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SESM2017: Thermodynamics > Properties and Processes > Flashcards

Properties and Processes Flashcards

1
Q

What is a system?

A

a quantity of matter or a region in space for study

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2
Q

Which system has no change in heat, work or mass?

A

isolated system

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3
Q

Which system has a change in heat, work and mass?

A

open system (control volume)

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4
Q

Which system has a change in heat and work but no change in mass?

A

closed system

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5
Q

What are extensive properties?

A

properties that depend on the amount of matter in a system

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6
Q

Give 4 examples of extensive properties

A
  • mass
  • volume
  • weight
  • length
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7
Q

What are intensive properties?

A

properties independent of matter in a system

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8
Q

What are specific properties?

A

properties per unit mass, always intensive

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9
Q

Give 4 examples of intensive properties

A
  • temperature
  • density
  • pressure
  • concentration
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10
Q

How do you denote an extensive property?

A

capitalised

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11
Q

How do you denote a specific property?

A

lowercase

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12
Q

How do you denote a molar property?

A

lowercase with a bar

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13
Q

What is internal energy?

A

the kinetic energy (and rotational and vibrational energy) of molecules

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14
Q

Which property is internal energy proportional to?

A

temperature

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15
Q

Is absolute or delta enthalpy more useful?

A

delta enthalpy

absolute enthalpy is deemed useless

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16
Q

Which equation is used to calculate the change in internal energy?

A

ΔU = Q - W

ΔU = change in internal energy
Q = heat
W = work

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17
Q

At a constant volume, what is the internal energy equal to?

A

heat

as there is no work

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18
Q

What is denoted by ΔU?

A

change in internal energy

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19
Q

What is denoted by Q?

A

heat

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20
Q

What is denoted by W?

A

work

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21
Q

What is denoted by ΔH?

A

enthalpy change

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22
Q

Under isobaric conditions, what is the change in enthalpy equal to?

A

heat

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23
Q

What equation is used to calculate change in enthalpy under isobaric conditions?

A

ΔH = ΔU + pΔV

ΔH = enthalpy change
ΔU = change in internal energy
p = pressure
ΔV = change in volume

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24
Q

At the same temperature, is the enthalpy change or internal energy change greater and why?

A

enthalpy change

there is the same increase in internal energy but work is done expanding under a constant pressure so more heat is needed

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25
Q

What is a state of equilibrium?

A

a state when all properties are uniform and unchanging

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26
Q

What do thermodynamic properties completely describe?

A

the state of the system

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27
Q

Which other four equilibrium are required to reach thermodynamic equilibrium?

A
  • thermal equilibrium
  • mechanical equilibrium
  • phase equilibrium
  • chemical equilibrium
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28
Q

What is thermal equilibrium?

A

no tendency to transfer heat

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29
Q

What is mechanical equilibrium?

A

no tendency to accelerate

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30
Q

What is phase equilibrium?

A

no tendency for phase changes

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31
Q

What is chemical equilibrium?

A

no tendency for further chemical change

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32
Q

What is the two property rule?

A

the state of a simple, compressible system can be completely specified by two independent, intensive properties

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33
Q

What is a simple compressible system?

A

a system with no external forces acting upon it

(e.g. gravity, electric)

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34
Q

What does it mean if two properties are independent of one another?

A

one property can be varied whilst the other is held constant

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35
Q

What is the ideal gas equation?

A

pV = mRT

p = pressure
V = volume
m = mass
R = specific gas constant
T = temperature

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36
Q

What are the 4 statements in the kinetic theory of gases?

A
  • all molecular colisions are elastic
  • volume occupied by molecules is negligible
  • intermolecular forces are negligible
  • molecules carry only translational kinetic energy
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37
Q

What is the difference between a ‘perfect’ and a ‘semi-perfect’ ideal gas?

A
  • ‘perfect’ ideal gas, heat capacity is constant
  • ‘semi-perfect’ ideal gas, heat capacity is a function of temperature
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38
Q

What is denoted by Cp?

A

heat capacity

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39
Q

What equation is used to calculate density in ideal gases?

A

𝜌 = p/RT

𝜌 = density
p = pressure
R = specific gas constant
T = temperature

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40
Q

What is Van der Waals model of real gases?

A

(p + a/V2)(V - b) = mRT

p = pressure
a = constant
V = volume
b = constant
m = mass
R = specific gas constant
T = temperature

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41
Q

In Van der Waals model of real gases, what is the purpose in the ‘a’?

A

constant that accounts for forces between molecules

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42
Q

In Van der Waals model of real gases, what is the purpose in the ‘b’?

A

constant to account for volume occupied by molecules themselves

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43
Q

What happens to Van der Waals model of real gases at larger volumes?

