chapter 10 - thermal properties

Question 1

heat

Answer 1

measure of the internal energy of molecules

Question 2

internal energy

Answer 2

potential energy + kinetic energy

Question 3

heat capacity

Answer 3

change in internal energy per unit change in temperature

Question 4

specific heat capacity

Answer 4

change in internal per unit mass for each unit change in temperature

Question 5

why does the temperature remain constant during change of state

Answer 5

energy transferred to the solid results in work done against the intermolecular forces of attraction. the average separation of the particles increases and thus potential energy increases, but kinetic energy remains constant, and temperature does not change

Question 6

latent heat of fusion

Answer 6

amount of energy transferred to change a substance between the solid and liquid states, at a constant temperature

Question 7

specific latent heat of fusion

Answer 7

amount of energy transferred per unit mass to change of substance between the solid and liquid states, at constant temperature

Question 8

latent heat of vapourisation

Answer 8

amount of energy transferred to change a substance between the liquid and gaseous states, at constant temperature

Question 9

why does the temperature remains constant during boiling

Answer 9

latent heat absorbed by liquid is used to break the intermolecular forces of attraction holding the molecules together. latent heat is also used to overcome the atmospheric pressure which is preventing the vapour molecules from escaping

Question 10

specific latent heat of vapourisation

Answer 10

amount of energy transferred per unit mass of a substance to change it between the liquid and gaseous states, at constant temperature

Question 11

formula for heat capacity

Answer 11

C (heat capacity) = Q (change in internal energy) / change in temperature

Question 12

formula for specific heat capacity

Answer 12

Q (heat energy) = m (mass) x c (specific heat capacity) x Δθ (change in temperature)

C (heat capacity) = c (specific heat capacity) x m (mass)

Question 13

determine specific heat capacity of solid

Answer 13

1. solid should be in a cylindrical block shape with 2 bores
2. find mass of block
3. wrap block with wool (lagging) to prevent hear loss to surroundings
4. connect voltage supply to an immersion heater and put into one of the bores of the solid block. place a thermometer in the other bore
5. record the initial temperature and switch on the power supply for t seconds
6. record ammeter reading I, and voltmeter reading V
7. after t seconds, switch off heater and record the highest temperature reached
   heat supplied by heater (VI x t) = mcΔθ

Question 14

factors affecting rate of evaporation

Answer 14

• temperature increase, rate of evaporation increase
• humidity decrease, rate of evaporation increase
• surface area of liquid increase, rate of evaporation increase
• pressure of liquid decrease, rate of evaporation increase
• wind speed increase, rate of evaporation increase
• boiling point decrease, rate of evaporation increase

Question 15

why does evaporation produce a cooling effect?

Answer 15

the particles in a liquid are moving in continuous random motion, with each particle moving at a different speed. the average kinetic energy is proportional to the temperature of the liquid. the fast moving particles, especially those near the surface are able to break the intermolecular force of the neighboring particles and the can escape into the air. the liquid is left with only the less energetic particles. the average kinetic energy of the molecules is thus lower, resulting in a decrease in temperature and hence a cooling effect is produced

Question 16

heat capacity vs specific heat capacity

Answer 16

specifc heat capcity is the same for every object with the same material

Question 17

explain what a specific heat capcity of 4.2 kJ/(kg K) means

Answer 17

4.2 kJ of thermal energy to raise the temperature of 1kg of an object by 1K

Question 18

explain what specifc latent heat of fusion of 334 KJ/kg means

Answer 18

object requires 334 kJ of thermal energy to change 1kg of the object between its solid and liquid states, at a constant temperature