Properties and Processes Flashcards
What is a system?
a quantity of matter or a region in space for study
Which system has no change in heat, work or mass?
isolated system
Which system has a change in heat, work and mass?
open system (control volume)
Which system has a change in heat and work but no change in mass?
closed system
What are extensive properties?
properties that depend on the amount of matter in a system
Give 4 examples of extensive properties
- mass
- volume
- weight
- length
What are intensive properties?
properties independent of matter in a system
What are specific properties?
properties per unit mass, always intensive
Give 4 examples of intensive properties
- temperature
- density
- pressure
- concentration
How do you denote an extensive property?
capitalised
How do you denote a specific property?
lowercase
How do you denote a molar property?
lowercase with a bar
What is internal energy?
the kinetic energy (and rotational and vibrational energy) of molecules
Which property is internal energy proportional to?
temperature
Is absolute or delta enthalpy more useful?
delta enthalpy
absolute enthalpy is deemed useless
Which equation is used to calculate the change in internal energy?
ΔU = Q - W
ΔU = change in internal energy
Q = heat
W = work
At a constant volume, what is the internal energy equal to?
heat
as there is no work
What is denoted by ΔU?
change in internal energy
What is denoted by Q?
heat
What is denoted by W?
work
What is denoted by ΔH?
enthalpy change
Under isobaric conditions, what is the change in enthalpy equal to?
heat
What equation is used to calculate change in enthalpy under isobaric conditions?
ΔH = ΔU + pΔV
ΔH = enthalpy change
ΔU = change in internal energy
p = pressure
ΔV = change in volume
At the same temperature, is the enthalpy change or internal energy change greater and why?
enthalpy change
there is the same increase in internal energy but work is done expanding under a constant pressure so more heat is needed
What is a state of equilibrium?
a state when all properties are uniform and unchanging
What do thermodynamic properties completely describe?
the state of the system
Which other four equilibrium are required to reach thermodynamic equilibrium?
- thermal equilibrium
- mechanical equilibrium
- phase equilibrium
- chemical equilibrium
What is thermal equilibrium?
no tendency to transfer heat
What is mechanical equilibrium?
no tendency to accelerate
What is phase equilibrium?
no tendency for phase changes
What is chemical equilibrium?
no tendency for further chemical change
What is the two property rule?
the state of a simple, compressible system can be completely specified by two independent, intensive properties
What is a simple compressible system?
a system with no external forces acting upon it
(e.g. gravity, electric)
What does it mean if two properties are independent of one another?
one property can be varied whilst the other is held constant
What is the ideal gas equation?
pV = mRT
p = pressure
V = volume
m = mass
R = specific gas constant
T = temperature
What are the 4 statements in the kinetic theory of gases?
- all molecular colisions are elastic
- volume occupied by molecules is negligible
- intermolecular forces are negligible
- molecules carry only translational kinetic energy
What is the difference between a ‘perfect’ and a ‘semi-perfect’ ideal gas?
- ‘perfect’ ideal gas, heat capacity is constant
- ‘semi-perfect’ ideal gas, heat capacity is a function of temperature
What is denoted by Cp?
heat capacity
What equation is used to calculate density in ideal gases?
𝜌 = p/RT
𝜌 = density
p = pressure
R = specific gas constant
T = temperature
What is Van der Waals model of real gases?
(p + a/V2)(V - b) = mRT
p = pressure
a = constant
V = volume
b = constant
m = mass
R = specific gas constant
T = temperature
In Van der Waals model of real gases, what is the purpose in the ‘a’?
constant that accounts for forces between molecules
In Van der Waals model of real gases, what is the purpose in the ‘b’?
constant to account for volume occupied by molecules themselves
What happens to Van der Waals model of real gases at larger volumes?
it reduces to the standard ideal gas equation
What are the two best ways to model real substances?
- computer programs using advanced models
- data charts with empirical data
What is Dalton’s law of partial pressures?
p total = p a + p b
p = pressure
How do you calculate a partial pressure of a gas “i” in relation to the total pressure of a mixture?
p i = X i p
p i = partial pressure of “i”
X i = mole fraction of gas “i”
p = total pressure of mixture
How do you calculate the mole fraction of a gas “i” (X i ) in a mixture?
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
How do you calculate the enthalpy of a mixture of ideal gases contained in a fixed volume?
sum the enthalpy that each gas would have if it alone occupied the entire volume at the same temperature
Which equation can be used to calculate the enthalpy of a mixture of ideal gases contained in a fixed volume?
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”
How do you calculate a partial enthalpy of a gas “i” in relation to the total enthalpy of a mixture?
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”
How do you calculate the mass fraction of a gas “i” (Y i ) in a mixture?
Y i = m i / m total
Y i = mass fraction of gas “i”
m i = mass of gas “i”
m total = total mass
What is denoted by X i ?
mole fraction of a gas “i”
What is denoted by Y i ?
mass fraction of a gas “i”
Is heat input or heat output denoted as positive?
heat input is denoted as positive
Is work input or work output denoted as positive?
work output is denoted as positive
What is the 0th Law of Thermodynamics?
