DNA
• It passes through the cell membrane and undergoes ligand exchange, with chlorines replaced by water molecules.
• Nitrogen is a better ligand than water and forms dative covalent bonds to the platinum.
• This distorts the shape of DNA and prevents it replicating.

Selectivity

• Selectivity cancer cells grow and replicate faster than normal cells, so they are affected more

Adverse effects

• Adverse effects it also binds to healthy cells, especially fast-replicating ones such as hair follicles, causing hair loss
• Society must balance adverse effects against therapeutic value.
• New therapies aim to target cancer cells while leaving healthy cells untouched.

Section 12 of 25Ligand substitution

Ligand substitution

Ligand substitution (exchange)

Ligand substitution (exchange) replacement of one ligand in a complex by another

• It forms a new, more stable complex.
• Ligands can be partly or entirely substituted.

Effect of ligand size

• Water and ammonia ligands are similar in size, so coordination number and geometry do not change.
• Chloride ligands are larger than water, so coordination number and geometry change.

Aqueous ions

• Aqueous ions a transition metal ion in solution exists as a hexaaqua complex
• Cu²⁺(aq) is the blue [Cu(H₂O)₆]²⁺(aq).

Section 13 of 25Copper(II) with sodium hydroxide and ammonia

Reagent	Observation	Change
NaOH(aq), dropwise	Pale blue solution to pale blue precipitate	[Cu(H2O)6]2+ + 2OH− → Cu(H2O)4(OH)2(s) + 2H2O (partial substitution)
NaOH, excess	No change	Precipitate remains
NH3(aq), dropwise	Pale blue precipitate	Same hydroxide precipitate because aqueous ammonia contains OH−
NH3, excess concentrated	Precipitate dissolves to a deep blue solution	Cu(H2O)4(OH)2 + 4NH3 → [Cu(NH3)4(H2O)2]2+ + 2H2O + 2OH−

• Required knowledge
  • for copper(II) only the excess ammonia substitution and colour change need to be known

Section 14 of 25Copper(II) with concentrated hydrochloric acid

Reaction

• Reaction full substitution: [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O
• The colour changes from blue to yellow.

Changes

• Coordination number changes from 6 to 4, because the larger chloride ligands mean only four fit around the metal ion.
• Geometry changes from octahedral to tetrahedral.
• The colour changes from blue to yellow.

Reversibility

• Reversibility some blue [Cu(H₂O)₆]²⁺ remains, so the mixture can appear green
• Adding water displaces the chloride ligands and the pale blue solution returns.

Caution

• Caution solid copper chloride dissolved in water forms [Cu(H₂O)₆]²⁺, not [CuCl₄]²⁻

Section 15 of 25Chromium(III) substitution

• Starting complex
  • [Cr(H₂O)₆]³⁺ is pale purple, though often appears green because of impurities

Reagent	Observation	Equation
NaOH, dropwise	Dark green precipitate	[Cr(H2O)6]3+ + 3OH− → Cr(H2O)3(OH)3(s) + 3H2O
NaOH, excess	Dissolves to a dark green solution	Cr(H2O)3(OH)3 + 3OH− → [Cr(OH)6]3− + 3H2O
NH3, dropwise	Dark green precipitate (same hydroxide)	As for dropwise NaOH
NH3, excess concentrated	Dissolves to a purple solution	Cr(H2O)3(OH)3 + 6NH3 → [Cr(NH3)6]3+ + 3H2O + 3OH−

Section 16 of 25Precipitates with sodium hydroxide

Ion	Aqua ion	With OH−	Excess OH−
Cu2+	Pale blue solution	Pale blue precipitate, Cu(H2O)4(OH)2	No change
Fe2+	Pale green solution	Dark green precipitate, Fe(H2O)4(OH)2	No change
Mn2+	Pale pink solution	Pale brown precipitate, Mn(H2O)4(OH)2	No change
Cr3+	Pale purple solution	Dark green precipitate, Cr(H2O)3(OH)3	Dark green solution, [Cr(OH)6]3−
Fe3+	Yellow solution	Orange-brown precipitate, Fe(H2O)3(OH)3	No change

Equations

• [Fe(H₂O)₆]²⁺ + 2OH⁻ → [Fe(H₂O)₄(OH)₂](s) + 2H₂O.
• [Fe(H₂O)₆]³⁺ + 3OH⁻ → [Fe(H₂O)₃(OH)₃](s) + 3H₂O.

