Thermal Properties Of Matter

Question 1

Define internal energy of a system

Answer 1

Internal energy of a system is the sum of the total kinetic energy and potential energy of all the atoms or molecules in a system

KE- related to temperature of the system
PE- related to physical state (solid, liquid or gas)

Question 2

Effect of transfer of thermal energy

Answer 2

-change in temperature

OR

-change in state (always occurs at a constant temperature)

Question 3

Effect of transfer of thermal energy on internal energy of a system

Answer 3

Transfer of thermal energy into the body -> increases the internal energy

Transfer of thermal energy out of the body-> decreases the internal energy

Question 4

When two objects of two different initial temperatures are brought together…

Answer 4

Thermal energy transfer will take place between them. When these two objects reach the same temperature, we say that they are in thermal equilibrium

Question 5

Thermal equilibrium

Answer 5

Thermal energy is said to be conserved.
Thermal energy lost by the initially hotter object is equal to the sum of the internal energy gained by the colder object and heat loss to the environment

Question 6

Define heat capacity of a body

Answer 6

Heat capacity is the amount of thermal energy required to raise the temperature of the body by 1degree or 1kelvin, without a change in state

SI unit: Joules per Kelvin or Joules per Celsius

Question 7

Equation for heat capacity

Answer 7

C = Q divided by delta 0

C: Heat capacity
Q: thermal energy in joules
Delta 0: change in temperature

Useful eqn: Q = C delta 0

Question 8

Define specific heat capacity

Answer 8

Specific heat capacity of a substance is the amount of thermal energy required to raise the temperatures of unit mass of the substance by 1 degree or 1 kelvin, without any change in state

SI Unit: Joules per Kg per Kelvin/ Joules per kg per Celsius

Question 9

Equation for specific heat capacity

Answer 9

c= Q divided by (mass times delta 0)

*Note for specific heat capacity, it is (c) and not (C)
Always check the units for mass and in c are consistent throughout the equation

Question 10

Specific heat capacity in different states

Answer 10

Specific heat capacity of a substance is different when it is in solid, liquid or gaseous state

Always take note of the physical state of the substance at that instant

Question 11

Thermal energy and change in state

Answer 11

The transfer of thermal energy into or out of a body can cause a change in the physical state of the body

Question 12

Melting/solidification and boiling condensation are processes involving a change in state…

Answer 12

Without a change in temperature

Question 13

During melting and boiling, energy absorbed is used to…

Answer 13

Overcome intermolecular forces

Increases the internal energy of the material

Question 14

Energy involved in boiling/condensation compared to energy involved in melting/solidification

Answer 14

During boiling, extra work is done against atmospheric pressure

Energy involved in boiling/condensation is greater than energy involved in melting/solidification

Question 15

Define latent heat

Answer 15

Latent heat is the amount of thermal energy needed to change a substance from solid to liquid or liquid to gas without any change in temperature

Question 16

Equation for latent heat

Answer 16

Q

SI unit: joules

Question 17

Define specific latent heat

Answer 17

Specific latent heat is the amount of thermal energy needed to change the unit mass of a substance from solid to liquid (specific latent heat of fusion) or liquid to gas (specific latent heat of vaporization)

Question 18

Equation for specific latent heat

Answer 18

lf or lv= Q/m

SI unit: joules per kg

Q= mlf or mlv

Question 19

Evaporation

Answer 19

Evaporation is also a process where a substance undergoes a change in state

However, it occurs in a slightly randomized manner hence we seldom use latent heat to describe this process

Question 20

Similarities between boiling and evaporation

Answer 20

Both involve a change in state from liquid to gas

Question 21

Differences between boiling and evaporation.

Answer 21

Boiling occurs at boiling point while evaporation occurs at any temperature

Boiling occurs throughout the liquid while evaporation occurs at the surface of the liquid

In boiling bubbles can be seen. In evaporation no bubbles can be seen

Question 22

Evaporation through kinetic theory

Answer 22

Liquid molecules are in constant random motion and collide with each other continuously

When molecules with higher kinetic energy collide with molecules near the surface of the liquid, it may gain enough energy to overcome intermolecular forces of the liquid and leave the body

Question 23

Why does cooling occur when a liquid evaporates?

Answer 23

During evaporation, the fastest-moving molecules leave the liquid. The average kinetic energy of the molecules remaining in the liquid is decreased

Since temperature is directly proportional to average kinetic energy of molecules of the liquid, temperature falls and cooling occurs

Question 24

Gradient of graph is related to?

Answer 24

Specific heat capacity

The steeper the graph, the greater the specific heat capacity

Question 25

Why is the rate of cooling faster at the beginning?

Answer 25

Rate of cooling is faster when the temperature difference between the substance and its surroundings is larger

Question 26

Why does condensation take longer than freezing?

Answer 26

Latent heat of vaporization is usually larger than the latent heat of fusion