Fundamentals of Electrochemistry - W.H. Freeman

1504T_ch14_270-297 04/06/06 20:12 Page 278
Figure 14-7 Cell used to measure the
standard potential of the reaction
Ag e T Ag(s). This cell is hypothetical
because it is usually not possible to adjust the
activity of a species to 1.
H 2 (g)
e –
+0.799 V
– +
Salt bridge
(A H2 = 1) H +
Glass
tube
Bubbles
of H 2
Pt
(A H + = 1) Ag +
(A Ag + = 1)
Ag
Pt(s) H H + 2 (g, A = 1) (aq, A = 1) Ag + (aq, A = 1) Ag(s)
Standard hydrogen electrode
(S.H.E.)
Question What is the pH of the standard
hydrogen electrode?
We will write all half-reactions as reductions.
By convention, E° 0 for S.H.E.
Walther Nernst appears to have been the first
to assign the potential of the hydrogen
electrode as 0 in 1897. 10
We always attach the left-hand electrode to
the negative terminal of the potentiometer
and the right-hand electrode to the positive
terminal. The voltage on the meter is the
difference:
Voltage right-hand electrode potential
left-hand electrode potential
Challenge Draw a picture of the cell
S.H.E. Cd 2 (aq, A 1) 0 Cd(s) and show
the direction of electron flow.
The left half-cell, connected to the negative terminal of the potentiometer, is called the
standard hydrogen electrode (S.H.E.). It consists of a catalytic Pt surface in contact with an
acidic solution in which A H
1. A stream of H 2 (g) bubbled through the electrode saturates
the solution with H 2 (aq). The activity of H 2 (g) is unity if the pressure of H 2 (g) is 1 bar.
The reaction that comes to equilibrium at the surface of the Pt electrode is
(14-11)
We arbitrarily assign a potential of 0 to the standard hydrogen electrode at 25°C. The voltage
measured by the meter in Figure 14-7 can therefore be assigned to Reaction 14-10, which
occurs in the right half-cell. The measured value E° 0.799 V is the standard reduction
potential for Reaction 14-10. The positive sign tells us that electrons flow from left to right
through the meter.
We can arbitrarily assign a potential to Reaction 14-11 because it serves as a reference
point from which we can measure other half-cell potentials. An analogy is the arbitrary
assignment of 0°C to the freezing point of water. Relative to this freezing point, hexane boils
at 69° and benzene boils at 80°. The difference between boiling points is 80° 69° 11°.
If we had assigned the freezing point of water to be 200°C instead of 0°C, hexane boils at
269° and benzene boils at 280°. The difference between boiling points is still 11°. Regardless
of where we set zero on the scale, differences between points remain constant.
The line notation for the cell in Figure 14-7 is
Pt(s) 0 H 2 (g, A 1) 0 H (aq, A 1) Ag (aq, A 1) 0 Ag(s)
or
S.H.E. Ag (aq, A 1) 0 Ag(s)
By convention, the left-hand electrode (Pt) is attached to the negative (reference) terminal of
the potentiometer and the right-hand electrode is attached to the positive terminal. A standard
reduction potential is really a potential difference between the potential of the reaction
of interest and the potential of S.H.E, which we have arbitrarily set to 0.
To measure the standard potential of the half-reaction
we construct the cell
H (aq, A 1) e T 1 2 H 2(g, A 1)
Cd 2 2e T Cd(s)
S.H.E. Cd (aq, A 1) 0 Cd(s)
(14-12)
with the cadmium half-cell connected to the positive terminal of the potentiometer. In this
case, we observe a negative voltage of 0.402 V. The negative sign means that electrons
flow from Cd to Pt, a direction opposite that of the cell in Figure 14-7.
Appendix H contains standard reduction potentials arranged alphabetically by element.
If the half-reactions were arranged according to descending value of E° (as in Table 14-1),
278 CHAPTER 14 Fundamentals of Electrochemistry