Remembering formulae

• Remembering formulae the number of OH⁻ in the precipitate equals the charge on the metal ion

Section 17 of 25Precipitates with ammonia

Ion	With NH3	Excess NH3
Cu2+	Pale blue precipitate	Dark blue solution, [Cu(NH3)4(H2O)2]2+
Fe2+	Dark green precipitate	No change
Mn2+	Pale brown precipitate	No change
Cr3+	Dark green precipitate	Purple solution, [Cr(NH3)6]3+
Fe3+	Orange-brown precipitate	No change

Role of ammonia

• Role of ammonia aqua ions act as acids with ammonia
• Initially ammonia is a base, removing one H⁺ per ammonia molecule used: [Cu(H₂O)₆]²⁺ + 2NH₃ → [Cu(H₂O)₄(OH)₂](s) + 2NH₄⁺.
• With excess ammonia some ions undergo ligand substitution: [Cu(H₂O)₄(OH)₂] + 4NH₃ → [Cu(H₂O)₂(NH₃)₄]²⁺ + 2H₂O + 2OH⁻.

Colours

• Colours follow the published OCR colours of inorganic ions and complexes

Section 18 of 25Haemoglobin and oxygen transport

Haemoglobin and oxygen transport

Haem

• Haem a complex with iron(II) at its centre
• Oxygen forms a dative covalent bond to Fe(II), allowing oxygen molecules to be carried in the blood.
• Oxygen is a weak ligand, so it bonds weakly and breaks off easily to be released into cells.

Carbon monoxide poisoning

• CO is a better ligand than oxygen
• It binds strongly and irreversibly to Fe(II), stopping oxygen reaching cells.
• Replacing oxyhaemoglobin by carboxyhaemoglobin gives a darker red colour, a sign of poisoning.

Anaemia

• Anaemia too little haemoglobin because of blood loss or iron deficiency
• Iron deficiency can be treated with iron sulfate tablets in the diet.

Section 19 of 25Iron(II) and iron(III) redox

Iron(II) and iron(III) redox

Self-indicating

• Self-indicating transition metal ions change colour as oxidation state changes, so redox titrations often need no indicator

Oxidation of Fe²⁺ to Fe³⁺

• Oxidation of Fe²⁺ to Fe³⁺ by acidified manganate(VII)
• MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O.
• Deep purple MnO₄⁻ becomes almost colourless Mn²⁺.

Reduction of Fe³⁺ to Fe²⁺

• Reduction of Fe³⁺ to Fe²⁺ by reducing agents such as potassium iodide
• 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.
• Orange-brown Fe³⁺ becomes pale green Fe²⁺, but brown iodine can mask the colour change.

Section 20 of 25Chromium redox

1Reduction of dichromate

• Reduction of dichromate orange Cr₂O₇²⁻ is reduced to green Cr³⁺ by zinc in acid
• Cr₂O₇²⁻ + 14H⁺ + 3Zn → 2Cr³⁺ + 7H₂O + 3Zn²⁺.
• With excess zinc, Cr³⁺ is reduced further to pale blue Cr²⁺: Zn + 2Cr³⁺ → Zn²⁺ + 2Cr²⁺.
• Fe²⁺ is a weaker reducing agent than zinc and reduces dichromate only to Cr³⁺.
• E^⊗ values compare reducing agent strength and explain which reactions occur.

2Oxidation of Cr³⁺ to chromate(VI)

• Oxidation of Cr³⁺ to chromate(VI) low oxidation states are more easily oxidised in alkaline solution
• Hot alkaline hydrogen peroxide gives 3H₂O₂ + 2Cr³⁺ + 10OH⁻ → 2CrO₄²⁻ + 8H₂O.
• Dark green Cr³⁺ becomes yellow CrO₄²⁻.
• Chromium goes +3 to +6 and oxygen goes −1 to −2.

