CIE IGCSE Chemistry

Revision Notes

3.4 1 Macromolecules

Giant Covalent Structures

  • Diamond and graphite are allotropes of carbon which have giant covalent structures
  • This class of substances contains a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds forming a giant lattice structure
  • Giant covalent structures have high melting and boiling points as they have many strong covalent bonds that need to be broken down
  • Large amounts of heat energy are needed to overcome these forces and break down bonds

 

Diamond, Graphite & Fullerene, IGCSE & GCSE Chemistry revision notesDiamond, graphite and fullerene are examples of giant covalent structures

Uses of Giant Covalent Structures

Diamond

  • Each carbon atom bonds with four other carbons, forming a tetrahedron
  • All the covalent bonds are identical and strong with no weak intermolecular forces
  • Diamond thus:
    • Does not conduct electricity
    • Has a very high melting point
    • Is extremely hard and dense (3.51 g/cm3)
  • Diamond is used in jewellery and as cutting tools
  • The cutting edges of discs used to cut bricks and concrete are tipped with diamonds
  • Heavy-duty drill bits and tooling equipment are also diamond-tipped

Graphite

  • Each carbon atom is bonded to three others forming layers of hexagonal-shaped forms, leaving one free electron per carbon atom
  • These free electrons exist in between the layers and are free to move and carry charge, hence graphite can conduct electricity
  • The covalent bonds within the layers are very strong but the layers are connected to each other by weak intermolecular forces only, hence the layers can slide over each other making graphite slippery and smooth
  • Graphite thus:
    • Conducts electricity
    • Has a very high melting point
    • Is soft and slippery, less dense than diamond (2.25 g/cm3)
  • Graphite is used in pencils and as an industrial lubricant, in engines and in locks
  • It is also used to make non-reactive electrodes for electrolysis
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The Structure of Silicon(IV) Oxide (Silicon Dioxide)

  • SiO2 is a macromolecular compound which occurs naturally as sand and quartz
  • Each oxygen atom forms covalent bonds with 2 silicon atoms and each silicon atom in turn forms covalent bonds with 4 oxygen atoms
  • A tetrahedron is formed with one silicon atom and four oxygen atoms, similar as in diamond

 

Silicon-Dioxide, IGCSE & GCSE Chemistry revision notesDiagram showing the structure of SiO2 with the silicon atoms in blue and the oxygen atoms in red

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Diamond & Silicon(IV) Properties

  • SiO2 has lots of very strong covalent bonds and no intermolecular forces so it has similar properties to diamond
  • It is very hard, has a very high boiling point, is insoluble in water and does not conduct electricity
  • SiO2 is cheap since it is available naturally and is used to make sandpaper and to line the inside of furnaces

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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