Gases and Gas Laws - HQ

Question 1

Define

Evaporation

Answer 1

The change from liquid to gas at a temperature below the boiling point. (E.g., if water in a cup is left exposed to the atmosphere it eventually evaporates, even though it never reaches 100°C (i.e. water’s boiling point)

Question 1

Explain

Evaporation according to the Kinetic Theory of Matter

Answer 1

• The particles in a sample of a liquid possess a range of kinetic energies, at any particular instant a small fraction of particles will have kinetic energies considerably greater than the average value
• If a particle near the surface of the liquid has sufficient kinetic energy, it may be able to escape from the attractive forces of its neighbouring particles and become a gas (i.e. evaporation occurs at the surface of the liquid)
• Because the more energetic particles are the ones that tend to escape, this will lower the average kinetic energy of the particles remaining in the liquid
• Therefore, the temperature of the liquid also falls as evaporation takes place

Question 2

Vapour pressure

Answer 2

The pressure exerted by a vapour in equilibrium with the liquid from which it was derived (e.g. such as in a closed container that is at dynamic equilibrium)

Question 3

Dynamic equillibrium (vapour pressure)

Answer 3

• When a liquid is placed in a container that is then sealed, there are two processes happening at the same time
• The evaporation of the liquid. This occurs at a constant rate at a given temperature
• The condensation of the vapour (gas) to reform some of the liquid
• Eventually the rate of evaporation of the liquid equals the rate of condensation of the vapour, (i.e. the rate at which the particles are escaping from the liquid is equal to the rate at which they are returning to it)

Question 4

Factors affecting vapour pressure

Answer 4

• Strength of intermolecular forces
• Temperature of the liquid

Question 5

How does the strength of intermolecular forces affect vapour pressure?

Answer 5

The stronger these attractive forces, the greater the amount of kinetic energy required by the particles to escape from the liquid to form the gas. Therefore fewer gas particles are produced, leading to a smaller vapour pressure.

Question 6

How can the relative strengths of the intermolecular forces of different liquids be inferred from vapour pressure data?

Answer 6

The pressure of the vapour produced by water is much less than the pressure of the vapour produced by ethanol at the same temperature. Thus the intermolecular bonds between the water molecules must be stronger than those between the ethanol molecules

Question 7

How does temperature affect vapour pressure?

Answer 7

• As the temperature rises, the vapour pressure of a liquid increases
• This occurs because the average kinetic energy of the particles of the liquid increases with increasing temperature
• As a result, at a higher temperature a greater proportion of the particles will have sufficient kinetic energy to escape from the liquid to produce the vapour