States of Matter

Question 1

Define

ideal gas and the ideal gas equation

Answer 1

A gas whose volume varies in proportion to the temperature and in inverse proportion to the pressure.

pV = nRT

Question 2

List the basic assumptions of the kinetic theory as applied to an ideal gas

Answer 2

• gas molecules move rapidly and randomly
• volume of the molecules is negligible
• no forces of attraction or repulsion between molecules
• all collsions are elastic, so no k.e. is lost
• temperature is related to average k.e. of molecules

Question 3

Explain the limitations of ideality, and the conditions necessary for a gas to approach ideal behaviour

Answer 3

Real gases do not fit the assuptions of ideal gases as:

• there is attraction between molecules
• we cannot ignore the volume of the molecules

For a gas to approach ideal behaviour, the gas:

• must have a small atomic radius
• must have a low number of electrons so minimised induced dipole-dipole forces

This is why small noble gases such as helium and neon approach ideal behaviour
themselves

Question 4

Explain the limitations of ideality at very high pressures and very low temperatures

Answer 4

• molecules are close to each other
• volume of molecules not negligible relative to container
• induced dipole-dipole forces present, pulling molecules to each other and away from container walls

As a result:

• pressure is lower than expected from ideal gas
• effective volume is less than expected from ideal gas

Question 5

Describe, using a kinetic-molecular model, the liquid state, freezing and melting

Answer 5

• particles are close to each other
• enough k.e. to slide past each other randomly
• sometimes arranged in a slightly ordered way, but this is constantly changing
• intermolecular forces (IMF) ≈ kinetic forces (KF)

• when energy decreases, liquid freezes
  
  • as a solid, ​IMF > KF
• when a solid is heated, solid melts
  
  • IMF weakened

Question 6

Explain vaporisation and condensation

Answer 6

Vaporisation: change from liquid to vapour

• Evaporation
  
  • particles with most energy at liquid’s surface can escape
  • temperature below boiling point
• Boiling
  
  • IMF weaken enough and k.e. increases as temperature increases
  • particles become free from each other at all points in the liquid
  • happens at boiling point

Condensation: change from vapour to liquid

• molecules lose k.e. and come closer together
• experience increasing attraction

Question 7

Explain the term

vapour pressure

Answer 7

The pressure exerted by a vapour in equilibrium with a liquid.

i.e. liquid molecules ⇌ vapour molecules

In a closed container, the molecules return to the liquid at the same rate as the molecules escape the liquid. The vapour molecules exert a force on the walls of the container, causing pressure. Vapour pressure increases with temperature.

The temperature at which vapour pressure = atmospheric pressure is the b.p. of the liquid.

Question 8

Describe the lattice structure of an ionic solid

Answer 8

• Alternating anions and cations in cubic arrangement
• Each anion bonded to 6 cations; each cation bonded to 6 anions

Question 9

Describe the lattice structure of a simple molecular solid (such as iodine).

Answer 9