Chemistry - A Molecular Science

subscript is then used to distinguish substan

ces. For example, if we use 1 for Cu and 2 for

Fe, then OX

= Cu 1

2+ ions (the oxidized form of Cu), and RED

= Fe atoms (the reduced 2

form of Fe). A gain of electrons by OX

converts it into the reducing agent RED 1

. 1

Similarly, the loss of electrons by RED

converts it into an oxidizing agent OX 2

. Thus, as 2

shown in the following, a redox reaction contai

ns oxidants and reductants on both sides:

OX

+ RED 1

→ 2

RED

+ OX 1

2

The various terms introduced to this

point are summarized in the margin.

We saw in Section 9.7 that

G < 0 for all spontaneous processes carried out at Δ

constant temperature and pressure. Thus, elect

rons transfer spontaneously from Fe to Cu

2+^

because their free energy is lower on Cu than on

Fe. The following factors affect the value

of

G: Δ

1. the energy difference between

the donor and acceptor orbitals;

2. the bond energies of any bonds

that must be broken or formed; and

3. entropy effects.

The first factor usually dominates because or

bital energy differences are usually large,

bond energies often cancel in simple process

es, and entropy is seldom important in

reactions that do not involve gases. In general,

The acceptor (unfilled) orbitals on good oxid

ants are at relatively low energy, and the

electrons on good reductants are at relatively high energy.

However, the other factors can dictate the spont

aneous direction of electron transfer when

the orbital energies are not very different. Example 11.1

Use the diagrams for the valence orbitals of A and B in the margin to answer the following. Assume that orbital energy differences dominate

G. Δ

a) Which is the better reductant and which is the better oxidant?

The highest energy electrons on A (green circle in margin figure) are much higher in energy than those on atom B (red circle

), so A is the better reducing agent

. The lowest-

energy unfilled orbital on B (yellow ellipse) is lower in energy than the unfilled orbitals on A, which are too high in energy to be shown, so B is the better oxidizing agent

.

b) Will B oxidize A to A

4+; i.e., is A + 2B

→

A

4+ + 2B

2- spontaneous?

For this reaction to occur, all four valence electrons on A would have to transfer to the empty orbital on B. While the high-energy electrons (green circle) are higher in energy and would transfer, the low-energy electrons (red

circle) are at lower energy than the unfilled

orbital on B and will not transfer. B is not a strong enough oxidant to oxidize A to A

4+.

Cu

• Fe

Cu

• Fe

OX

OX

2+

2+

®

• 
  

RED

RED +

®

12

1

2

electrontransfer
OX

(^1) • Oxidizing agent or oxidant • Substance that is reduced during reaction • Electron acceptor • That half of redox couple 1 in the higher oxidation state
RED
(^2)

• Reducing agent or reductant • Substance that is oxidized during the reaction • Electron donor • That half of redox couple 2 in the lower oxidation state RED

(^1)

• Reduced form of OX

(^1)
OX
(^2) • Oxidized form of RED
(^2)
OX
/RED 1
and OX 1
/RED 2
(^2)
• Redox couples involved in the reaction (^) Energy
A
B
A
B
A
2B
X
Example 11.1
Example 11.1a
Example 11.1b
Chapter 11 Electron Transfer and Electrochemistry