Oxidation and Reduction Section 1

Review of Terms

In Section 8.3 those reactions that involved a transfer of electrons were classified as oxidation-reduction reactions. Many definitions were given. In this chapter on oxidation-reduction, we will remind you of those definitions. For a more complete discussion of the material, reread Section 8.3.

When an iron nail is immersed in a solution of copper(II) ion, a thin coating of copper is deposited on the nail and iron(II) ion is found in the solution. The equation for this reaction is:

Note that we have used a net ionic equation to describe the reaction. The essential components of an oxidation-reduction reaction are much more easily identified in a net ionic equation than in a formula equation. We will use almost exclusively net ionic equations in this chapter.

Below each component of the equation we have written its oxidation number. We have done so to stress that a reaction is classified as oxidation-reduction if in its course two elements change oxidation number; one must increase in oxidation number, and the other must decrease in oxidation number. Table 14.1 reviews the rules for assigning oxidation numbers. A complete discussion of the subject is in Section 6.2.

**TABLE 14.1** Rules for assigning oxidation numbers
Substance	Oxidation number
any uncombined element	0
a monatomic ion	the charge on the ion
oxygen (except in peroxides)	-2
in a compound	the sum equals zero
in a polyatomic ion	the sum equals the charge on the ion

In the reaction whose equation is shown above, iron has been oxidized: its oxidation number has increased from 0 to +2. If an element's oxidation number increases, the element must have lost electrons, which is the criterion for oxidation. Therefore, iron was oxidized in the reaction. Table 14.2 lists the criteria of oxidation and reduction and states that the substance oxidized is the reducing agent in the reaction. Therefore, iron is the reducing agent in this reaction.

Copper(II) ion is reduced in this reaction. It meets the requirements of having decreased in oxidation number and gained electrons. Copper(II) ion is the oxidizing agent in this reaction.

**Table 14.2** Criteria for oxidation and reduction
Substance oxidized	Substance reduced
loses electrons	gains electrons
attains a more positive oxidation number	attains a more negative oxidation number
is the reducing agent	is the oxidizing agent

We will see later that, in the balanced equation for this or any other oxidation-reduction reaction, the number of electrons lost by the substance oxidized equals the number of electrons gained by the substance reduced. Electrons can neither be plucked from the air to use in an equation nor left over after a reaction.

Example:

When hydrogen sulfide is bubbled through an acidic solution of iron(III) chloride, iron(II) ion and uncombined (free) sulfur are obtained. The net ionic equation for this reaction is:

2 Fe³⁺ + HS^- 2 Fe²⁺ + S(s) + H⁺

For this reaction identify the substance oxidized, the substance reduced, the oxidizing agent, and the reducing agent.

Solution

1. Assign an oxidation number to each element each time it occurs in the equation.

2 Fe³⁺ + HS^- 2 Fe²⁺ + S + H⁺

+3 +1, -2 +2 0 +1

2. Iron has changed from +3 to +2. Iron has decreased in oxidation number, therefore it has gained electrons and has been reduced. The iron(III) ion is the oxidizing agent.

3. Sulfur has changed from -2 in the bisulfide ion, HS^-, to free sulfur with an oxidation number of 0. The sulfur has lost electrons; it has been oxidized. The bisulfide ion is the reducing agent.

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