Polymers and giant covalent structures explained

Bonding, structure and the properties of matter • Bonding and substance properties

Flashcards

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How does graphite appear in bonding diagrams?

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Graphite appears as layers of hexagonal carbon rings with each carbon bonded to three others and delocalised electrons within layers.

Key concepts

What you'll likely be quizzed about

Definition of a polymer

A polymer is a large molecule formed from repeating small units (monomers) joined by covalent bonds in long chains. Polymers show repeating structural units in diagrams, often written inside brackets with an 'n' to indicate many repeats. Chain length and side groups affect physical properties by changing intermolecular forces.

Recognising polymers in diagrams

Polymers appear as long chains or repeated motifs connected by single covalent bonds. A displayed polymer diagram often shows a repeating unit bracketed with 'n' or a section of the chain that repeats. Absence of discrete, separate molecules and presence of extended chains indicate a polymer rather than simple molecules.

Types of polymer bonding and effects

Covalent bonds link monomers along the polymer backbone; weaker intermolecular forces (van der Waals, dipole interactions, hydrogen bonds where present) act between chains. Strong covalent backbones cause chemical stability; stronger intermolecular forces cause higher melting points and greater rigidity. Cross-linking causes a network of covalent bonds between chains and increases hardness and thermal resistance.

Definition of giant covalent structure

A giant covalent structure consists of a continuous network of atoms each bonded covalently to several neighbours, forming an extended lattice. No molecules exist; the structure extends throughout the solid. Representative examples include diamond, graphite and silica (silicon dioxide).

Recognising giant covalent structures in diagrams

Giant covalent diagrams show atoms linked in a continuous pattern with covalent bonds to multiple neighbours. Diamond diagrams show each carbon bonded to four others in a tetrahedral network; graphite diagrams show layers of hexagonal rings with delocalised electrons between layers. The absence of discrete molecules and evidence of an extended network identify a giant covalent structure.

Cause→effect: bonding to properties

Many strong covalent bonds throughout a giant lattice cause very high melting and boiling points and general insolubility. In graphite, delocalised electrons cause electrical conductivity along layers and weak forces between layers cause lubricating properties. In polymers, long chains and intermolecular forces control flexibility, melting behaviour and solubility.

Key notes

Important points to keep in mind

Polymers show repeated units and long chains; diagrams often use brackets with 'n' to indicate repetition.

Addition polymers result from broken double bonds in monomers; condensation polymers form with loss of small molecules.

Giant covalent structures form continuous networks; no discrete molecular units exist in diagrams.

Many strong covalent bonds across a lattice cause very high melting points and insolubility.

Graphite conducts due to delocalised electrons in layers; diamond does not conduct because all electrons are in localized bonds.

Intermolecular forces between polymer chains control flexibility, melting point and solubility.

Cross-linking adds covalent bonds between chains and increases hardness and thermal stability.

Branching reduces packing and often lowers melting point and density compared with straight chains.

Look for atoms bonded to multiple neighbours and an extended pattern to identify giant covalent lattices.

Distinguish discrete molecules from networks: separate molecules indicate molecular solids; continuous bonding indicates giant covalent.