Lectures 3 and 4

Question 1

What is a pure substance?

Answer 1

A substance that is homogeneous (i.e. uniform thermodynamic property throughout). The chemical composition is the same in all phases.

Question 2

What are the three phases of matter?

Answer 2

Solid
Liquid
Gas

Question 3

Describe the molecules in solids, liquids and gases respectively.

Answer 3

• solid: molecules oscillate about their “fixed” positions
• liquid: groups of molecules float about each other
• gas: gas molecules move randomly

Question 4

The transformation from

• solid to liquid
• liquid to gas
• gas to liquid
• liquid to solid

Answer 4

• melting
• evaporation
• condensation
• freezing

Question 5

Describe the refrigeration process

Answer 5

• Refrigerant gas compressed to high P and T through a compressor
• Gas travels through condenser coils and becomes a liquid. Heat is rejected to the room.
• Refrigerant liquid travels through expansion valve, decreasing T
• Cold liquid removes heat from refrigerator. Liquid refrigerant receives heat and liquid evaporates to a gas, and this then goes back into the compressor.

Question 6

Describe the steam power plant

Answer 6

• Liquid water is pumped from near atmospheric P to a higher P
• Compressed water is heated in a boiler. Water is evaporated into steam
• High T, P steam is sent to turbine. Steam expands and used to turn turbine blades. Turbine connected to electric generator and this produces electricity to grid. T, P of steam reduces significantly and condensation of steam to water
• Steam leaves turbine and sent to condenser where heat is removed and remaining steam condenses to have 100% liquid which is sent back to pumps

Question 7

What must be added or removed for a phase change?

Answer 7

Energy

Question 8

What is Gibb’s Phase Rule

Answer 8

F = C - phi + 2

Question 9

What is F in Gibb’s Phase Ruler

Answer 9

The number of intensive parameters that must be specified in order to completely define the system or number of variables that can be changed independently and still maintain equilibrium

Question 10

What is phi in Gibb’s phase rule?

Answer 10

The number of phases. Phases are mechanically separable constituents

Question 11

What is C in Gibb’s phase rule?

Answer 11

The minimum number of components.

Question 12

What does the number 2 specify in Gibb’s Phase Rule?

Answer 12

Two intensive parameters - usually temperature and pressure

Question 13

Which systems does Gibb’s Phase rule apply to?

Answer 13

Systems in equilibrium

Question 14

What are the regions of interest on P-v diagrams?

Answer 14

Compressed liquid, saturated liq-vap mixture, superheated vapour

Question 15

What is the dome on a P-v diagram?

Answer 15

It is the lines separating these phases.

Question 16

What is the quality of a two phase liquid-vapour mixture?

Answer 16

x = mass (vapour) / mass (total)
x = mass (vapour)/ (mass(liquid) + mass (vapour))

Question 17

What is the critical point?

Answer 17

It is the point beyond which the liquid and vapour phases are indistinguishable. They have the same density.

Question 18

What is the triple point?

Answer 18

The co-existence of solid, liquid and vapour.

Question 19

What is sublimation?

Answer 19

Transformation from vapour to solid.

Question 20

What is the definition of enthalpy?

Answer 20

H = u + Pv

Question 21

How does the specific volume of liquid compare to that of vapour?

Answer 21

The specific volume of vapour is always greater than the specific volume of liquid

Question 22

What is the saturation pressure?

Answer 22

Pressure at which the liquid and vapour phases are in equilibrium at a given temperature

Question 23

What is the saturation temperature?

Answer 23

The temperature at which the liquid and vapour phases are in equilibrium at a given pressure

Question 24

Alternative equations for the quality involving variable such as u and h

Answer 24

u = uf + x*(ug-uf)
h = hf + x*(hg-hf)

Question 25

Define compressed liquid

Answer 25

State with the pressure greater than the saturated pressure. Or state with lower temperature than the saturation temperature

Question 26

Define superheated vapour

Answer 26

State with the pressure lower than the saturated pressure. Or state with higher temperature than saturation temperature

Question 27

What is required if the exact conditions are not in the table?

Answer 27

Linear interpolation

Question 28

Equation for linear interpolation

Answer 28

x desired = x1 + Tdesired - T1/ T2 - T1 * (x2 - x1)

Question 29

What are the ideal gas equations of state?

Answer 29

Pv = RT, PV = mRT, P = rhoRT

Question 30

When can a gas be considered ideal?

Answer 30

When the gas pressure is very small compared to the critical pressure. Or the gas temperature is twice the critical temperature and the pressure is less than 10 times the critical pressure.

Question 31

What common vapour is often considered an ideal gas but it is not?

Answer 31

Water vapour

Question 32

Common gases that can be treated as ideal gases

Answer 32

Air, N2, H2, CO2, CO, Ar

Question 33

What temperature range is the ideal gas law valid for?

Answer 33

Less than 1000 K

Question 34

What is the specific heat?

Answer 34

It is the amount of energy required to increase the temperature of a unit mass by a single degree

Question 35

What is the specific heat capacity at constant volume?

Answer 35

Energy required to raise the temperature of a unit mass of a substance by one degree as the volume is kept constant

Question 36

What is the specific heat capacity at constant pressure?

Answer 36

Energy required to raise the temperature of a unit mass of substance by one degree as the pressure is maintained at constant

Question 37

Relation of Cv and dT

Answer 37

Cv = du/ dt

Question 38

Relation of Cp and dT

Answer 38

Cp = dh/ dT

Question 39

For sensible heat what are the equations?

Answer 39

Cv ( T2 - T1)

Cp (T2 - T1)

Question 40

Relation between Cp, Cv and R

Answer 40

Cp = Cv + R

Question 41

The ratio of specific heat capacities

Answer 41

k = Cp/ Cv