PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 251

• Calculation from the GiauqueG-functions, which are usually tabulated for individual
  chemical substances either as(
  −

G◦m,i(T)−Hm◦,i(298)

T

)

or

(

−

G◦m,i(T)−Hm◦,i(0)

T

)

,

where (

−

G◦m,i(T)−Hm◦,i(298)

T

)

=Sm◦,i(T)−

Hm◦,i(T)−Hm◦,i(298)

T

(8.32)

or (

−

G◦m,i(T)−Hm◦,i(0)

T

)

=Sm◦,i(T)−

Hm◦,i(T)−Hm◦,i(0)

T

. (8.33)

For the calculation of ∆rG◦or the equilibrium constants we need to know ∆rH◦(298) in the
first case, or ∆rH◦(0) in the second case.

lnK=

1

R

[k

∑

i=1

νi

(

−

G◦m,i(T)−Hm◦,i(298)

T

)

−

∑k

i=1

νi

∆fHi◦(298)

T

]

(8.34)

or

lnK=

1

R

[k

∑

i=1

νi

(

−

G◦m,i(T)−Hm◦,i(0)

T

)

−

∑k

i=1

νi

∆fHi◦(0)

T

]

. (8.35)

Example

Calculate the equilibrium constant of water vapour formation

0 =H 2 O(g)−H 2 (g)−

1

2

O 2 (g)

at 400 K if:

substance −

(G◦(400)−H◦(298)

400

)

/J mol−^1 K−^1

H 2 131. 721

O 2 205. 029

H 2 O 190. 079

and∆fH◦(298)for water vapour is− 241. 827 kJ mol−^1.