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]

S2.1Structure 2 — Models of bonding and structure

The ionic model — IB Chemistry SL Revision Notes & Topical Questions

5 min read 13 parts Lesson 1 of 6 1 quiz

📚 IB Chemistry SL 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

S2.1 The ionic model — IB CHEMISTRY SL notes: Explain how ions form, deduce ionic formulae and names, and explain the structure and properties of ionic lattices.Electron transfer — ionic bonding involves the transfer of electrons from a metal to a non-metal, usually giving both a…

Noble-gas configuration
Transition metals
Variable charges
Non-metal cations
Definition
Neutrality

Part 1 of 11Forming ions

Forming ions

Electron transfer

Electron transfer ionic bonding involves the transfer of electrons from a metal to a non-metal, usually giving both a full outer shell.

Cations

Cations metals lose valence electrons to form positive ions.

Anions

Anions non-metals gain electrons to form negative ions.

Noble-gas configuration

Na⁺ has the configuration of neon [2,8] and Cl⁻ that of argon [2,8,8].

Transition metals

Transition metals form ions of different charges, such as Fe²⁺ and Fe³⁺, and Cu⁺ and Cu²⁺.

Bond type

Bond type ionic bonding forms when the electronegativity difference is large, typically metal plus non-metal.

Part 2 of 11Charges of simple ions

Group	Charge	Example
1	+1	Na⁺
2	+2	Mg²⁺
13	+3	Al³⁺
15	−3	N³⁻
16	−2	O²⁻
17	−1	Br⁻

Variable charges

Variable charges transition metal charges are shown by Roman numerals (Stock notation), e.g. copper(II) oxide contains Cu²⁺.

Non-metal cations

NH₄⁺ and H⁺ are positive ions of non-metals.

Part 3 of 11Polyatomic ions

Definition
- ions made of more than one type of atom.

Ion	Formula
Ammonium	NH₄⁺
Hydroxide	OH⁻
Nitrate	NO₃⁻
Hydrogencarbonate	HCO₃⁻
Carbonate	CO₃²⁻
Sulfate	SO₄²⁻
Phosphate	PO₄³⁻

Part 4 of 11Formulae and names of ionic compounds

Neutrality
- the total positive charge equals the total negative charge, so the compound has zero overall charge.

Identify the cation and the anion.

Deduce each charge from the group, the Roman numeral or the polyatomic ion list.

Balance the charges using the simplest ratio.

Write the cation first, with brackets around any polyatomic ion needed more than once, e.g. Ca(OH)₂ and (NH₄)₂SO₄.

Naming

Naming cation first, then the anion, with the suffix –ide for simple anions, e.g. lithium sulfide, calcium nitride and sodium hydride.

Formula unit

Formula unit an ionic formula is the empirical formula because ionic compounds form giant lattices, not molecules.

Part 5 of 11The ionic bond and lattice

Ionic bond

Ionic bond the electrostatic attraction between oppositely charged ions.

Lattice

Lattice a giant, regular, repeating three-dimensional arrangement of alternating cations and anions that is electrically neutral.

Non-directional bonding

Non-directional bonding each ion attracts all neighbouring ions of opposite charge in every direction.

Coordination number

Coordination number in NaCl each Na⁺ is surrounded octahedrally by 6 Cl⁻ and vice versa.

Part 6 of 11Lattice enthalpy

Definition

Definition the enthalpy change when one mole of a solid ionic compound is separated into its gaseous ions, e.g. NaCl(s) → Na⁺(g) + Cl⁻(g), ΔH = +790 kJ mol⁻¹.

Sign

Sign always endothermic and positive, because energy is needed to separate the ions.

Trend

Trend it increases with larger ionic charges and smaller ionic radii, which give greater charge density and stronger attraction.

Order

MgO > NaCl > KBr.

Cations

Cations are smaller than their parent atoms because the outer shell is lost and the same nuclear charge attracts fewer electrons.

