PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 368

11.4.1.3 Ions

The same standard states are chosen for ions in a solution as for an undissociated electrolyte
(i.e. unit molar concentrations or unit molalities). For an infinitely diluted solution equation
(11.37) applies.

Note:The significant difference between the description of an undissociated substance and

that of ions is due to the fact that the activity coefficients of the ions are considerably

different from unity even in dilute solutions.

11.4.2 Mean molality, concentration, activity and activity coefficient

Let us consider a strong electrolyte of the molalitymwhich dissociates into ions according to
equation (11.1)
Kν+Aν−=ν+Kz

• +ν−Az
  −
  .

The mean molality of the ionsm±is defined by the relation

m±=

[(

m+

)ν+(

m−

)ν−]^1 ν

, (11.38)

whereν=ν++ν−,m+=ν+mandm−=ν−m. From this it follows that

m±=m

[(

ν+

)ν+(

ν−

)ν−]ν^1

. (11.39)

The mean concentrationc±, the mean activitiesa[±c],a[±m]and the mean activity coefficients

γ[±c],γ±[m]are defined in a similar way.

c±=

[(

c+

)ν+(

c−

)ν−]^1 ν

, a±=

[(

a+

)ν+(

a−

)ν−]^1 ν

, γ±=

[(

γ+

)ν+(

γ−

)ν−]^1 ν

. (11.40)

Note:The mean activities and mean activity coefficients are introduced because the ac-

tivities and activity coefficients of ions are experimentally unavailable.