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
What is an example of the 0th Law of Thermodynamics?
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
Which type of system does the 1st Law of Thermodynamics apply to?
closed systems
What understanding does the 1st Law of Thermodynamics provide us with?
whether a process is energetically feasible
What is the 1st Law of thermodynamics?
energy cannot be created or destroyed; it can only be transferred or converted from one form to another
Which type of system does the steady flow energy equation (SFEE) apply to?
open system (control volume)
What is the steady flow energy equation (SFEE)?
(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
What are the units of temperature in thermodynamics problems?
Kelvin (K)
What does x. (x dot) represent
rate of change of x
x with respect to time
What is entropy?
a measure of disorder or uncertainty
What equation formally describes a change in entropy?
dS = Qrev / T
dS = infinitesimal change in entropy
Qrev = reversible heat input into a system
T = temperature
What equation represents the 2nd Law of Thermodynamics in general?
Δstotal = Δssystem + Δssurroundings >= 0
Δs = change in entropy
What equation represents the 2nd Law of Thermodynamics in an isolated system?
Δs >= 0
Δs = change in entropy
What is the 2nd Law of Thermodynamics?
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
Why are heat engine efficiencies limited?
some energy is always lost as waste heat due to the 2nd Law of Thermodynamics
How does a heat engine work?
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
What equation can be used to calculate the net work done of a heat engine or heat pump?
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
Which equation proves heat engines can never be 100% efficient?
Wnet < QH
as shown from the 2nd Law of Thermodynamics
Wnet = net work done
QH = heat absorbed from high temperature resevoir
Which equations can be used to calculate the efficiency of a heat engine?
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
What does η represent?
efficiency
How does a heat pump work?
transfers heat from a cooler space to a warmer space by using mechanical work
effectively working in reverse of a heat engine
What does adiabatic mean?
no heat transfer
What does isobaric mean?
constant pressure
What does isochoric mean?
constant volume
What does isothermal mean?
constant temperature
What does isenthalpic mean?
constant enthalpy
What does isentropic mean?
constant entropy
What is a reversible adiabatic process also known as?
an isentropic process
What are the three isentropic relations of perfect gases?
pvγ = constant
Tvγ-1 = constant
T / p(γ-1)/γ = constant
p = pressure
v = volume
T = temperature
γ = heat capacity ratio
How do you calculate the heat capacity ratio (γ)?
γ = cp / cv
γ = heat capacity ratio
cp = heat capacity at constant pressure
cv = heat capacity at constant volume
What is γ?
heat capacity ratio
What is cp?
heat capacity at constant pressure
What is cv?
heat capacity at constant volume
Is work done dependent on the path taken?
yes
Give three examples of state functions.
- internal energy (U)
- enthalpy (H)
- entropy (S)
What is a state function?
a property that depends only on the current state of a system, not on the path or process used to reach that state
Are state functions path dependent?
no
What is a quasi-equilibrium process?
a process so slow, it is always assumed to be at equilibrium
How much energy is wasted in a reversible process?
none
reversible processes give the maximum energy possible
Why do reversible processes not exist in real life?
every process increases enthalpy meaning no wasted energy is impossible
due to the 2nd Law of Thermodynamics
What is path dependency?
processes where the outcome depends on the specific path taken, not just the initial and final states
What defines a reversible process?
can be reversed without any net change in the system or surroundings, as a quasi-equilibrium process without increasing entropy
What characterises a non-reversible process?
increases entropy and cannot return to its original state without changes in the surroundings
How is entropy change affected in a reversible reaction?
no entropy change
entropy increase in system is equal to entropy decrease in surroundings
How is entropy change affected in an irreversible reaction?
entropy change > 0
entropy increase in system is more than entropy decrease in surroundings
What is the equation for entropy, in an irreversible system?
dS >= Q/T
irreversibilities result in increase in entropy
dS = infinitesimal entropy change
Q = heat
T = temperature
What is the equation for entropy in an isolated system?
dS >= 0
as dQ = 0
dS = infinitesimal entropy change
What is the TdS equation for internal energy?
TdS = dU + pdV
T = temperature
dS = infinitesimal entropy change
dU = infinitesimal internal energy change
p = pressure
dV = infinitesimal volume change
What is the TdS equation for enthalpy?
TdS = dH − Vdp
T = temperature
dS = infinitesimal entropy change
dH = infinitesimal enthalpy change
V = volume
dp = infinitesimal pressure change
What is the 3rd Law of Thermodynamics?
as the temperature of a system approaches absolute zero (0K), the entropy of a pure substance approaches a zero.
Which two energies are related in the SFEE?
- energy exchange between control volume & surroundings
- energy change within the material entering/exiting control volume
For turbines and compressors, what does the SFEE simplify down to?
-W .= m .Δh
