Chemistry - A Molecular Science

reactions and forcing the transferred electrons to move through an external circuit. The marriage of electrical conduction through a circuit and redox chemistry is called electrochemistry

. The apparatus used is called an

electrochemical cell

and each half-

reaction usually takes place in its own

half-cell

. Thus, electrochemical cells contain two

half-cells just as redox reactions contain two half-reactions.

Electrochemical cells allow us to gain contro

l of the free energy of the electrons in a

redox reaction. We can extract work from s

pontaneous redox reactions in electrochemical

cells called

galvanic cells

, or we can use an external power supply to change the relative

free energies of the reactants and force non-spontaneous reactions to proceed in electrochemical cells called

electrolytic cells

. When a battery is discharging, it is a

galvanic cell, but when it is recharging, it is

an electrolytic cell. Galvanic cells are the

major focus of this chapter,

but electrolytic cells are discussed briefly in Section 11.8.

The maximum work that can be obtained from the electrons in a redox reaction equals

the free energy decrease in the reaction;

i.e

.,^

Work done by electrons = -

ΔG Eq.

11.1

Electrochemical cells were investigated a

bout 60 years prior to the discovery of the

electron, so the early electrochemists expr

essed the work done in terms of charge and

electrical potential rather than electrons a

nd energy change. The absolute value of the

charge on a mole of electrons is called the

faraday

and given the symbol

F

. A faraday is

Avogadro’s number times the absolute valu

e of the charge on a single electron.

1 F

= (6.022

x^10

23 electrons/mol)(1.602

x

10

-19

C/electron) = 96,500 C/mol.

The work done by n moles of electrons (a charge of n

F coulombs) being transferred

through an electrical potential

E is given in Equation 11.2.

Work done by electrons = n

FE

Eq

11.2

nF

is the magnitude of the charge in coulombs that is transferred, and

E is the electrical

potential difference through which the electrons move expressed in volts. A

volt (V)

is a

joule per coulomb (1V = 1 J/C), so Equation 11.2 gives the work done by the electrons in joules. Rearranging Equation 11.2 shows that th

e electrical potential equals the work done

by the electrons divided by the num

ber of coulombs. In other words,

E^

is the work that can

be done by each coulomb of charge

. Combining Equations 11.1 and 11.2 gives us the

following relationship between the free energy of the redox reaction and the voltage that would be measured in the corresponding electrochemical cell:

Chapter 11 Electron Transfer and Electrochemistry