14. Thermal Properties of Matter

Question 1

Internal Energy

Definition

Answer 1

The internal energy of a substance is the sum of the kinetic energy due to the random motion of the particles and potential energy due to forces of attraction between the particles

Internal kinetic energy - temperature
Internal potential energy - state

Question 2

Heat capacity (C)

Definition

Answer 2

Heat capacity (C) is the amount of heat required to raise the temperature of a substance by 1K (or 1°C)

Unit: J K^-1

Question 3

Specific Heat Capacity (c)

Definition

Answer 3

Specific heat capacity (c) is the amount of heat required to raise the temperature of a unit mass of a substance by 1K (or 1°C).

Unit: J kg^-1 K^-1

Question 4

Melting

Defintion

Answer 4

Melting is the change from the solid state to the liquid state when energy is transferred to the substance, without a change in temperature.

Question 5

Solidification/Freezing

Definition

Answer 5

Soldification/Freezing is the change from the liquid state to the solid state when energy is transferred out of the substance, without a change in temperature.

Question 6

Boiling

Definition

Answer 6

Boiling is the change from the liquid state to the gaseous state when energy is transferred to the substance without a change in temperature.

Question 7

Condensation

Definition

Answer 7

Condensation is the change from the gaseous state to the liquid state when energy is transferred out of the substance without a change in temperature.

Question 8

Latent Heat

Definition

Answer 8

Latent heat is the energy released or absorbed to change the state of a substance, at constant temperature.

2 types of latent heat:
Latent heat of fusion & Latent heat of vaporisation

Question 9

Describe latent heat during melting in terms of behavior of particles

Answer 9

During melting, the energy absorbed is used to do work against the forces of attraction between the partlces.
The particles now have a wider range of movement, and the average distance between the particles increases.
This increases the internal potential energy of the substance, whereas the internal kinetic energy remains constant.

Question 10

Describe latent heat during boiling in terms of behaviour of particles

Answer 10

During boiling, the energy absorbed is used to do work against the forces of attraction between the particles, so they can move much further apart,
as well as to do work against atmospheric pressure, in order to escape to the surroundings.
This increases the internal potential energy of the substance, whereas the internal kinetic energy remains constant.

Question 11

Describe latent heat during freezing in terms of behaviour of particles

Answer 11

During freezing, the energy released to the surroundings causes the forces of attraction to reform around the particles.
The particles now are restricted to fixed positions, and the average distance between particles decreases.
This decreases the internal potential energy of the substance, whereas the internal kinetic energy remains constant.

Question 12

Describe latent heat during condensation in terms of behaviour of particles

Answer 12

During condensation, the energy released to the surroundings causes the forces of attraction to reform around the particles.
The average distance between particles decreases.
This decreases the internal potential energy of the substance, whereas the internal kinetic energy remains constant.

Question 13

Latent Heat of Fusion (L_f)

Definition

Answer 13

Latent heat of fusion is the amount of heat required/released to change a substance between the solid and liquid states, at constant temperature.

Unit: J

Question 14

Specific Latent Heat of Fusion (l_f)

Definition

Answer 14

Specific latent heat of fusion is the amount of heat required/released to change a unit mass of a substance between the solid and liquid states, at constant temperature.

Unit: J kg^-1

Question 15

Latent Heat of Vaporisation (L_v)

Definition

Answer 15

Latent heat of vaporisation is the amount of heat required/released to change a substance between the liquid and gaseous states, at constant temperature.

Unit: J

Question 16

Specific Latent Heat of Vaporisation (l_f)

Definition

Answer 16

Specific latent heat of vaporisation is the amount of heat required/released to change a unit mass of a substance between the liquid and gaseous states, at constant temperature.

Unit: J kg^-1

Question 17

Evaporation

Definition

Answer 17

Evaporation is the process where a liquid changes into a gas without boiling.

Question 18

6 Differences between Boiling and Evaporation

Answer 18

Differences between boiling and evaporation:
1. Boiling occurs at a fixed temperature while evaporation occurs at any temperature
2. Boiling is relatively fast while evaporation is relatively slow
3. Boiling occurs throughout the liquid while evaporation takes place only at the liquid surface
4. Bubbles are formed in the liquid during boiling while no bubbles are formed in the liquid during evaporation
5. Tempearture remains constant during boiling while temperature may change during evaporation
6. An external heat source is required during boiling while an external heat source is not required during evaporation

Question 19

Describe how evaporation occurs in terms of behaviour of particles

Answer 19

1. The particles in a liquid are always moving randomly at different speeds.
2. Particles at the surface of the liquid can escape to the atmosphere, proided they have sufficient kinetic energy to overcome the attractive forces of the other liquid particles and do work against atmospheric pressure.
3. The less energetic particles are left behind and the average kinetic energy of the particles in the liquid decreases. Therefore, the average temperature of the liquid decreases. This results in a cooling effect of evaporation.

Question 20

6 Factors affecting rate of evaporation

Answer 20

Factors affecting rate of evaporation:
1. Temperature
2. Exposed surface area
3. Humidity of surrounding air
4. Motion of surrounding air
5. Pressure
6. Nature of liquid