Anions

Anions are larger than their parent atoms because extra electrons increase repulsion.

Part 7 of 11Physical properties of ionic compounds

Property	Behaviour	Explanation
Melting and boiling point	High	Many strong electrostatic attractions act in all directions; MgO melts at about 2850 °C and NaCl at about 801 °C
Volatility	Low	Strong ionic bonding; negligible vapour pressure at room temperature
Conductivity as solid	None	Ions are fixed in the lattice
Conductivity molten or aqueous	Conducts	Ions are free to move and carry charge; electrons do not move
Brittleness	Brittle	A shifted layer brings like charges together and repulsion splits the crystal
Solubility	Usually soluble in water, insoluble in non-polar solvents	Ion-dipole attractions with water release energy to separate ions

Part 8 of 11Dissolving an ionic solid in water

Water molecules approach the surface of the lattice.

The δ− oxygen atoms attract cations and the δ+ hydrogen atoms attract anions.

Ion-dipole attractions release energy that helps overcome the lattice.

Ions are pulled out and become surrounded (hydrated) by water.

The mobile hydrated ions allow the solution to conduct electricity.

Part 9 of 11Comparing structure types

Structure	Particles	Melting point	Conductivity	Solubility
Giant ionic	Ions	High	Only molten or aqueous	Generally soluble
Giant metallic	Positive ions in a sea of electrons	Moderately high to high	Solid and liquid	Insoluble
Simple covalent	Small molecules	Low	None	Usually insoluble unless polar
Giant covalent	Atoms	Very high	None (except graphite)	Insoluble

Part 10 of 11Bonding continuum

Bonding continuum

Idea

Idea no compound is 100% ionic, so bonding is described on a continuum.

Bonding triangle

Bonding triangle places compounds using average electronegativity and electronegativity difference to classify ionic, covalent and metallic character.

Electronegativity difference

Electronegativity difference a large difference indicates predominantly ionic bonding.
- (roughly above 1.8)

Part 11 of 11More from the specification

1Enthalpy changes of covalent bonds

Enthalpy changes of covalent bonds endothermic bond cleavage absorbs energy whereas exothermic formation of new covalent bonds releases energy.

2Formation of binary ionic compounds

Formed when metallic elements transfer valence electrons to non-metallic elements like chlorine, producing cations and anions.

3Omnidirectional electrostatic forces

Strong electrostatic attractions act equally in all three dimensions between oppositely charged ions in the crystal lattice.

4Sodium chloride lattice model

Each Na⁺ ion is octahedrally coordinated by six surrounding Cl⁻ ions in a face-centred cubic giant ionic lattice.

5Physical properties of ionic solids

High melting points, brittle cleavage along planes, and electrical conduction only when molten or dissolved in water.

6Chloride

Salt of hydrochloric acid.

7Element

A substance made of only one type of atom.

8Electrostatic force

Force of attraction between opposite charges.

Key terms

Term	Meaning
Ion	An atom or group of atoms with an overall charge from losing or gaining electrons.
Ionic bond	The electrostatic attraction between oppositely charged ions.
Ionic lattice	A giant regular arrangement of alternating cations and anions.
Polyatomic ion	A charged species of two or more covalently bonded atoms.
Lattice enthalpy	The enthalpy change when one mole of a solid ionic compound separates into gaseous ions.
Coordination number	The number of nearest oppositely charged ions around a given ion.
Electronegativity	The ability of an atom to attract a shared pair of electrons in a bond.

Avoid these mistakes

Saying ionic compounds consist of molecules — they are giant lattices with formula units.

Saying solid ionic compounds conduct because electrons move — only mobile ions conduct, and only when molten or aqueous.

Forgetting brackets around a polyatomic ion — the formula is Ca(NO₃)₂, not CaNO₃2.

Treating lattice enthalpy as exothermic — as defined here it is endothermic and positive.

Last updated: 15 October 2026