Chemistry - A Molecular Science

Chapter 11 Electron Transfer and Electrochemistry

are gained in the reduction half-reaction. Summing the two half-reactions yields the net reaction.

Electrons are neither created nor destroyed in a redox process, so

the number of

electrons lost in the oxidation half-reaction

must equal the number gained in the reduction

half-reaction

. Electrons appear in half-reactions, but

they never appear in a net reaction!

Although we will use tabulated half-reactions to construct balanced redox reactions, it

is informative to see why some half reactions contain atoms that are not involved in the electron transfer. Consider the half-reaction for the reduction of NO

1- 3

to NO.

NO

1-^3

→

NO

The nitrogen atom is being reduced from +5 in NO

1- 3

to +2 in NO, so three electrons are

required to reduce NO

1- 3

to NO.

3e

1- + NO

1- 3

→

NO

However, neither the charge nor the number

of oxygen atoms is balanced in the above

equation. Water serves as a source of ox

ygen atoms in aqueous solutions, so H

O 2

molecules must be added to the right side

to balance the oxygen atoms. However, the

addition of water also introduces hydrogen atoms. Recall from Section 10.5 that an Arrhenius acid is a substance that produces H

1+ ions in water, so H

1+ ions can be used to

balance hydrogen atoms and charge in acidic

solutions. The half-reaction for the reduction

of NO

1- 3

in acid is 1-3e

+ 4H

1+ + NO

1- 3

→

NO + 2H

O 2

Both the atoms and the charge are now balanced. Nitrate ion in the presence of H

1+ is

nitric acid, a strong acid and a strong oxidant. The above half-reaction would be used in any reaction involving nitric acid as an oxidant.

We have seen that Fe spontaneously redu

ces copper(II) ion. We know from experience

that iron reacts with water to form rust, but

the copper in water pipes does not corrode.

How can we predict whether a redox reaction is

spontaneous? To answer that question,

we introduce an important field of chemistry known as electrochemistry and then use it to quantitatively rate the oxidizing and reducing powers of reactants.

11.3

GALVANIC CELLS Recall that free energy is released in all spont

aneous processes, and it is the energy that is

free to do work. When steel wool

is placed in a solution of Cu

2+, the released free energy

of the spontaneous redox reaction simply warm

s the solution. However, the released free

energy can be harnessed to extract work from the reaction by separating the two half-