A

it reduces to the standard ideal gas equation

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44
Q

What are the two best ways to model real substances?

A
  • computer programs using advanced models
  • data charts with empirical data
45
Q

What is Dalton’s law of partial pressures?

A

p total = p a + p b

p = pressure

46
Q

How do you calculate a partial pressure of a gas “i” in relation to the total pressure of a mixture?

A

p i = X i p

p i = partial pressure of “i”
X i = mole fraction of gas “i”
p = total pressure of mixture

47
Q

How do you calculate the mole fraction of a gas “i” (X i ) in a mixture?

A

X i = n i / n total

X i = mole fraction of gas “i”
n i = total moles of gas “i”
p = total number of moles

48
Q

How do you calculate the enthalpy of a mixture of ideal gases contained in a fixed volume?

A

sum the enthalpy that each gas would have if it alone occupied the entire volume at the same temperature

49
Q

Which equation can be used to calculate the enthalpy of a mixture of ideal gases contained in a fixed volume?

A

ħ = ∑ X i ħ i (T)
h = ∑ Y i h i (T)

ħ = total molar enthalpy
X i = mole fraction of gas “i”
ħ i = partial molar enthalpy of gas “i”

h = total enthalpy
Y i = mass fraction of gas “i”
h i = partial enthalpy of gas “i”

50
Q

How do you calculate a partial enthalpy of a gas “i” in relation to the total enthalpy of a mixture?

A

ħ i = X i ħ
h i = Y i h

ħ = total molar enthalpy
X i = mole fraction of gas “i”
ħ i = partial molar enthalpy of gas “i”

h = total enthalpy
Y i = mass fraction of gas “i”
h i = partial enthalpy of gas “i”

51
Q

How do you calculate the mass fraction of a gas “i” (Y i ) in a mixture?

A

Y i = m i / m total

Y i = mass fraction of gas “i”
m i = mass of gas “i”
m total = total mass

52
Q

What is denoted by X i ?

A

mole fraction of a gas “i”

53
Q

What is denoted by Y i ?

A

mass fraction of a gas “i”

54
Q

Is heat input or heat output denoted as positive?

A

heat input is denoted as positive

55
Q

Is work input or work output denoted as positive?

A

work output is denoted as positive

56
Q

What is the 0th Law of Thermodynamics?

A

if two systems, A and B, are each in thermal equilibrium with a third system, C, then A and B are in thermal equilibrium with each other

57
Q

What is an example of the 0th Law of Thermodynamics?

A

If you have a thermometer, both the surroundings and mercury are in thermal equilibrium with the glass exterior of the thermometer so the surroundings and mercury are also in thermal equilibrium

58
Q

Which type of system does the 1st Law of Thermodynamics apply to?

A

closed systems

59
Q

What understanding does the 1st Law of Thermodynamics provide us with?

A

whether a process is energetically feasible

60
Q

What is the 1st Law of thermodynamics?

A

energy cannot be created or destroyed; it can only be transferred or converted from one form to another

61
Q

Which type of system does the steady flow energy equation (SFEE) apply to?

A

open system (control volume)

62
Q

What is the steady flow energy equation (SFEE)?

A

(Q . - W .) = ΣOUT m .(eh + 1/2u 2 + gz) - ΣIN m .(eh + 1/2u 2 + gz)

Q. = rate of heat tranfer
W. = rate of work done
m. = rate of mass transfer
ΣOUT/IN = sum of energy changes into/out of the system
eh = enthalpy
u = kinetic energy
gz = gravitational potential energy

63
Q

What are the units of temperature in thermodynamics problems?

A

Kelvin (K)

64
Q

What does x. (x dot) represent

A

rate of change of x

x with respect to time

65
Q

What is entropy?

A

a measure of disorder or uncertainty

66
Q

What is a change in enthalpy formally described as?

A

dS = Qrev / T

dS = infinitesimal change in entropy
Qrev = reversible heat input into a system
T = temperature

67
Q

What equation represents the 2nd Law of Thermodynamics in general?

A

Δstotal = Δssystem + Δssurroundings >= 0

Δs = change in entropy

68
Q

What equation represents the 2nd Law of Thermodynamics in an isolated system?

A

Δs >= 0

Δs = change in entropy

69
Q

What is the 2nd Law of Thermodynamics?

A

the total entropy of an isolated system can never decrease over time

it either remains constant in ideal (reversible) processes or increases in real (irreversible) processes

70
Q

Why are heat engine efficiencies limited?

A

some energy is always lost as waste heat due to the 2nd Law of Thermodynamics

71
Q

How does a heat engine work?

