two electrons flow into the cathode co

mpartment shown in Figure 11.2, a Cu

2+ ion is

reduced to Cu metal. The reduction results in excess SO

2- 4

ions in solution. The excess

negative charge must then be neutralized either by two K

1+ ions flowing from the salt

bridge into the cathode compartment or by a su

lfate ion flowing into the salt bridge. The

excess charge of the Fe

2+ ion formed in the anode compartment can be balanced by

two chloride ions flowing from the salt bridge or by the Fe

2+ ion flowing into the bridge.

-^

The load is simply a device that uses the energy released by the transferred electrons; it can be anything operated with a battery. A

voltmeter, which measures the electrical

potential difference between the cathode and a

node, is the load in Figure 11.2. One

terminal is labeled ‘Hi’ or ‘+’ and one labeled ‘Lo’ or ‘-’ and the cell potential is determined as

(^) E
=^
E Hi
-E^
Lo
, which is +0.78 V in the cell shown in Figure 11.2.
Electrons are negatively charged, so th
eir free energy depends upon the electrical
potential they experience; the more positive
the potential, the lower their free energy.*
Thus, electrons flow spontaneously from lower (more negative) electrical potential to higher (more positive) electrical potential because doing so lowers their free energy.

• The effective nuclear charge (Sec
  tion 3.2) represents the electrical
  potential experienced by the valence electrons. Recall that orbital energies decrease as Z
  increases. This is equivalent to saying eff
  that orbital energies decrease as the electrical potential they experience becomes more positive.
  † E
  , not
  ΔE
  is used even though it is the potential difference because ,
  is defined as the potential difference. E
  The potential difference experienced by electrons flowing from point A with an
  electrical potential of
  EA
  to point B where the electrical potential is
  EB
  is defined as their
  final electrical potential (
  EB
  ) minus their initial electrical potential (
  EA
  );
  i.e
  .,
  = E
  EB

• EA

†. If

EB

is more positive than

EA

, then

0 (the electrical potential increases) and the electrons E

flow spontaneously from A to B. The same c

onsiderations hold for electrochemical cells.

Electrons flow from those species that are being

oxidized at the anode to those species that

are being reduced at the cathode, so the cell potential is

= E

Ecathode

• 
  

Eanode

. T

he potential

of the electron after it is transferred is related to the

cathode half-cell potential

, E

cathode

,

while the potential of the electron before

it is transferred is related to the

anode half-cell

potential

, E

anode

. If

(^) Ecathode

(^) Eanode
, then the electrons experi
ence an increase in their
electrical potential (
0), and they flow spontaneously from the anode to the cathode. E
All reactants in the cell shown in Figure 11.
2 are in their standard state, so the cell
potential is the potential difference betw
een the cathode and the anode under
standard
conditions. Consequently it is called the
standard cell potential,
o (^) E
cell
.
oE
cell

o (^) E
cathode

-^ E

o anode

Eq. 11.4

o (^) E
cathode
and
E
oanode
are the standard half-cell potentials

. The extent of reaction depends

upon

GΔ

o, which can be determined from Equation 11.3 to be oGΔ

= -n

FE

o

Eq. 11.5

GΔ

o < 0 if

o (^) E
cell
is positive, so a reaction is extensive if the standard cathode half-cell
potential (
o (^) E
cathode
) is greater than the standa
rd anode half-cell potential (
o (^) E
anode
). Thus, we
Chapter 11 Electron Transfer and Electrochemistry