C2.2 Structure and Properties of Substances

A simple molecule consists of a few atoms joined together by covalent bonds

Structure:

• Weak intermolecular forces (attractive forces between molecules)
  • Relatively easy to overcome or break in state changes
• Strong covalent bonds between atoms
  • Do not break in state changes

Properties

• Relatively low melting and boiling points
• Tend to be liquids ot gases at room temperature
• Insulator/poor conductor of electricity
  • Have no overall electric charge

Structure

• Regular structures/giant ionic lattices
  • Strong electrostatic forces
    • In all directions
    • Between oppositely charged ions

Properties

• High melting and boiling points 
  • Large amounts of energy needed to break strong ionic bonds
• Solid at room temperature
• Can conduct electricity when melted or dissolved in water
  • Charged ions free to move around

Both macromolecules and simple molecules contain convalent bonds

• General properties
  • Solids at room temperature
  • Very high melting points
    • Atoms linked together by strong covalent bonds
• Diamond
  • Very hard
    • Each atom bonded to four others with strong covalent bonds
• Graphite
  • Soft and sliperry, layers slide over each other
    • Each atom bonded to three others with strong covalent bonds between atoms in a layer
    • Weak intermolecular forces and no covalent bonds between layers
  • Conducts heat and electricity
    • Similar to metals as one electron from each carbon atom is delocalised
• Fullerenes
  • Structure based on hexagonal rings of carbon atoms. 
  • Drug delivery into the body, lubricants, catalysts, nanotubes for reinforcing materials

Properties

• Can be bent and shaped
  • Layers of atoms in the giant lattice can slide over each other
• Can conduct heat and electricity
  • Have free delocalised electrons in their structures

Alloys

• Harder than pure metals
  • Mixture of metals
  • Different sized atoms distort layers
    • More difficult for layers to slide over each other
• Shape memory alloys
  • Can return to their original shape after being deformed
  • e.g. Nitinol in dental braces

Large molecules made when small molecules (monomers) join together, e.g. Poly(ethene) and poly(propene)

Thermosoftening

• Individual, tangled polymer chains
• No cross links
• Soften and melt when heated 
  • Chains slide over each other easily
• Weak intermolecular forces between chains

Thermosetting

• Polymer chains with cross links between them
• Do not soften or melt when heated

Properties of polymers depend on monomers used and reaction conditions. LDPE (low density poly(ethene)) and HDPE (high-density poly(ethene)) are made at different temperatures and pressures with different catalysts, so have different properties and uses 

Refers to structures 1-100nm in size (1nm is one millionth of a mm) 

Nanoparticles:

• Have:
  • Only a few hundred atoms
  • A high surface area to volume ratio
  • Properties different from the same substance in larger pieces
• Can be used as:
  • Computers, catalysts, coatings
  • Highly selective sensors to detect substances
  • Stronger and lighter construction materials
  • Cosmetics such as sunscreens and deodorants

Nanotubes

• Made of carbon
• Structure similarnto a graphite layer rolled into a tube
• Very strong
• Conduct electricity