PHYSICAL CHEMISTRY IN BRIEF

CHAP. 6: THERMODYNAMICS OF HOMOGENEOUS MIXTURES [CONTENTS] 162

S Symbols: The values of thermodynamic quantities in the standard state of an ideal gas at the

standard pressure are denoted using the superscript◦, e.g.Hm◦,i, Sm◦,i,···.

The values of thermodynamic quantities in the standard state of a pure component at the

temperature and pressure of the system are denoted using the superscript•, e.g.Hm•,i, Sm•,i,···.

The values of thermodynamic quantities in the standard state of a hypothetical pure sub-

stance at infinite dilution will be denoted using the superscript[x], e.g.Hm[x,i], Sm[x,i],···.

The values of thermodynamic quantities in the standard state of a unit molar concentration

will be denoted using the superscript[c], e.g.Hm[c],i, S[mc],i,···.

The values of thermodynamic quantities in the standard state of unit molality will be

denoted using the superscript[m], e.g.Hm[m,i], Sm[m,i],···.

Note:For the activity and the activity coefficients, however, symbolsa[ix],a[ic],a[im],γ[ 1 x],

γ[ic],γ[im]are used in a different meaning [see6.5.4,6.5.5].

Figure6.3illustrates the standard values of enthalpy of component 1 in a binary mixture.

6.5.4 Activity

The activity of theithcomponent in a mixture is defined by the relation

ai=

fi

fist

, (6.89)

wherefistis the fugacity in the chosen standard state [see6.1.2].
U Main unit:dimensionless quantity.
An alternative definition of activity follows from the relation for fugacity (6.74)

ai= exp

(

μi−μsti

RT

)

= exp

(

Gi−Gstm,i

RT

)

. (6.90)

• The dependence of activity on temperature is expressed by the relation

lnai(T 2 ) = lnai(T 1 ) +

∫T 2

T 1

(

∂lnai

∂T

)

p,n

dT , (6.91)