PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 252

Solution

−∆rG◦= 400 (190. 079 −

1

2

205. 029 − 131 .721) + 241827 = 224164. 2 J mol−^1 ,

K= exp(−

∆rG◦

RT

) = 1. 9 × 1029.

• Calculation from thestandard electrode potentialof galvanic cells

If a given reaction can be carried out using a reversible electrochemical cell, then it is also
possible to calculate ∆rG◦from the standard electrode potential of the cellE◦because we have

∆rG◦=−zF E◦, (8.36)

whereFis the Faraday constant andzis the number of electrons exchanged during one mole
of reaction [see11.2.2].

8.4.3 Calculation from the equilibrium constants of other reactions.

For the calculation of ∆rGstjust like ∆rH◦, reactions may be combined [see Hess’s law,5.1.2]

Example

Calculate the equilibrium constant of the reaction

0 =CO 2 (g) + 2H 2 (g)− C(s)− 2 H 2 O(g),

ifK 1 is the equilibrium constant of the reaction

0 = CO 2 (g)−C(s)−O 2 (g)

andK 2 is the equilibrium constant of the reaction

0 = H 2 O(g)−H 2 (g)−

1

2

O 2 (g).