Physical Chemistry , 1st ed.

where again we omit the units for clarity. It is easy to see that the sum of the

two partial pressures equals 0.750 atm. The equilibrium constant is calculated

using these pressures:

K

(p

p

I

I

/

2 /

p

p

°

°

)^2



(0

0

.

.

5

2

1

3

5

5

)^2

1.13

The value of the equilibrium constant suggests that there is about the same

amount of products as reactants. The molar amounts as well as the equilib-

rium partial pressures also support this.

The extent can be determined from the initial and equilibrium amounts

of molecular iodine:

    

    0.00314 mol

This is consistent with a reaction whose equilibrium positions itself about

halfway between pure reactants and pure products.

5.4 Solutions and Condensed Phases

Up to this point the equilibrium constants have been expressed in terms of
partial pressures. However, for real gases the fugacities of the species should be
used. If the pressures are low enough, the pressures themselves can be used,
since at low pressures the pressure is approximately equal to the fugacity. But
many chemical reactions involve phases other than the gas phase. Solids, liq-
uids, and dissolved solutes also participate in chemical reactions. How are they
represented in equilibrium constants?
We answer this by defining activity aiof a material in terms of its standard
chemical potential °iand its chemical potential iunder nonstandard pres-
sures:

i°iRTln ai (5.11)

Comparison of this equation with equation 4.58 shows that for a real gas, ac-
tivity is defined in terms of the fugacity as

agasf

p

ga

°

s (5.12)

Reaction quotients (and equilibrium constants) are more formally written in
terms of activities, rather than pressures:

iproducts

a^ i

Q  (5.13)

jproducts

aj^ j

This expression applies no matter what the state of the individual reactant or
product.
For condensed phases (that is, solids and liquids) and dissolved solutes,
there are different expressions for activity, although the definition from equa-
tion 5.11 is the same for all materials. For condensed phases, the activity of a

2.86  10 ^3 mol 6.00  10 ^3 mol



 1

5.4 Solutions and Condensed Phases 129