CHEM-E6100 Fundamentals of chemical thermodynamics
Transcript of CHEM-E6100 Fundamentals of chemical thermodynamics
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CHEM-E6100Fundamentals of chemical thermodynamics
Week 4, Fall 2021
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Contents
• Summary about state functions
• Size of a system (Ch 5.6)
• Fundamental equation of thermodynamics
• Chemical potential (Ch 5.7)
• Concept of activity
• General criterion of equilibrium
• Maxwell’s relation (Ch 5.9)
• Impact of pressure on properties of substances
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Summary of the state functions
• It was shown earlier that the change of internal energy (U) can be expressed as:
dU = TdS – PdV
• With our focus on the chemical equilibria, we get the relations between the key variables from their definitions as:
• H = U + PVor in differential form dH = TdS + VdP
• A = U – TS or in differential form dA = -SdT – PdV
• G = H- TS or in differential form dG = -SdT + VdP
• d(TS)=TdS + SdT
• d(PV)=PdV+VdP
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Summary of the state functions
• It was shown earlier that the change of internal energy (U) can be expressed as:
dU = TdS – PdV
• With our focus on the chemical equilibria, we get the relations between the key variables from their definitions as:
• H = U + PVor in differential form dH = TdS + VdP
• A = U – TS or in differential form dA = -SdT – PdV
• G = H- TS or in differential form dG = -SdT + VdP
• d(TS)=TdS + SdT
• d(PV)=PdV+VdP
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dH= ?
dA=?
dG=?
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Size of a system
• The previous study concerned ‘rational’ state variables and their relations
with the (practical) state variables
• How do the state functions behave when the size of the system (also its
amount(s)) vary?
• The general mathematical expression for a function of three variables (in
general amounts +2) gives us:
• An infinitesimal change of any function F = F(P,T,nk) can be expressed by the (differential) changes of its variables T, P and nk as
dF = (F/T)P,nkdT + (F/P)T,nkdP + k(F/nk)P,T,njdnk, kj
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Size of a system• The infinitesimal change of any function F = F(P,T,nk)
dF = (F/T)P,nkdT + (F/P)T,nkdP + k(F/nk)P,T,njdnk, kj
is called total differential of function F
• It is obtained as a sum of the tangents of function F in respect of every
variable, when the other variables are constant, multiplied by its
difference
• For example tangent = (F/T), difference = dT
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Chemical potential
• Change of Gibbs energy is governed (in closed systems) by the
differential equation
dG = -SdT + VdP
• When we write accordingly the change of Gibbs energy as a function of
temperature and pressure as well as the amounts (of components), we get
dG = (G/T)P,nkdT + (G/P)T,nkdP + k(G/nk)P,T,njdnk, kj
(G/T)P,nk = -S and (G/P)T,nk = V
• The third term is the key variable in chemical equilibria, the chemical
potential of component k () and it is simply partial differential of Gibbs
energy and amount = (G/nk)P,T,nj
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Chemical potential
• Correspondingly as we get for Gibbs energy
dG = -SdT + VdP + kkdnk
• Internal energy, enthalpy and Helmholtz energy are of the form
• dU = TdS - PdV + k(U/nk)S,V,njdnk
• dH = TdS - VdP + k(H/nk)S,P,njdnk
• dA = -SdT + PdV + k(A/nk)T,V,njdnk
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Chemical potential
• Because the change of components amounts always reflects in the
system as an equally large change in its state of energy we can
conclude that
• (G/nk)T,P,nj = k
• k = (U/nk)S,V,nj = (H/nk)S,P,nj = (A/nk)T,V,nj
• The chemical potential describes the change of substance’s state of
energy when its amount varies
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By combining the 1st law and 2nd law for an open system in internal equilibrium and in equilibrium with the surroundings, gives the fundamental equation of thermodynamics
i is the chemical potential of particle I
Including non-expansion work, such as in a galvanic cell gives:
𝑑𝑈 = 𝑇𝑑𝑆 − 𝑃𝑑𝑉 + 𝜇𝑖𝑑𝑛𝑖
𝑑𝑈 = 𝑇𝑑𝑆 − 𝑃𝑑𝑉 + 𝜇𝑖𝑑𝑛𝑖 + 𝑑𝑊𝑟𝑒𝑣 (𝑛𝑜𝑛 −𝑃𝑉)
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Rearranging the fundamental equation gives the definition of Gibbs energy
U-TS+PV = ini
G = U-TS+PV
G = ini
The Gibbs energy function compresses all main state functions and variables of thermodynamics into one expression
