Physical Chemistry , 1st ed.

guide about the behavior of an electrochemical system as temperature changes.

Since Fis a relatively large number, the change in E° is slight as the temperature

changes, but there can be a noticeable effect for some common electrochemical

reactions.

Example 8.5

Estimate Efor the following reaction at 500 K:

2H 2 (g) O 2 (g) →2H 2 O (g)

This is the chemical reaction for fuel cells, which among other uses provide

electrical power to the space shuttle.

Solution

First, we determine E° under standard conditions. The above reaction can be

broken down into the half-reactions

2 (H 2 (g) →2H^ 2e^ ) E°0.000 V

O 2 (g) 4H^ 4e^ →2H 2 O () E°1.229 V

The standard EMF for the reaction is therefore 1.229 V.

S° for the reaction is determined by looking up S° values for H 2 ,O 2 , and

H 2 O (all in the gaseous state) in Appendix 2. We get

rxnS°2(188.83) [2(130.68) (205.14)] KJ

rxnS°

88.84 KJ

for the molar reaction. The change in temperature is 500 K 298 K 

202 K. Using equation 8.27, we can estimate the change to E°:

E°



n

S

F

°

T (202 K)

E° 0.0465 V

so that the approximate voltage of the reaction at 500 K is

E1.229 0.0465

E1.183 V

This is a slight but noticeable decrease.

We can easily rearrange equation 8.26 to get an expression for S°:

S°nF

E

T

°

 (8.29)

Now that we have expressions for G° and S° , we can find an expression for

H°. Using the original definition for G(that is,GH TS), we get

nFE°H° TnF

E

T

°



We rearrange this algebraically to get

H°
nFE° T

E

T

°

 (8.30)

This equation allows us to calculate H° for a process using electrochemical

information.

88.84 KJ



(4 mol e^ )(96,485 moCle )

220 CHAPTER 8 Electrochemistry and Ionic Solutions