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]

2.5Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Group 7 — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 11 parts Lesson 5 of 53 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.

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Lesson summary

2.5 Group 7 — trends, displacement, redox reactions and halide tests — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Explain physical and chemical trends down Group 7, describe displacement and disproportionation reactions, and use halide reactions with sulfuric acid and silver nitrate.T…

Cold dilute alkali (about 15 °C)
Hot alkali (about 70 °C)
Water treatment
Halide ion
Displacement reaction
Volatility

Part 1 of 9Physical trends of the halogens

The halogens F, Cl, Br, I (and At) have the outer configuration ns²np⁵ and exist as diatomic molecules X₂.

Uses

Uses chlorine for water purification and bleaching, bromine for flame retardants and fire extinguishers, iodine as an antiseptic and disinfectant.

Colour darkens down the group.

Chlorine is a pale green gas, bromine a red-brown liquid and iodine a grey-black solid with purple vapour.

Melting and boiling points increase and volatility decreases down the group.

Halogens are simple molecules with weak London forces between X₂ molecules.
More electrons give larger instantaneous dipoles and stronger London forces.
More energy is needed to separate the molecules.

Fluorine is the most volatile and iodine the least.

Part 2 of 9Electronegativity and oxidising power

Electronegativity decreases down the group.

Atomic radius increases, so the incoming electron is further from the nucleus and more shielded.

A halogen atom gains an electron (X + e⁻ → X⁻), so it is reduced and acts as an oxidising agent.

Oxidising power decreases down the group: F₂ > Cl₂ > Br₂ > I₂.

Reactivity with hydrogen decreases down the group; the reaction becomes less vigorous.

Part 3 of 9Halogen displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its halide.

Chlorine oxidises bromide and iodide; bromine oxidises iodide only; iodine oxidises neither.

Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq).
Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq).

Br⁻ and I⁻ go from −1 to 0 (oxidised); Cl₂ and Br₂ go from 0 to −1 (reduced); the metal ion is a spectator.

Observations

Bromine forms a yellow-orange solution; iodine forms a brown solution.
With hexane or cyclohexane, bromine is yellow-orange and iodine is purple in the organic layer.
The organic solvent makes the colours easier to see because the halogens are more soluble in it.

Part 4 of 9Redox reactions with metals and iron(II)

Redox reactions with metals and iron(II)

Halogens oxidise metals to ionic halides: 2Na + Cl₂ → 2NaCl (0 to +1) and Ca + Br₂ → CaBr₂ (0 to +2).

Chlorine and bromine oxidise iron(II) to iron(III).

Cl₂ + 2Fe²⁺ → 2Cl⁻ + 2Fe³⁺.
Br₂ + 2Fe²⁺ → 2Br⁻ + 2Fe³⁺.

Iodine is too weak an oxidising agent to oxidise Fe²⁺; instead Fe³⁺ oxidises iodide: 2I⁻ + 2Fe³⁺ → I₂ + 2Fe²⁺.

Part 5 of 9Disproportionation of chlorine

1Disproportionation

Disproportionation the same species is both oxidised and reduced.

2Cold dilute alkali (about 15 °C)

Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O.
1Chlorine goes 02+1 (oxidised) and 03−1 (reduced)
NaClO is the active ion in bleach.

3Hot alkali (about 70 °C)

3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O.
1Chlorine goes 02+5 and 03−1

4Water treatment

Cl₂ + H₂O ⇌ HCl + HClO.
Chloric(I) acid kills bacteria.
HClO ⇌ H⁺ + ClO⁻, and ClO⁻ also sterilises the water.

Part 6 of 9Halides with concentrated sulfuric acid

Reactions give toxic gases and must be done in a fume cupboard.

General step: H₂SO₄(l) + X⁻ → HX(g) + HSO₄⁻.

3Chloride

NaCl + H₂SO₄ → HCl + NaHSO₄; misty fumes of HCl and no further reaction.

4Bromide

HBr is formed, then oxidised by H₂SO₄.
2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O.
Observe misty fumes, reddish-brown Br₂ and choking SO₂.

5Iodide

HI is oxidised more extensively.
2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O.
6HI + H₂SO₄ → 3I₂ + S + 4H₂O (yellow solid sulfur).
8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O (bad-egg smell).
Observe purple iodine vapour.

Reducing power of the halide ions increases down the group: Cl⁻ < Br⁻ < I⁻.

The larger ion holds its outer electrons less strongly, so it loses them more easily.

Halide with concentrated H₃PO₄ gives the hydrogen halide without oxidation.

Part 7 of 9Silver nitrate and ammonia test for halides

Silver nitrate and ammonia test for halides

Acidify with dilute nitric acid, then add AgNO₃(aq): Ag⁺(aq) + X⁻(aq) → AgX(s).

Nitric acid is added first to remove carbonate ions.

Halide ion	Precipitate colour	Dilute NH₃	Concentrated NH₃
Cl⁻	White	Dissolves	Dissolves
Br⁻	Cream	Insoluble	Dissolves
I⁻	Pale yellow	Insoluble	Insoluble

AgF is soluble, so silver nitrate cannot test for fluoride.

Part 8 of 9Hydrogen halides, fluorine and astatine

Hydrogen halides, fluorine and astatine

Halogens react with hydrogen: Cl₂ + H₂ → 2HCl.

Hydrogen halides are acidic gases.

They give steamy fumes in moist air.
They dissolve in water to form acids: HCl → H⁺ + Cl⁻.
They give white smoke with ammoniaNH₃ + HCl → NH₄Cl.

Predict reactions of fluorine and astatine from the group trends, allowing for exceptions; HAt + H₂O → H₃O⁺ + At⁻.

Part 9 of 9More from the specification

Element

A substance made of only one type of atom.

Ionic equation

An equation showing only the ions that react.

Sulfur dioxide

An acidic gas made when sulfur-containing fuels burn.

Key terms

Term	Meaning
Halogen	A Group 7 element, existing as diatomic molecules X2.
Halide ion	The X- ion formed when a halogen atom gains one electron.
Displacement reaction	A reaction in which a more reactive halogen oxidises the halide of a less reactive halogen.
Disproportionation	A reaction in which the same element is simultaneously oxidised and reduced.
Volatility	How easily a substance evaporates; low boiling point means high volatility.
Chlorate(I) ion	ClO-, the active ion in bleach.
Reducing agent	A species that donates electrons and is itself oxidised.

Avoid these mistakes

Saying reactivity increases down Group 7 — it decreases because the incoming electron is further and more shielded.

Using the wrong direction for halide reducing power — it increases down the group.

Forgetting that disproportionation needs one species both oxidised and reduced — chlorine goes to −1 and +1 or +5.

Not adding nitric acid before silver nitrate — carbonates would give a false precipitate.

Saying the halogens have hydrogen bonds — only London forces act between X2 molecules.

Last updated: 18 October 2026