Chapter 11: Thermal Properties of Matter

Question 1

Define internal energy.

Answer 1

The internal energy of a substance is the total energy of all the particles in the substance. It consists of two components, which are internal KE and internal PE.

Question 2

Define internal KE.

Answer 2

Internal KE is due to the motion of the particles, and is directly related to temperature — the higher the temperature, the more vigorous the motion of the particles.

Question 3

Define internal PE.

Answer 3

Internal PE is due to the stretching and compression of the interatomic or intermolecular bonds as particles move. Amount of PE stored in the bonds depends on:

1) the forces between the particles
2) how far apart they are

Question 4

Define heat capacity and state the formula involving heat capacity.

Answer 4

Heat Capacity C is the amount of thermal energy required to raise the temperature of a substance by 1K or 1° C.

C (J/ °C) = Q (thermal energy required, J)/ ⨺θ (change in temperature, °C)

Question 5

Define specific heat capacity and state the formula involving it.

Answer 5

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

Q = mc(⨺θ) (mass of substance is in kg)

Question 6

Use the kinetic model of matter to explain what happens when a solid melts.

Answer 6

1) The particles in a solid are held together by strong bonds. These particles are closely packed together. The internal potential energy of a solid is lower than that of a liquid.
2) Thermal energy is absorbed to break the strong bonds between the particles. The particles can move out of their fixed positions, and hence are further apart from one another.
3) The change of state from a solid to a liquid occurs.

Question 7

Explain the changes in temperature that occur when a solid melts.

Answer 7

1) The temperature of a substance changes only when there is no change of state.
2) During the point where change in state occurs from a solid to a liquid, the thermal energy absorbed is used to break the strong bonds between the particles of the solids.
3) Only the internal PE of the particles is increased.
4) Since none of the thermal energy supplied is converted to internal KE, there is no change in temperature during melting.

Question 8

Use the kinetic model of matter to explain what happens when a liquid freezes.

Answer 8

1) The intermolecular bonds between liquid particles are weaker than those in a solid. The particles are further apart. Hence, the internal potential energy of a liquid is higher than a solid.
2) Thermal energy is released to form strong intermolecular bonds. The particles are held in fixed positions, and hence are closer to one another.
3) The change of state from a liquid to a solid occurs.

Question 9

Explain the changes in temperature that occur when a liquid freezes.

Answer 9

1) The temperature of a substance changes only when there is no change of state.
2) When the particles of a liquid come together in freezing, strong bonds are formed. As the strong bonds form, internal PE decreases, and thermal energy is released and lost to the surroundings.
3) Since the internal KE does not change, the temperature does not continue to fall when a liquid is freezing (i.e. during solidication)

Question 10

Why doesn’t the temperature of a liquid rise as it boils and changes into steam?

Answer 10

The thermal energy supplied was used to separate the water molecules as well as to provide energy for the molecules to push back on the surrounding atmosphere (to escape into the air).

Question 11

Define latent heat and state the two types of latent heat.

Answer 11

Latent heat is the energy released or absorbed by a substance during a change of state, without a change in its temperature. There are two types of latent heat:

1) Latent heat of fusion (for melting and solidification)
2) Latent heat of vaporisation (for boiling and condensation)

Question 12

Define latent heat of fusion.

Answer 12

The amount of thermal energy required to change a substance from solid state to liquid state, without a change in temperature.

Question 13

Define specific latent heat of fusion, and state the formula involving it.

Answer 13

The amount of thermal energy required to change unit mass (e.g. 1 kg) of the substance from solid state to liquid state, without a change in temperature.

Lf (J) = lf (J/ kg) x m (kg)

Question 14

Define latent heat of vaporisation.

Answer 14

The amount of thermal energy required to change a substance from liquid state to gaseous state, without a change in temperature.

Question 15

Define specific latent heat of vaporisation, and state the formula involving it.

Answer 15

The amount of thermal energy required to change unit mass (e.g. 1 kg) of the substance from liquid state to gaseous state, without a change in temperature.

Lv (J) = lv (J/ kg) x m (kg)

Question 16

State the differences between boiling and evaporation.

Answer 16

1) Occurs at a particular temperature (B) VS Occurs at any temperature (E)
2) Relatively fast (B) VS Relatively slow (E)
3) Takes place throughout the liquid (B) VS Takes places only at the liquid surface (E)
4) Bubbles are formed in the liquid (B) VS No bubbles are formed in the liquid (E)
5) Temperature remains constant (B) VS Temperature may change (E)
6) External thermal energy source required (B) VS External thermal energy source not required (E)

Question 17

Use the kinetic model of matter to explain how evaporation occurs.

Answer 17

1) Molecules in a liquid are always moving at different speeds. (i.e. they have diff KEs)
2) At the surface, the liquid molecules that have enough energy to overcome the downward attractive forces of the other liquid molecules and the atmospheric pressure escape into the atmosphere.
3) The less energetic molecules are left behind. The average kinetic energy of the molecules in the liquid decreases, and therefore the average temperature of the liquid decreases.

Question 18

State the factors that affect the rate of evaporation.

Answer 18

1) Temperature (↑ temp, ↑ rate)
2) Humidity of surrounding air (↑ water vapour present in the air, ↓ rate)
3) Surface area of liquid (SA ↑, rate ↑)
4) b.p. of liquid (↓ b.p., ↑ rate)
5) Movement of air (↑ movement, ↑ rate)
6) Pressure (↓ pressure, ↑ rate)