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]

S2.4Structure 2 — Models of bonding and structure

From models to materials — IB Chemistry SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 4 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

S2.4 From models to materials — IB CHEMISTRY SL notes: Use the bonding triangle to classify bonding, relate properties to bonding, explain alloys and describe addition polymers.Three models — ionic, covalent and metallic bonding explain properties, but real substances are more co…

Intermediate bonding
Other examples
Property link
Average electronegativity
Electronegativity difference
Data booklet

Part 1 of 9Bonding as a continuum

Three models

Three models ionic, covalent and metallic bonding explain properties, but real substances are more complex.

Intermediate bonding

Intermediate bonding most substances lie between the extremes, e.g. AlCl₃ melts at only 192 °C because its bonding is polar covalent, not purely ionic.

Other examples

Al₂O₃ is ionic with some covalent character, and SiO₂ is polar covalent.

Property link

Al₂O₃ melts at 2072 °C because of its stronger ionic bonding.

Part 2 of 9Using the bonding triangle

1Average electronegativity

χ_mean = (χ_A + χ_B) ÷ 2, plotted on the x-axis, with no unit.

2Electronegativity difference

Δχ = |χ_A − χ_B|, plotted on the y-axis, with no unit.

Region	Position	Δχ and mean
Ionic	Apex of the triangle	Large Δχ
Covalent	Bottom right	Small Δχ, high mean
Metallic	Bottom left	Small Δχ, low mean
Polar covalent	Between ionic and covalent	Intermediate Δχ

Elements

Elements have Δχ = 0 and lie on the x-axis according to their electronegativity.

NaCl

χ(Na) = 0.9 and χ(Cl) = 3.2 give a mean of 2.05 and a difference of 2.3, placing it near the ionic apex.

Data booklet

Data booklet electronegativities are in Section 9 and the bonding triangle in Section 17, and it is used qualitatively.

Part 3 of 9Bonding, structure and properties

Find the Pauling electronegativities of both elements.

Calculate the average and the difference.

Plot the point on the bonding triangle.

Read the region and describe the character.

Predict melting point, conductivity and solubility from the bonding type.

Bonding, structure and properties

Metals

Metals conductive and malleable, used for wiring.

Ionic ceramics

Ionic ceramics high melting point, hard, insulating and brittle, used for furnace linings.

Covalent network solids

Covalent network solids very hard like diamond, used in cutting tools.

Polymers

Polymers light, flexible and insulating, used for packaging.

Part 4 of 9Alloys

Definition

Definition a mixture of a metal with other elements, usually metals but sometimes a non-metal such as carbon.

Bonding

Bonding alloys keep metallic bonding with delocalised electrons, and the non-directional bond lets atoms of different sizes sit in the lattice.

Hardness

Hardness atoms of different sizes disrupt the regular layers, so layers cannot slide easily.
- (or small interstitial atoms such as C in steel)

Properties

Properties alloys are harder and stronger but less malleable, may resist corrosion better, and usually conduct less well and may melt over a range.

Alloy	Components	Property and use
Brass	Copper and zinc	Strong and corrosion resistant; door handles and instruments
Steel	Iron and carbon	Very strong; construction and cars
Stainless steel	Iron, chromium, nickel, carbon	Corrosion resistant; cutlery and surgical instruments
Solder	Lead and tin	Low melting point; joining metals
Bronze	Copper and tin	Hard and corrosion resistant; medals and ship fittings

Part 5 of 9Polymers

Polymer

Polymer a macromolecule made of many repeating units (at least 50) joined by covalent bonds.

Monomer

Monomer a small molecule that joins repeatedly to others to form a polymer.

Copolymers

Copolymers contain two or more different monomer units, such as nylon and proteins.

Natural polymers

Natural polymers proteins, starch and DNA, made from monomers such as amino acids and nucleotides.

Synthetic polymers

Synthetic polymers plastics such as poly(ethene) and nylon.

Properties of plastics

Properties of plastics low density, unreactive, water resistant and strong because of covalent bonds and loosely packed chains.