3Chromate to dichromate

• Chromate to dichromate dilute sulfuric acid: 2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O
• Yellow chromate becomes orange dichromate.

4Skills

• Skills full equations need not be memorised, but you should construct and interpret redox equations from half equations and oxidation numbers

Section 21 of 25Copper redox and disproportionation

1Reduction of Cu²⁺ to Cu⁺

• Reduction of Cu²⁺ to Cu⁺ with excess iodide ions
• 2Cu²⁺ + 4I⁻ → 2CuI(s) + I₂.
• Pale blue Cu²⁺ gives a white precipitate of CuI and brown iodine; iodide is oxidised and Cu²⁺ reduced.

2Disproportionation of Cu⁺

• Disproportionation of Cu⁺ copper(I) oxide with hot dilute sulfuric acid gives a brown precipitate of copper and a blue solution of copper(II) sulfate
• Cu₂O + H₂SO₄ → Cu + CuSO₄ + H₂O.
• Copper is reduced from +1 to 0 and oxidised from +1 to +2 at the same time, which is disproportionation.

Section 22 of 25Qualitative analysis of anions

1Procedure

• Procedure when testing one solution for several anions, carry out the tests in the same fixed order each time

2Silver halides and ammonia

• Silver chloride dissolves in dilute ammonia.
• Silver bromide dissolves in concentrated ammonia.
• Silver iodide is insoluble in concentrated ammonia.

Section 23 of 25Qualitative analysis of cations

Transition metal ions

• Transition metal ions adding aqueous sodium hydroxide gives characteristic precipitates, used to identify an unknown metal ion
• Cu²⁺ pale blue, Fe²⁺ dark green, Mn²⁺ pale brown, Cr³⁺ dark green (dissolving in excess) and Fe³⁺ orange-brown.

Ammonium ions

• Ammonium ions warm with hydroxide ions: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
• The ammonia turns damp red litmus paper blue.

Section 24 of 25Key terms

Term	Meaning
Transition element	A d-block element that forms at least one stable ion with an incomplete d subshell
Ligand	A molecule or ion that donates a lone pair to a metal ion to form a coordinate bond
Coordination number	The number of coordinate bonds to the central metal ion
Complex ion	A central metal ion surrounded by ligands
Bidentate ligand	A ligand forming two coordinate bonds
Cisplatin	A square planar platinum complex used as an anti-cancer drug

Term	Meaning
Ligand substitution	Replacement of one ligand in a complex by another
Hexaaqua complex	A complex with six water ligands, such as [Cu(H2O)6]2+
Amphoteric hydroxide	A hydroxide that dissolves in both acid and excess alkali, such as Cr(OH)3
Cisplatin	A square planar platinum complex that binds to DNA and is used against cancer
Partial substitution	Replacement of only some of the ligands

Term	Meaning
Haemoglobin	An iron(II) complex protein that carries oxygen in the blood
Disproportionation	A reaction in which the same element is both oxidised and reduced
Carboxyhaemoglobin	Haemoglobin with carbon monoxide bound to the iron(II)
Self-indicating	A reagent whose own colour change shows the end point
Anaemia	A condition caused by too little haemoglobin

Section 25 of 25Exam technique

1

Calling every d-block element a transition element — Sc and Zn do not form ions with an incomplete d subshell

2

Removing 3d electrons before 4s when forming ions — 4s electrons go first

3

Saying a ligand accepts electron pairs — it donates a lone pair

4

Counting a bidentate ligand as one coordinate bond — it forms two

1

Writing [CuCl₄]²⁻ for copper chloride dissolved in water — it forms [Cu(H₂O)₆]²⁺

2

Saying the copper hydroxide precipitate dissolves in excess sodium hydroxide — it does not

3

Forgetting that aqueous ammonia first gives the hydroxide precipitate

4

Saying the coordination number stays 6 when chloride replaces water — it becomes 4 and the shape tetrahedral

1

Saying carbon monoxide binds weakly to haemoglobin — it binds strongly and irreversibly

2

Saying iron(II) reduces dichromate to Cr²⁺ — only zinc is strong enough

3

Describing disproportionation without stating both oxidation and reduction of the same element

4

Testing for halides before removing carbonate and sulfate