H=U+PV → G = H-TS
A similar function called Helmholtz energy can also be defined
A = U-TS
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The chemical potential, i, of species i is related to Gibbs energy by
Chemical potential is the same as the partial molar Gibbs energy at constant P, T, and nj
For a pure substance
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𝜇𝑖 = (𝜕𝐺
𝜕𝑛𝑖)𝑇,𝑃,𝑛𝑗≠𝑖
𝜇𝑖∗ =𝐺∗
𝑛𝑖= 𝐺𝑚∗
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Activity is closely related to chemical potential
Chemical potentials are the driving force for distribution of components between phases of variable compositions
Activity can be thought of as the parameter that gives the effective “availability” of component i for reaction in the system
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The relation between chemical potential and activity is given by
The activity is a measure of the difference in the chemical potential between the studiedstate and the reference state
Choice of reference state is therefore necessaryand critical for activity determintation 14
𝜇𝑖=𝜇𝑖𝑜 + 𝑅𝑇 ln𝑎𝑖
Chemical potential of i in a phase
Standard state chemicalpotential of i in a phase
Activity of i in a phase
Phase in question may be given by the superscript
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𝑎𝑖 = 𝛾𝑖𝑥𝑖
For practical purposes various standard statesare used for different types of phases
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Ideal gas: 𝑎𝑖 =𝑝𝑖𝑝𝑖𝑜
1 bar of pure ideal gas i at T
Aqueous solutions: 𝑎𝑖 =𝑐𝑖𝑐𝑖𝑜
Activity coefficient, γ=1 for ideal solutions
Solutions in general:
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General criterion of equilibrium
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General criterion of equilibrium
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Gibbs energy and the equilibrium constant
(Ch. 11.2)• Law of mass action
• jA+kB ⇌ lC+mD
• K = equilibrium constant, a =activity
• Example: 1CO + 1 H2O ⇌ 1CO2 + 1 H2
• ∆G°rxn= -RT lnK or K=e-DG/RT
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Gibbs energy and equilibrium constant
– Reaction: CO + H2O ⇌ CO2 + H2
– K=[a(CO2)· a(H2)] / [a(CO) · a(H2O)]
– K=e-DG/RT; DG is the change in Gibbs energy for
the reaction (GCO2+GH2-GCO-GH2O)
– DG<0: reaction favors CO2 + H2
G
x=T,n,ξ
dG/dx<0
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Thermodynamic concepts
– Reaction: CO + H2O ⇌ CO2 + H2
– K=[a(CO2)· a(H2)] / [a(CO) · a(H2O)]
– K=e-DG/RT; DG is the change in Gibbs energy for
the reaction (GCO2+GH2-GCO-GH2O)
– DG<0: reaction favors CO2 + H2
– DG>0: reaction favors CO + H2O
G
x=T,n,ξ
dG/dx>0
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Thermodynamic concepts
– Reaction: CO + H2O ⇌ CO2 + H2
– K=[a(CO2)· a(H2)] / [a(CO) · a(H2O)]
– K=e-DG/RT; DG is the change in Gibbs energy for
the reaction (GCO2+GH2-GCO-GH2O)
– DG<0: reaction favors CO2 + H2
– DG>0: reaction favors CO + H2O
– DG=0: chemical equilibrium
G
x=T,n,ξ
dG/dx=0
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Conditions for Chemical Equilibrium
Minimum of Total Gibbs Energy
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Water-gas shift reaction
• Syngas (synthesis gas), a mixture of CO + H2, can be used to produce electricity or for diesel production in the Fischer-Tropsch processes
• Syngas is produced by steam reforming of CH4/natural gas or gasification of woody biomass
• Syngas (CO+H2) will react with CO2 or H2O
CO+H2O⇌ CO2+H2
• Reaction is used to control/change CO/H2 ratio in syngas
• Affects the gas composition in gasification processes
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24/50
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
600 700 800 900 1000 1100 1200
Mo
l fra
ctio
n
Temperature [°C]
H2
CH4
H2O
CO
CO2
Series6
CH4
CO2, H2
CO, H2O
Eq
uim
ola
r co
mp
osit
ion
Water-gas shift reaction (1 bar)
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25/50
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
600 700 800 900 1000 1100 1200
Mo
l fra
ctio
n
Temperature [°C]
H2
CH4
H2O
CO
CO2
Series6
CH4
CO2, H2
CO, H2O
Eq
uim
ola
r co
mp
osit
ion
Water-gas shift reaction (1 bar)
CO + H2O is favored by
higher temperatures
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Consider the water gas shift reaction
CO2 + H2 = CO + H2O
Note:
- three elements (components): C, H, O
- one phase: gas
- no change in number of molecules during reaction
When is this single reaction valid?
