PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 250

and

∆rG◦=

∑k

i=1

νiG◦m,i, (8.27)

where the summation is performed over all substances involved in the reaction.

• Calculation from∆fHi◦andS◦m,i
  Using this data we may calculate ∆rH◦and ∆rS◦for a given temperature [see5.1]. Hence

∆rG◦= ∆rH◦−T∆rS◦ (8.28)

Note:The same procedure could be also applied in the preceding case.

Note:At the standard temperature, ∆fHi◦equalsHm◦,ifor the most stable modification.

∆fHi◦of elements are zero at all temperatures.

• Calculation from∆fG◦iorG◦m,i
  We again calculate ∆rG◦from relation (8.27) or, alternatively

∆rG◦=

∑k

i=1

νi∆fG◦i (8.29)

and the equilibrium constant from relation (8.25).

Note:∆fGiof elements in their most stable modification equals zero,G◦m,iof elements is

not zero, not even at the standard temperature (if this is not zero absolute temperature)

due to the non-zero entropy.

• Calculation from thelogarithms of the equilibrium constants of formation

Kf,i= exp

(

−

∆fG◦i

RT

)

. (8.30)

For the equilibrium constant of the given reaction it holds

lnK=

∑k

i=1

νilnKf,i. (8.31)