كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

5.2Unit 5. Transition Metals and Organic Nitrogen Chemistry

Transition Metals & Complexes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 12 parts Lesson 2 of 43 1 quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

0%read

Lesson summary

5.2 Transition Metals & Complexes — configurations, ligands, colour, shapes, cisplatin and haemoglobin — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Define transition metals and write their configurations, describe ligands and complex ions, explain colour, ligand exchange, the chelate …

Exceptions
Coordination number
Type of ligand
Oxidation state
Ligand exchange (substitution)
Chelate effect

Part 1 of 10Transition metals and their configurations

Transition metal

Transition metal a d-block element that forms at least one stable ion with an incomplete d subshell.

Scandium and zinc are d-block but not transition metals.

Sc only forms Sc³⁺, [Ar] 3d⁰.
Zn only forms Zn²⁺, [Ar] 3d¹⁰.

The first transition series is therefore titanium to copper.

4s fills before 3d, but 4s electrons are lost first when ions form.

Repulsion in occupied orbitals raises the 4s above the 3d, making 4s the outer subshell.
Fe [Ar] 3d⁶4s² → Fe²⁺ [Ar] 3d⁶.
Mn(III) is 1s²2s²2p⁶3s²3p⁶3d⁴.

Exceptions

Cr is [Ar] 3d⁵4s¹ and Cu is [Ar] 3d¹⁰4s¹, because half-full and full 3d are more stable.

Part 2 of 10General properties and oxidation states

Transition metals have properties unlike other metals.

Variable oxidation states, because 3d and 4s energies are close.

Roman numerals show the oxidation state, e.g. Fe(II) and Fe(III).
Loss of the 4s electrons makes +2 a common state.

They form complex ions.

They form coloured compounds.

They behave as catalysts.

Part 3 of 10Complex ions, ligands and coordination number

Complex

Complex a central metal ion surrounded by ligands joined by dative covalent bonds.

Ligand

Ligand a molecule or ion with a lone pair that it donates to the metal ion.
- (a Lewis base, like a nucleophile)
Monodentate ligands form one dative bond: H₂O, NH₃, Cl⁻, OH⁻, CN⁻.
Bidentate ligands form two: 1,2-diaminoethane (en) and ethanedioate (ox).
Multidentate ligands form more, e.g. EDTA⁴⁻, which is hexadentate.

Coordination number

Coordination number the number of dative bonds from ligands to the central metal ion.

The charge on a complex is the sum of the metal charge and the ligand charges; square brackets group the metal ion and ligands.

Naming

Cationprefix for number of ligands / ligand / metal / oxidation number, e.g. hexaaquairon(II), hexaamminecobalt(II).
Anionthe metal ends in -ate and may use a Latin stem, e.g. tetrachlorocuprate(II), tetracyanonickelate(II).
Ammonia as a ligand is ammine, with a double m.

Part 4 of 10Origin of colour

A coloured compound absorbs part of the visible spectrum; the colour seen is the complementary colour of the colours absorbed.

In an isolated ion the five 3d orbitals have the same energy.

When ligands bond, the d orbitals split into two sets of different energy.

An electron in the lower level absorbs a photon from the visible region and is promoted to the higher level.
ΔE = hν, so a larger gap absorbs higher-frequency light; h = 6.63 × 10⁻³⁴ J s.

Ions with d⁰ or d¹⁰ configurations cannot have d–d transitions and are colourless: Sc³⁺, Zn²⁺, Cu⁺.

Part 5 of 10Factors changing the colour

Factors changing the colour

The splitting energy ΔE depends on:

Type of ligand

Type of ligand a ligand with greater charge density causes greater splitting.
[Cu(H₂O)₆]²⁺ is light blue and [Cu(NH₃)₄(H₂O)₂]²⁺ is dark blue.

Identity of the metal ion and its nuclear charge.

Oxidation state

Oxidation state a higher oxidation number gives a stronger interaction with the ligands.
[Fe(H₂O)₆]²⁺ absorbs red and appears green;
- [Fe(H₂O)₆]³⁺ absorbs blue and appears orange (yellow-brown due to hydrolysis).

Coordination number and shape

[Cu(H₂O)₆]²⁺ is light blue and [CuCl₄]²⁻ is yellow.

Ion	d electrons	Colour
[Cu(H₂O)₆]2+	d9	Blue
[Fe(H₂O)₆]2+	d6	Pale green
[Fe(H₂O)₆]3+	d5	Pale violet
[Cr(H₂O)₆]3+	d3	Violet
[Zn(H₂O)₆]2+	d10	Colourless

Part 6 of 10Ligand exchange

Ligand exchange

Ligand exchange (substitution)

Ligand exchange (substitution) one ligand in a complex is replaced by another, forming a more stable complex.

Ligands of similar size (H₂O, NH₃) exchange with no change in coordination number or shape.

[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O gives a deep blue solution.

Cl⁻ is larger, so exchange changes the coordination number from 6 to 4.

[Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O: blue to green (a mixture) to yellow.
[Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O: pink to blue.
The copper oxidation state is unchanged although the charge goes from +2 to −2.