At some overall composition O2, CH4 and C-s may start to form.
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The total composition is determined entirely by the
amounts of
the three system components.
At any state:
Ctot = nCO + nCO2
Htot = 2nH2 + 2nH2O
Otot = nCO + nH2O + 2nCO2
The amount of the three components can
all vary freely in the range
1 < Otot /Ctot < 2 + 0.5 Htot
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At given total amounts of the components there always
will be a mixture of the four species CO2, H2, CO and H2O.
Two extreme positions of the reaction can be recognized.
CO2 + H2 = CO + H2Oextreme left
no CO or H2O
extreme right
no CO2 or H2
the equilibrium state is located somewhere between these extremes
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Mixture of CO2, H2O, CO, H2
50 mol C, 50 mol H, 90 mol O
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8 1.0
Extent of reaction
Mo
l fr
acti
on
CO2
H2
CO
H2O
extreme left extreme right
The concept of extent of reaction can be used to determine the position
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The ”start situation” does not have to be one of extremes.
In technical application it can be any state satisfying the
mass balance.
The ”start situation” does not affect the equilibrium state,
it is not necessary to define it, unless simultaneous
energy balance calculations are needed.
Definitions like ”before” and ”after” are sometimes misleadingly
used in equilibrium calculations to determine the mass balance
restrictions.
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The system is in chemical equilibrium when its Gibbs Energy
is at the lowest possible value.
We search for the combination of amounts of CO2,CO, H2O and H2
with the special feature that a small change in mol fraction for any of
species leads to a higher total Gibbs Energy for the system.
The total Gibbs Energy [J/mol] at constant temperature T is
TGtot =xCO2oGCO2 + xCO
oGCO + xH2OoGH2O + xH2
oGH2 +
RT(xCO2lnxCO2 + xCOlnxCO +xH2OlnxH2O +xH2lnxH2) + GE
standard values
ideal mixing term excess Gibbs Energy
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Standard Gibbs energies, 0G
-1000
-800
-600
-400
-200
0
200 600 1000 1400 1800 2200
Temperature [K]
Sta
nd
ard
Gib
bs E
nerg
y [
kJ/m
ol]
C(s) + 0.5 O2(g) = CO(g)
C(s) + O2(g) = CO2(g)
H2(g) + 0.5 O2(g) = H2O(g)
H2(g
)
H2(g
)
CO(g)
H2O(g
)
CO2(g)
Standard Gibbs Energies are tabulated and relate to
conventions of reference state
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-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.0 0.2 0.4 0.6 0.8 1.0
-6
-5
-4
-3
-2
-1
0
1
2
3
4
The total Gibbs Energy can be expressed as a function of extent of
reaction and can be numerically calculated and derived in the whole
range
TGtot
dTGtot
dx
constant T
eq. stateR
ela
tive G
ibbs E
nerg
y
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The eq. composition is simultaneously determined
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.0 0.2 0.4 0.6 0.8 1.0
-6
-5
-4
-3
-2
-1
0
1
2
3
4
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.4 0.6 0.8 1.0
CO2
H2
H2O
CO
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Conclusions
Equilibrium state can be determined if we know
- all possible species and phases
- standard Gibbs energy for all species
- total amount of all system components
- non-ideality (excess Gibbs Energy function)
We do not need to use or know
- reaction equations
- equilibrium constants
- state ”before” equilibrium is established
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Maxwell relations
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Summary of the state functions
• It was shown earlier that the change of internal energy (U) can be
expressed as:
dU = TdS – PdV
• With our focus on the chemical equilibria, we get the relations between the
key variables from their definitions as:
• H = U + PV or in differential form dH = TdS + VdP
• A = U – TS or in differential form dA = -SdT – PdV
• G = H- TS or in differential form dG = -SdT + VdP
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Maxwell relations
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Maxwell relations
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Maxwell relations
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Maxwell relations
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Maxwell relations
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