Biomedical Engineering Reference
In-Depth Information
TABLE 3.2
Species Activity Based on Different Choices of Standard-State Activity
State at (
T
,
P
)
Standard state at (
T
,
P[
1 bar)
At low and moderate pressures
Pure gas
G
j
P
Pure gas
a
j
z
1
bar
p
j
Pure gas
G
j
Species in a gaseous mixture
a
j
z
1
bar
Pure liquid
G
j
Pure liquid
a
j
z
1
C
j
C
g
j
;
as
C
j
Pure liquid
G
j
Species in a liquid mixture
a
j
z
g
j
/ 1
C
/ 1
C
j
g
j
1
G
j
¼
G
j
ð
T
;
P
¼ 1
bar
;
C
j
¼ 1
M
Þ
a
j
z
M
;
g
j
/ 1
as
C
j
/ 0
G
j
T
C
j
C
¼ 1
C
j
C
g
j
;
G
j
¼
;
P
¼ 1
bar
;
a
j
z
g
j
/ 1
as
C
j
/ 0
Pure solid
G
j
Pure solid
a
j
z
1
Pure solid
G
j
Species in a solid mixture
a
j
z
1
C
n
þ
j
C
n
j
G
j
Dissolved electrolyte in solution
A
j
¼ n
þ
A
jþ
þ n
A
j
¼ n
þ
G
jþ
ð
T
;
P
¼ 1
bar
;
C
jþ
¼ 1
M
Þ
þ
Þ
n
þ
þn
ðg
Þ
n
þ
þn
a
j
z
ð1
M
þ n
G
j
ð
T
;
P
¼ 1
bar
;
C
j
¼ 1
M
Þ
We define the standard state of a liquid as a
j
¼
1 and for gases as an ideal gas pressure of l
bar, p
j
¼
1. For ideal liquid solutions (activity coefficients of unity), we write
C
j
C
j
a
j
¼
(3.76)
0
j
where
C
is either the total concentration or 1 mol/L (as shown in
Table 3-1
). At chemical
equilibrium
!
C
j
C
j
!
n
j
Y
N
S
D
G
R
RT
K
eq
¼ exp
¼
(3.77)
j
¼1
and for gases
!
Y
N
S
DG
R
RT
p
n
j
j
K
eq
¼ exp
¼
(3.78)
j
¼1
In these expressions, K
eq
is dimensionless, while p
j
has dimensions; this equation is
still correct because we implicitly write each partial pressure as p
j
/1 bar which is
dimensionless.
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