Chemistry - A Molecular Science

Chapter 11 Electron Transfer and Electrochemistry

11.1

ELECTRON TRANSFER OR REDOX REACTIONS The solution of CuSO

in Figure 11.1a has a deep blue color due to the presence of 4

aqueous Cu

2+ ions. When steel wool, which is primarily iron, is immersed into such a

solution and allowed to sit for a time, the co

lor of the solution begins to pale, and iron

atoms in the steel dissolve to be replaced with a red-brown coating of metallic copper (Figure 11.1b). Our conclusion is that the Cu

2+ ions are converted into Cu atoms as the

blue color fades and metallic copper forms. The

steel wool dissolves as the Fe atoms in the

steel wool enter the solution as Fe

2+ ions. The chemical equation for the reaction is

Fe(s) + Cu

2+(aq)

→

Fe

2+(aq) + Cu(s)

In the reaction, an Fe atom donates two electrons to a Cu

2+ ion, so Fe is an

electron donor

and Cu

2+

is an

electron acceptor

. The loss of two electrons increases the oxidation state

of Fe from 0 to +2, and the gain of two electrons by Cu

2+ reduces its oxidation state from

+2 to 0. The loss and gain of electrons are such important processes as are the substances involved in the electron transfer that they are given special names,

Oxidation

is a loss of electrons. Oxidation incr

eases the oxidation state of an atom.

Reduction

is a gain of electrons.* Reduction reduces the oxidation state of an atom.

Oxidants

or

oxidizing agents

are substances that oxidize other substances. They do so

by accepting electrons from the other subs

tance. The gain of electrons reduces them.

Reductants

or

reducing agents

are substances that reduce other substances. They do so

by donating electrons to the other substanc

e. The loss of electrons oxidizes them.

Electron transfer reactions always involve both red

uction and ox

idation, so they are

frequently referred to as

redox reactions

.

Steel (Fe) wool

Blue solutionof Cu ions

2+

Colorless solutionof Fe ions

2+

Red coating ofCu on steel wool

Allow steel wool to sit insolution for severalminutes and remove

(a) (b)

Figure 11.1 Fe + Cu

2+^

→ Fe

2+ + Cu

(a) Steel wool (Fe) and a blue solution of Cu

2+. (b) Steel wool with a

red-brown coating of metallic copper and a solution of Fe

2+. The

slight blue tint of the solution is due to unreacted Cu

2+.

* There are several mnemonics that aid in remembering these

terms. For example, “OIL RIG” stands for

xidation O

s I

oss of L

electrons, and

Reduction

Is

Gain of electrons.

Electrons transfer from a set valence

orbitals on the reductant, called the

donor

orbitals

, into a set of unfilled orbitals on the oxidant called the

acceptor orbitals

. Thus,

electron transfer usually empties the donor or

bitals, but once emptied, these orbitals can

accept electrons;

i.e

., a redox reaction converts the reductant into an oxidant. Similarly,

once the acceptor orbitals have accepted electrons, the orbitals have electrons they can donate; i.e., the acceptor orbitals are convert

ed into donor orbitals. The oxidant and

reductant that are converted into one another form a

redox couple

. We represent redox

couples as oxidant/reductant. Each redox reac

tion has two redox couples. The two couples

in the reaction we are studying are the Fe

2+/Fe couple and the Cu

2+/Cu couple.

The abbreviations OX and RED, are used for oxidant and reductant, respectively. A