A

absorbing heat from a high-temperature reservoir, converting part of this heat into mechanical work, and releasing the remaining heat to a low-temperature reservoir

72
Q

What equation can be used to calculate the net work done of a heat engine or heat pump?

A

Wnet = QH - QL

as shown from the 1st Law of Thermodynamics

Wnet = net work done
QH = heat absorbed from high temperature resevoir
QL = heat expelled to low temperature resevoir

73
Q

Which equation proves heat engines can never be 100% efficient?

A

Wnet < QH

as shown from the 2nd Law of Thermodynamics

Wnet = net work done
QH = heat absorbed from high temperature resevoir

74
Q

Which equations can be used to calculate the efficiency of a heat engine?

A

η = Wnet/QH
η= (QH - QL)/QH
η = 1 - QL/QH

η = efficiency
Wnet = net work done
QH = heat absorbed from high temperature resevoir
QL = heat expelled to low temperature resevoir

75
Q

What does η represent?

A

efficiency

76
Q

How does a heat pump work?

A

transfers heat from a cooler space to a warmer space by using mechanical work

effectively working in reverse of a heat engine

77
Q

What does adiabatic mean?

A

no heat transfer

78
Q

What does isobaric mean?

A

constant pressure

79
Q

What does isochoric mean?

A

constant volume

80
Q

What does isothermal mean?

A

constant temperature

81
Q

What does isenthalpic mean?

A

constant enthalpy

82
Q

What does isentropic mean?

A

constant entropy

83
Q

What is a reversible adiabatic process also known as?

A

an isentropic process

84
Q

What are the three isentropic relations of perfect gases?

A

pvγ = constant
Tvγ-1 = constant
T / p(γ-1)/γ = constant

p = pressure
v = volume
T = temperature
γ = heat capacity ratio

85
Q

How do you calculate the heat capacity ratio (γ)?

A

γ = cp / cv

γ = heat capacity ratio
cp = heat capacity at constant pressure
cv = heat capacity at constant volume

86
Q

What is γ?

A

heat capacity ratio

87
Q

What is cp?

A

heat capacity at constant pressure

88
Q

What is cv?

A

heat capacity at constant volume

89
Q

Is work done dependent on the path taken?

A

yes

90
Q

Give three examples of state functions.

A
  • internal energy (U)
  • enthalpy (H)
  • entropy (S)
91
Q

What is a state function?

A

a property that depends only on the current state of a system, not on the path or process used to reach that state

92
Q

Are state functions path dependent?

A

no

93
Q

What is a quasi-equilibrium process?

A

a process so slow, it is always assumed to be at equilibrium

94
Q

How much energy is wasted in a reversible process?

A

none

reversible processes give the maximum energy possible

95
Q

Why do reversible processes not exist in real life?

A

every process increases enthalpy meaning no wasted energy is impossible

due to the 2nd Law of Thermodynamics

96
Q

What is path dependency?

A

processes where the outcome depends on the specific path taken, not just the initial and final states

97
Q

What defines a reversible process?

A

can be reversed without any net change in the system or surroundings, as a quasi-equilibrium process without increasing entropy

98
Q

What characterises a non-reversible process?

A

increases entropy and cannot return to its original state without changes in the surroundings

99
Q

How is entropy change affected in a reversible reaction?

A

no entropy change

entropy increase in system is equal to entropy decrease in surroundings

100
Q

How is entropy change affected in an irreversible reaction?

A

entropy change > 0

entropy increase in system is more than entropy decrease in surroundings

101
Q

What is the equation for entropy, in an irreversible system?

A

dS >= Q/T

irreversibilities result in increase in entropy

dS = infinitesimal entropy change
Q = heat
T = temperature

102
Q

What is the equation for entropy in an isolated system?

A

dS >= 0

as dQ = 0

dS = infinitesimal entropy change

103
Q

What is the TdS equation for internal energy?

A

TdS = dU + pdV

T = temperature
dS = infinitesimal entropy change
dU = infinitesimal internal energy change
p = pressure
dV = infinitesimal volume change

104
Q

What is the TdS equation for enthalpy?

A

TdS = dH − Vdp

T = temperature
dS = infinitesimal entropy change
dH = infinitesimal enthalpy change
V = volume
dp = infinitesimal pressure change

105
Q

What is the 3rd Law of Thermodynamics?

A

as the temperature of a system approaches absolute zero (0K), the entropy of a pure substance approaches a zero.

106
Q
A
107
Q

Which two energies are related in the SFEE?

A
  • energy exchange between control volume & surroundings
  • energy change within the material entering/exiting control volume
108
Q

For turbines and compressors, what does the SFEE simplify down to?

A

-W .= m .Δh

SESM2017: Thermodynamics (1 decks)

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