S2.4 From models to materialsIB Chemistry HL: Revision notes
Section 1
The bonding triangle
Bonding is a continuum between ionic, covalent and metallic models. The bonding triangle plots average electronegativity (horizontal) against electronegativity difference (vertical). Ionic vertex at the top (large ΔEN), metallic vertex at the bottom left (low average, small ΔEN), covalent vertex at the bottom right (high average, small ΔEN).
Example: SnCl₄ (Sn 2.0, Cl 3.2): average 2.6, ΔEN 1.2 → polar covalent → molecular, liquid at room temperature, non-conductor. The position reflects the relative contributions of each bonding type and lets you predict properties.
A large average electronegativity alone does not make a compound ionic — ionic character depends on the difference.
Section 2
Alloys and composites
An alloy is a mixture of a metal with other metals or non-metals (steel, brass, bronze, duralumin). Non-directional metallic bonding lets atoms of different sizes sit in the lattice; they disrupt the layers, so layers cannot slide easily and the alloy is harder and stronger, while still conducting.
A composite combines distinct materials, e.g. reinforced concrete: concrete (ionic/covalent network) resists compression; steel (metallic) resists tension.
Section 3
Plastics and addition polymers
Polymers are macromolecules made of repeating monomers. Plastics are unreactive and non-biodegradable (strong non-polar C–C and C–H bonds), insulators (no mobile charges) and soften on heating (weak London forces between chains).
Addition polymers form when one bond of each monomer's C=C breaks. Repeating units: ethene → –[CH₂–CH₂]–; propene → –[CH₂–CH(CH₃)]–; chloroethene → –[CH₂–CHCl]–. No other product forms.
Section 4
Condensation polymers (HL)
Condensation polymers form when functional groups on the monomers react, joining the monomers and releasing a small molecule (usually H₂O, or HCl when an acyl chloride is used). Each monomer needs two reactive groups (or one monomer has both, as in lactic acid).
- Polyamide: amine –NH₂ + carboxylic acid –COOH → amide link –CONH– + H₂O.
- Polyester: alcohol –OH + carboxylic acid –COOH → ester link –COO– + H₂O.
From a diamine + dicarboxylic acid, each repeating unit contains two links and releases two H₂O molecules.
Section 5
Drawing polyamide and polyester repeating units (HL)
Remove H from each amine or alcohol group and OH from each acid group, join the remaining parts, and put brackets round one of each monomer.
- Nylon-6,6 from H₂N(CH₂)₆NH₂ + HOOC(CH₂)₄COOH → –[NH(CH₂)₆NHCO(CH₂)₄CO]–
- PET from HOOC–C₆H₄–COOH + HO–CH₂CH₂–OH → –[CO–C₆H₄–CO–O–CH₂CH₂–O]–
- PLA from CH₃CH(OH)COOH → –[O–CH(CH₃)–CO]–
- Kevlar from H₂N–C₆H₄–NH₂ + ClOC–C₆H₄–COCl → –[NH–C₆H₄–NH–CO–C₆H₄–CO]– with HCl released
Polyamides such as Kevlar form hydrogen bonds between N–H and C=O on neighbouring chains, giving great strength. Ester and amide links can be hydrolysed, so many condensation polymers are more biodegradable than addition polymers.
Count atoms: repeating unit = sum of both monomers minus the small molecules released (two per unit for a diamine–diacid pair).
Must know
- Bonding triangle: average EN against ΔEN; predict properties from position.
- Alloys: different-sized atoms stop layers sliding; composites combine complementary materials.
- Plastics: strong covalent chains, weak forces between chains, no mobile charges.
- Addition polymers: C=C opens, no by-product.
- (HL) Condensation polymers: functional groups react, small molecule lost; polyamides (–CONH–) and polyesters (–COO–).
That's the notes covered.
Carry on to the next subtopic.