Chemistry - A Molecular Science

Chapter 11 Electron Transfer and Electrochemistry

G = -nΔ

FE

= -96,500n

Eq E

11.3

Equation 11.3 indicates that the cell potential (

(^) E) becomes more positive as the free energy
change of the redox reaction becomes more
negative, which means that the amount of
work that can be done by
each
electron increases as the cell potential becomes more
positive.
A galvanic cell converts chemical potential energy into electrical energy by physically
separating the two half-reactions and incorpor
ating them into an electrical circuit.
Electrochemical reactions occur
at the surfaces of metals, the
electrodes
that are placed
into each half-cell. Electrons are injected into the external circuit in the oxidation half-cell (anode
) as the donor is oxidized and withdrawn from the circuit in the reduction half-cell
(cathode
) as the acceptor is reduced. A typical galvanic cell consists
of four components:
anode half-cell,
cathode half-cell
, liquid junction
, and
load
(^).

• The anode half-cell houses the oxidation couple (Fe

2+/Fe). The anode is always the

half-cell in which oxidation occurs

. Oxidation is the loss of electrons, so electrons leave

the anode during reaction. In the cell shown in Figure 11.2, the anode half-cell consists of a piece of metal (Fe) immersed in 1 M FeSO

(Fe 4

2+). The metal serves as the anode

(the electrode for the oxidation half-reaction). Iron atoms at the surface of the anode each give up two electrons, which leave the anode and enter the electrical circuit. The resulting Fe

2+ ions enter the solution as the iron electrode

dissolves

. The iron anode is

an

active electrode

because it participates in the reaction.

+

+0.78 V

Fe

Fe

Fe + 2e

®

2+

-1

Cu + 2e

Cu

2+

1-®

Anode

Cathode

1-Cl

1+K

1 M FeSO

4

1 M CuSO

4

Cu

SaltBridge

Lo-

Hi+

1-e

1-e

2e

1-

Figure 11.2 A galvanic cell The galvanic cell using the Cu

2+/Cu and Fe

2+/Fe redox couples.

Negative charge flows counterclockwise through the above cell with the electrons carrying the charge in the external circuit and the ions carrying it through the so

lution and the salt bridge.

• The cathode half-cell houses the reduction couple

(Cu

2+/Cu). The cathode is always

the half-cell in which reduction occurs

.^ Reduction is the gain of

electrons, so electrons

enter the cathode during reaction. In the cell

shown in Figure 11.2, the cathode half-cell

consists of a piece of metal (Cu) immersed in 1 M CuSO

(Cu 4

2+). The metal servers as

the cathode (the electrode for the reduction half-reaction). Electrons move from the anode to the cathode, where they transfer to Cu

2+ ions in solution at the electrode

surface. The metallic copper that re

sults from the reduction of the Cu

2+ ions deposits on

and becomes a part of the cathode, whic

h is also an active electrode.

• The liquid junction serves as a barrier to prevent mixing of the two half-cell solutions

while allowing free movement of ions between them. A common liquid junction is a porous polymeric or ceramic material that is

crisscrossed by very small, open channels

through which ions can freely migrate. A spec

ial type of liquid junction is the salt bridge,

which typically contains a gelled saturated KCl solution held within an open glass tube. As with any liquid junction, the salt bridge

electrically “connects” the two half-cell

solutions to allow charge to flow, while keeping the solutions physically separate. Each electron that flows through the circuit carri

es one unit of negative charge out of the

anode compartment and into the cathode com

partment. However, electrical neutrality

must be maintained in each of these compartm

ents, and the salt bridge is a reservoir of

ions that can be used to maintain electric

al neutrality. An electron flowing into the

cathode can be balanced by either an anion fl

owing out of the cathode and into the salt

bridge or by a cation flowing out of the bridge and into the cathode. For example, when