SI unit
Name
Symbol
Definition
standard reaction enthalpy standard reaction entropy reaction quotient
ArH
Ar H = V HB
J mol
A
Ar S = V SB
J mol' K1 10,12,13
equilibrium constant equilibrium constant, pressure basis
B
Notes 10,12, 13,14
B
Q
Q = fl aBB
K, K K = exp(
—
ArGe/RT)
1
15
1
10,13,16
K,,
K,, = fl pBv
Par
13,17
concentration basis
K
K = fl c
(mol
m ) 13,17
molality basis
Km
Km fl B'
fugacity
f,
VB
= 2B lim (PB/2B)T
(mol kg' ) 13,17 Pa
9
p—*o
fugacity coefficient
Henry's law constant
4B =fB/PB kH
lim (JB/XB)
1
Pa
9, 18
1
9,19
a,,,,B = Ym,BB/1fl
1
a B = y, B CB/C
1
aX,B =
1
9, 20 9, 20 9, 20
kH,B =
XB
0
= (afB/xB)XB=O activity coefficient
referenced to Raoult's law f referenced to Henry's law molality basis concentration basis mole fraction basis ionic strength, molality basis concentration basis
'm,
I' I
= OB/XB
YX,BXB
'm = mBZB2
mol kg1
I = CBZB2
mol m
(15) This quantity applies in general to a system which is not in equilibrium. (16) This quantity is equal to the value of Q in equilibrium, when the affinity is zero. It is dimensionless and its
value depends on the choice of standard state, which must be specified. ISO [Si] and the IUPAC Thermodynamics Commission [24] recommend the symbol K and the name 'standard equilibrium constant', but some thermodynamicists prefer the symbol K and the name 'thermodynamic equilibrium constant'. (17) These quantities are not in general dimensionless. One can define in an analogous way an equilibrium constant in terms of fugacity K1, etc. At low pressures K,, is approximately related to K by the equation K and similarly in dilute solutions K is approximately related to K by K K/(c ); K,,/(p however, the exact relations involve fugacity coefficients or activity coefficients [24]. The equilibrium constant of dissolution of an electrolyte (describing the equilibrium between excess solid phase and solvated ions) is often called a solubility product, denoted K01 or K (or K,1 or K as appropriate). In a similar way the equilibrium constant for an acid dissociation is often written Ka, for base hydrolysis Kb, and for water dissociation K. (18) Henry's law is sometimes expressed in terms of molalities or concentrations and then the corresponding units of the Henry's law constant are Pa kg mol1 or Pa m3 mol 1, respectively. (19) This quantity applies to pure phases, substances in mixtures, or solvents. (20) This quantity applies to solutes.
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