chemistry a-levels

• Kinetic assumptions made when dealing with an ideal gas

  • The gas contains a large number of molecules moving in random directions at random speeds
  • Electrostatic forces between molecules is negligible, except during collisions
  • Collisions are perfectly elastic
  • Time of collisions between molecules is negligible compared to time between collisions
  • The molecules of a gas occupy negligible volume compared to the total volume of the gas
• Ideal gas behaviour

  • Low pressure
  • High temperature
• Limitations of an ideal gas at very low temperatures and very high pressures

  • Intermolecular forces are no longer negligible and have to be considered
  • Molecular size is also no longer negligible and has to be considered
• Ideal gas equation

  pV = nRT

  p - pressure (Pa)

  V - volume (m3)

  n - number of moles (mol)

  R - gas constant (8.314 J K-1 mol-1)

  T - temperature (K)
• Using the ideal gas equation to find molecular mass

  M = m/n

  M - molecular mass

  n - number of moles (mol)

  m - mass (g)
• Kinetic-molecular model of the liquid state

  • Particles are close together but not regularly arranged
  • Particles have a little more kinetic energy than in a solid
  • There are fewer electrostatic forces between particles than in a solid, allowing particles to move past each other and flow
• Melting in terms of the kinetic-molecular model

  Solid → Liquid

  Increasing the temperature of the surroundings causes particles to absorb energy meaning they gain more kinetic energy

  Eventually, the particles gain enough energy to disrupt the regular arrangement and become a liquid
• Vaporisation in terms of the kinetic-molecular model

  Liquid → Gas

  Heat energy causes particles in a liquid to move fast enough to break all forces of attraction between them and become a gas
• Vapour pressure

  When a liquid evaporates in a closed container, the gaseous particles move around above the liquid. When these particles collide with the walls of the container, they exert a pressure called the vapour pressure.
• Structure of a solid ionic compound

  • Regular, repeating arrangement (lattice)

  Caused by the electrostatic attraction between the oppositely charged ions
• Lattice structure of iodine

  • Iodine is an example of a simple molecular lattice

  Iodine, I2 molecules form a larger structure due to intermolecular forces (Van der Waals Forces) between molecules

  The structure is described as face centred cubic
• Allotrope

  Allotropes are different physical forms of an element in the same state
• Structure of a fullerene

  • Lattice structure

  E.g. a buckminsterfullerene (C60) is a molecule consisting of 60 carbon atoms arranged in pentagons and hexagons
• Nanotube

  A graphene sheet rolled up into a tube (single sheet of carbon atoms covalently bonded together)
• Structure of diamond

  • Giant covalent lattice

  Each carbon atom is covalently bonded to four other carbon atoms

  Extremely strong structure

  Bond shape and angle around each carbon: Tetrahedral, 109.5°
• Structure of graphite

  • Giant covalent lattice

  Made from layers of carbon arranged in hexagonal rings

  There are weak london forces between layers

  Each carbon atom bonds covalently to 3 other carbon atoms

  One delocalised electron per carbon
• Structure of graphene

  • Giant covalent lattice

  Single layer of graphite

  Each carbon atom is bonded to 3 other carbon atoms to create a hexagonal ringed structure

  One delocalised electron per carbon
• Graphene

  • Giant covalent lattice
  • Single layer of graphite
  • Each carbon atom is bonded to 3 other carbon atoms to create a hexagonal ringed structure
  • One delocalised electron per carbon
• Silicon(IV) oxide

  • Similar 3D structure to diamond
  • Silicon and oxygen atoms covalently bonded together
• Ice

  • Open lattice structure
  • Hydrogen bonds hold water molecules apart in hexagonal rings
• Metal (e.g. copper)

  • Giant metallic lattice with positive ions packed closely together with delocalised electrons
  • In copper, each atom is surrounded by 12 other copper atoms
• Diagram of metallic bonding

  • Positive charges = ions
  • Negative charges = electrons
• Hydrogen bonding

  Increases the boiling and melting points of a substance
• Hydrogen bonding

  Increases the viscosity of a substance
• Hydrogen bonding

  Creates surface tension in water
• Boiling point

  • A high boiling point indicates a giant structure (ionic metallic or giant covalent)
  • A low boiling point indicates simple molecules (or atoms for noble gases)
• Solubility

  • Compounds that are soluble in water tend to be ionic
  • If a soluble compound has a low boiling point, it may be small and very polar or able to form hydrogen bonds
• Electrical conductivity

  • If a solid substance conducts electricity, it is likely to be a metal, graphene or graphite
  • If a substance only conducts when molten or dissolved, it is an ionic compound
• Appearance/malleability

  • If a substance is brittle, it is likely to be ionic or giant covalent
  • If a substance is shiny, malleable and ductile, it is likely to be a metal