Concise Physical Chemistry

c14 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

232 ELECTROCHEMICAL CELLS

m1/2

0.0 0.1 0.2 0.3 0.4 0.5

γ±

0.75

0.80

0.85

0.90

0.95

1.00

FIGURE 14.3 The mean activity coefficient of HCl as a function ofm^1 /^2. The valueγ±=

− 0 .789 calculated in Exercise 14.2 is the penultimate point on the curve reading from left to

right. The standard value isγ±=− 0 .797.

becomes

logγ±=

−E+E◦+ 0. 1184

0. 1184

=

−. 353 +. 2224 + 0. 1184

0. 1184

=− 0. 10304

γ±=− 0. 789

Experimental values ofEat several specific values ofmlead to specific values of

γ±as a function ofm^1 /^2 in Fig. 14.3. The values are less than 1.0 as predicted by

the Debye–Huckel theory. The function is not linear except at low concentrations. ̈

Experimental points deviate from the linear function.

Problem 14.1

Suppose we construct a Daniell cell (Section 14.1), conventional in every way except

that we replace the Zn^2 +(aq) and Zn(rod) with Cd^2 +(aq) and a Cd(s) rod.

(a) Which is the cathode and which is the anode, the copper cup or the Cd rod?

(b) What is the approximate voltage? (It is approximate because we don’t know

the exact concentrations of the Cu(aq) and Cd(aq) solutions.)

Problem 14.2

Many applications of electrochemistry have been in chemical analysis. Suppose

aZn;Zn^2 +concentration cell is set up in which the Zn^2 +concentration in one cell