Dissolving solid copper chloride in water gives [Cu(H₂O)₆]²⁺, not [CuCl₄]²⁻.

Oxidation of the metal ion can also change a complex: pale green [Fe(H₂O)₆]²⁺ slowly oxidises in air to yellow [Fe(H₂O)₆]³⁺.

Part 7 of 10The chelate effect

Chelate effect

Chelate effect replacing monodentate ligands with bidentate or multidentate ligands gives a more stable complex.

ΔG is negative and the driving force is entropy.

ΔG = ΔH − TΔS_system.

Reactions between aqueous ions have small enthalpy changes because similar bonds are broken and made.
The number of particles increases, so ΔS_system is positive.

[Co(H₂O)₆]²⁺ + EDTA⁴⁻ → [CoEDTA]²⁻ + 6H₂O turns two reactant species into seven product species.

[Cu(H₂O)₆]²⁺ + 3en → [Cu(en)₃]²⁺ + 6H₂O turns four particles into seven.

Part 8 of 10Shapes and isomerism of complexes

Coordination number	Shape	Bond angle	Example
6	Octahedral	90°	[Fe(H₂O)₆]2+
4	Tetrahedral	109.5°	[CuCl₄]2-
4	Square planar	90°	cis-[Pt(NH₃)₂Cl₂]
2	Linear	180°	[Ag(NH₃)₂]+

Octahedral

Octahedral six ligands, which can be six monodentate, three bidentate or one hexadentate ligand.

Tetrahedral

Tetrahedral four large ligands, most commonly four chloride ions, which are too big for six to fit.

Predict shapes by counting the electron pairs donated by the ligands, ignoring the d electrons and the overall charge.

Cis–trans isomerism occurs in octahedral and square planar complexes with two pairs of different ligands.

In [Co(NH₃)₄(H₂O)₂]²⁺ the water ligands are adjacent in the cis isomer and opposite in the trans isomer.
[Ni(en)₂(H₂O)₂]²⁺ also shows it.

Optical isomerism occurs in octahedral complexes with bidentate ligands.

The isomers are non-superimposable mirror images with no plane of symmetry.
Examples are [Ni(en)₃]²⁺ and [Ni(en)₂(H₂O)₂]²⁺.

Part 9 of 10Square planar complexes and cisplatin

Square planar complexes and cisplatin

Four-coordinate complexes can be square planar, e.g. with CN⁻ ligands or d⁸ ions such as Pt²⁺; the bond angles are 90°.

Cisplatin, cis-[Pt(NH₃)₂Cl₂], is an anticancer drug used for testicular, ovarian, cervical, breast, lung and brain cancers.

It passes through the cell membrane and chlorine ligands are exchanged for water.
Nitrogen atoms on DNA bases are better ligands than water and bond to platinum.
The DNA is distorted and cannot replicate; the trans isomer is ineffective.

Adverse effects

It also binds to healthy cells, mainly those that replicate quickly, such as hair follicles, causing hair loss.
Society must balance adverse effects against therapeutic value.

Part 10 of 10Haemoglobin and ligand exchange

Haem is a complex with Fe²⁺ at its centre; haemoglobin has four haem groups.

Each Fe²⁺ is held by four nitrogen atoms of the porphyrin ring (a multidentate ligand) with a fifth dative bond from the protein globin.

The sixth site binds an O₂ molecule reversibly, so oxygen is carried round the body.
O₂ is a poor ligand and bonds weakly, so it breaks off easily at cells.

Carbon monoxide is toxic because it is a better ligand than O₂.

It binds more strongly to Fe²⁺ by forming stronger dative bonds (about 200 times more strongly), preventing oxygen transport.
Carboxyhaemoglobin is a darker red than oxyhaemoglobin.

Anaemia

Anaemia too little haemoglobin from blood loss or iron deficiency, which can be treated with iron sulfate tablets.

Key terms

Term	Meaning
Transition metal	A d-block element that forms at least one stable ion with an incomplete d subshell.
Ligand	A molecule or ion with a lone pair that forms a dative covalent bond to a central metal ion.
Coordination number	The number of dative bonds from ligands to the central metal ion.
Bidentate ligand	A ligand that forms two dative bonds, e.g. en or ethanedioate.
d-orbital splitting	The separation of the five d orbitals into sets of different energy by ligands.
Ligand exchange	A reaction in which one ligand in a complex is replaced by another.
Chelate effect	The extra stability of complexes with multidentate ligands, driven by increased entropy.
Complementary colour	The colour opposite the absorbed colour on the colour wheel; the colour observed.

Avoid these mistakes

Saying scandium and zinc are transition metals — their stable ions have empty or full d subshells.

Removing 3d electrons before 4s when forming ions — 4s are lost first.

Saying colour arises only from the metal — the ligands, oxidation state and shape all change ΔE.

Writing [CuCl₄]2- when copper chloride dissolves in water — the aqua complex forms.

Explaining the chelate effect by enthalpy — it is driven by the increase in entropy.

Last updated: 18 October 2026