IB Chemistry HL — Structure 2.4: From Models to Materials

This is the Higher Level lesson. The core S2.4.1–S2.4.5 (bonding triangle, alloys, addition polymers) is shared with SL; the HL-only content — condensation polymers (S2.4.6) — carries a purple HL badge.

Core — SL & HL

HL AHL only

👆 bonding triangle · polymers

1. Bonding is a continuum S2.4.1–2

Ionic, covalent and metallic bonding are not separate boxes — they blend into one another. The bonding triangle maps any compound using two numbers from electronegativity (EN): the difference in EN (up the triangle → more ionic) and the average EN (right → more covalent, left → more metallic). Pick a substance and see where it lands:

Bonding triangle plotter

Pick a substanceIts position is set by the average electronegativity (x) and the electronegativity difference (y).

2. Alloys S2.4.3

An alloy is a mixture of a metal with one or more other metals or non-metals (e.g. steel = iron + carbon; bronze = copper + tin). The added atoms are a different size, so they disrupt the neat layers and stop them sliding — making the alloy harder and stronger than the pure metal, while the delocalised electrons still hold it together.

Pure metal — layers slide (soft)

Alloy — bigger atoms block sliding (hard)

3. Polymers & addition polymerisation S2.4.4–5

Polymers are giant molecules (macromolecules) built from many small repeating units called monomers. In addition polymerisation, monomers each contain a C=C double bond; the double bond opens up and the monomers join into a long chain with no atoms left over. Pick a monomer to see its repeating unit:

4. Condensation polymers HL

In a condensation polymer, the monomers join through their functional groups and a small molecule (usually water) is lost every time a new link forms. Two families you must know:

Because a small molecule is released, condensation differs from addition polymerisation (which loses no atoms). The reverse reaction — hydrolysis — adds water back to break the links.

Common mistakes examiners see

What two quantities position a compound on the bonding triangle?✗ Melting point and density. ✓ The difference in electronegativity (vertical) and the average electronegativity (horizontal).

Why is an alloy usually harder than the pure metal?✗ The extra atoms form stronger covalent bonds. ✓ The different-sized atoms distort the layers so they can no longer slide over each other easily.

What happens to the C=C bond during addition polymerisation?✗ It stays as a double bond in the polymer. ✓ The double bond opens (becomes a single bond) so each carbon can bond to the next monomer — no atoms are lost.

Is a polymer a giant covalent network like diamond?✗ Yes, both are giant structures. ✓ No — a polymer is made of long molecules held to each other by intermolecular forces; diamond is one continuous covalent network.

What is the key difference between addition and condensation polymerisation?✗ They are the same thing. ✓ Addition loses no atoms (monomers have C=C); condensation releases a small molecule (usually water) each time a link forms between functional groups. HL

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