MNSR 21 - Heat

Question 1

States/Phases of Matter

Answer 1

Intermolecular forces and thermal energy of molecules determines whether object is in solid, liquid, or gas state

Question 2

Changing State

Answer 2

As temperature increases, molecules get more energy, can vibrate at greater amplitudes, material undergoes thermal expansion, can overcome some of the intermolecular forces and object can change state

Solid - liquid = fusion
Liquid - gas = vaporization

Question 3

Heat & temperature

Answer 3

• Temperature and heat are closely related but they are not the same thing!
• Temperature is a property which indicates the relative “hotness” of an object.

Question 4

Heat

Answer 4

• Heat is a measure of thermal energy which is transferred between systems due to a temperature difference.
• Heat energy is always transferred from a higher temperature to a lower temperature

Question 5

Thermometer

Answer 5

A thermometer is used to measure temperature.
It is calibrated to a reference material and its THERMOMETRIC PROPERTY. i.e a mercury thermometer and the thermal expansion of the mercury
An increase in the height of a column of mercury can be related to a change in temperature
Mercury/Alcohol expands/contracts linearly within that temp range (if temp goes up by certain amount, expands/contracts by same amount)

Question 6

Calorimeter

Answer 6

Device used to measure heat

Question 7

Thermometric Property

Answer 7

As the mercury/alcohol heats up in the thermometer, it will expand in a linear fashion for this temperature region and after calibration, a thermometer can be made.

Question 8

Heat Capacity

Answer 8

The amount of energy required to increase the temperature of a substance by 1°C

Question 9

Specific Heat Capacity, C

Answer 9

Energy required to increase the temperature of 1kg of a substance by 1°C.
𝑄=𝑚𝑐∆𝑇
Q = amount of energy (J)
m = mass of the substance (kg)
c = specific heat capacity (J/kg.K)
∆𝑇 = change in temperature °C.

Question 10

How much heat energy is required to raise the temperature of 3L of water from 20 to 80°C? SHC of water is 4200 J/kg.K

Answer 10

Q = 𝑚𝑐∆𝑇
m = 3 kg (1L water = 1kg)
∆𝑇 = 80-20 °C = 353-293 K = 60 K
C = 4200 J/kg.K
Q = (3)(4200)(60) = 7.56𝑥10^5 J

Question 11

Evaporation

Answer 11

Some liquid molecules have enough KE to escape from the bulk liquid.
The rate of evaporation increases with temperature.

Question 12

Vapor Pressure

Answer 12

Molecules evaporating from the liquid surface generate a VP
At equilibrium, number of molecules evaporating off is equal to the number of vapor molecules re-entering the liquid

Volatile compound turns from liquid to gas very easily, has a high VP (e.g. perfume turns into gas)

VP increases as temperature increases

Question 13

Boiling Point

Answer 13

Temperature at which VP = Patm
If Patm decreases -> BP decreases -> lower temperature required (BP at Everest is 90-95 degrees)
At BP, visible evaporation occurs throughout the bulk of the liquid
BP is the only temp at which a liquid and vapor can coexist in equilibrium for a given pressure

Question 14

Latent Heat

Answer 14

• To change from solid to liquid (at same temp) = Latent Heat of Fusion must be supplied to complete the phase change
• To change from liquid to gas (at same temp) = Latent Heat of Vaporization must be supplied to complete the phase change

Takes a lot more energy to turn from liquid to gas than solid to liquid = latent heat of vaporization generally much larger than latent heat of fusion.

Question 15

Specific Latent Heat

Answer 15

• Specific Latent Heat of Fusion, Lf = energy required to convert 1kg of solid to 1kg of liquid (at the same T)
• Specific Latent Heat of Vaporisation, Lv = energy required to convert 1kg of liquid to 1kg of gas (at the same T)

Question 16

Evaporation of Sweat

Answer 16

Takes more energy to evaporate than melt
Each water molecule that evaporates from skin removes a significant amount of heat energy

Same thing, birds require twice as much heat to melt snow from solid to liquid at 0 C than is needed to heat it from 0 C to their body temp of 40 C - takes them so much energy

Question 17

Phase Diagram

Answer 17

Temp on x, Pressure on y
Temperature at which phase change occurs varies with absolute pressure
- Triple Point characterizes the only conditions under which all 3 phases can exist simultaneously

Question 18

Positive Slope of Liquid-Gas Boundary

Answer 18

Can boil water at less than T<100 C is the gas pressure above the liquid surface is lowered

Question 19

In-flight cabin pressure

Answer 19

2/3 atmospheric pressure, water boils <100 C

At 1500m deep in the sea, hydrothermal vents heat water up to 340 C, but water does not boil

Question 20

H2O is unusual, negative phase boundary between Solid & Liquid

Answer 20

Freezing/melting point temp increases as pressure is reduced

Question 21

Freeze Drying

Answer 21

By reducing air pressure, water will freeze at temps greater than 0 C
Water is removed from food/sample by sublimation

Question 22

Sublimation

Answer 22

Substance goes directly from solid to gas phase

Question 23

Plasma

Answer 23

Formed when molecules are given so much energy they become electrically charged ionized gas
e.g. neon lights, aurora, disinfection

Question 24

Bose-Einstein Condensate

Answer 24

At micro-Kelvin Temperatures, the Pauli-Exclusion Principle is relaxed.
Atoms occupy identical ground states and get as close to each other as Heisenberg Uncertainty Principle allows.

Question 25

Q) For H2O, phase boundary between solid and liquid has a ______ slope, meaning that the ____ temperature ____ as pressure is reduced

Answer 25

Negative, melting point, increases