CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 22. Oxidation-Reduction Reactions

TABLE22.1:(continued)

Oxidation Reduction

Complete loss of electrons (ionic reaction) Complete gain of electrons (ionic reaction)

Shift of electrons away from an atom in a covalent bond Shift of electrons toward an atom in a covalent bond

Gain of oxygen Loss of oxygen

Loss of hydrogen in a molecular compound Gain of hydrogen in a molecular compound

Increase in oxidation number Decrease in oxidation number

Sample Problem 22.2: Identifying Oxidation and Reduction

Use changes in oxidation number to determine which atoms are oxidized and which atoms are reduced in the
following reaction. Identify the oxidizing and reducing agent.

Fe 2 O 3 (s) +3CO(g)→2Fe(s) + 3CO 2 (g)

Step 1: Plan the problem.

Use the oxidation number rules to assign oxidation numbers to each atom in the balanced equation. Coefficients do
not affect oxidation numbers. The oxidized atom increases in oxidation number, and the reduced atom decreases in
oxidation number.

Step 2: Solve.

+ 3

Fe 2

− 2

O 3 (s)+ 3

+ 2

C

− 2

O(g)→ 2

0

Fe(s)+ 3

+ 4

C

− 2

O 2 (g)

The element carbon is oxidized because its oxidation number increases from +2 to +4. The iron(III) ion within the
Fe 2 O 3 is reduced because its oxidation number decreases from +3 to 0. Carbon monoxide (CO) is the reducing agent
since it contains the element that is oxidized. Fe 2 O 3 is the oxidizing agent since it contains the ion that is reduced in
the reaction.

Practice Problem

1. Identify which atom is oxidized and which atom is reduced in the following equations.
   a. Zn(s) + H 2 SO 4 (aq)→ZnSO 4 (aq) + H 2 (g)
   b. 2Ag 2 O(s)→4Ag(s) + O 2 (g)

Reactions That are not Redox Reactions

As we have seen, a redox reaction must involve a change in oxidation number for at least two of the atoms involved in
the reaction. The oxidized element increases in oxidation number, while the reduced element decreases in oxidation
number. Single-replacement reactions are generally redox reactions because two different elements appear as free
elements (with an oxidation number of zero) on one side of the equation and as part of a compound on the other side.
Therefore, its oxidation number must change. Combustion reactions are redox reactions because elemental oxygen
(O 2 ) acts as the oxidizing agent and is, itself, reduced. Most combination and decomposition reactions in which a
compound is either formed from, or broken down into, its pure elements are redox reactions.

As seen in the following video, when aluminum metal comes into contact with bromine, a vigorous reaction occurs
and produces aluminum bromide. Combination reactions are redox reactions because free elements are made into a
compound. http://www.youtube.com/watch?v=MZzjww5jQPo (1:14)