Part 6 of 9Addition polymerisation

1Process

Process monomers with a C=C bond join as the π bond breaks and new C–C single bonds form.

2Product

Product only the polymer forms, so the atom economy is 100%.

3Empirical formula

Empirical formula the polymer has the same empirical formula as its monomer, and M_r of the polymer is about n × M_r of the monomer.

4Repeat unit

Repeat unit the smallest group that repeats, containing two main-chain carbons, shown in square brackets with n and continuation bonds.

Draw the monomer with the C=C bond and all four substituents.

Change the C=C into a C–C single bond.

Add a continuation bond to each of the two carbons.

Keep the substituents in the same positions.

Enclose the unit in square brackets with n outside.

Monomer	Polymer	Repeat unit
Ethene, CH₂=CH₂	Poly(ethene)	–[CH₂–CH₂]–
Propene, CH₂=CHCH₃	Poly(propene)	–[CH₂–CH(CH₃)]–
Chloroethene, CH₂=CHCl	PVC	–[CH₂–CHCl]–
Tetrafluoroethene, CF₂=CF₂	PTFE	–[CF₂–CF₂]–
Phenylethene, CH₂=CHC₆H₅	Polystyrene	–[CH₂–CH(C₆H₅)]–

Repeat unit versus monomer
- the unit in brackets is the repeat unit, and the monomer has C=C where the unit has C–C.

Part 7 of 9Properties of addition polymers

Properties of addition polymers

Chain length

Chain length longer chains and stronger intermolecular forces raise the melting point and strength.

Polar side groups

Polar side groups such as C–Cl in PVC give stronger dipole-dipole forces between chains.

Branching

Branching branched chains pack less closely, giving lower density and melting point, as in LDPE compared with HDPE.

Side groups

Side groups bulky or polar groups change flexibility and rigidity.

Part 8 of 9Plastics and the environment

Advantages

Advantages plastics are durable, cheap and of low density.

Persistence

Persistence they are non-biodegradable because strong, non-polar C–C and C–H bonds make them chemically stable, so they accumulate as pollution.

Origin

Origin most are derived from crude oil.

Burning

Burning releases toxic gases, e.g. HCl from PVC.

Responses

Responses recycling with resin identification codes, bioplastics and reduced use.

Composites

Composites combine a matrix and a reinforcement for improved properties, e.g. reinforced concrete and fibreglass.

Part 9 of 9More from the specification

1Triangular bonding diagram (van Arkel-Ketelaar)

A visual continuum mapping bonding type based on average electronegativity and electronegativity difference.

2Bonding continuum extremes

Pure covalent (e.g. Cl₂) lies at the bottom right, pure metallic (e.g. Na) at the bottom left, and ionic at the apex.
- (e.g. CsI or NaCl)

3Intermediate bonding character

Compounds like aluminium chloride (AlCl₃) occupy intermediate positions exhibiting polar covalent character and sublimation.

4Addition polymerisation

Synthetic plastics formed when alkene monomers link through repeated double bond addition without elimination products.

5Addition polymerisation

Monomers join with opening of C=C

6Sodium chloride

Compound of sodium and chlorine; common salt.

Key terms

Term	Meaning
Bonding continuum	The idea that bonding varies gradually between ionic, covalent and metallic extremes.
Bonding triangle	A diagram locating a substance by average electronegativity (x-axis) and electronegativity difference (y-axis).
Alloy	A mixture of a metal with other elements that keeps metallic bonding.
Polymer	A macromolecule made of many repeating units joined by covalent bonds.
Addition polymerisation	Monomers with C=C join with no other product to form a polymer.
Repeat unit	The smallest part of a polymer that repeats along the chain.

Avoid these mistakes

Saying alloys are chemically compounds — they are mixtures with metallic bonding.

Giving the monomer instead of the repeat unit inside brackets — the repeat unit has C–C, not C=C.

Saying a compound is 100% ionic — bonding is a continuum.

Forgetting the continuation bonds through the brackets in a repeat unit.

Last updated: 14 October 2026