Module 5

Lesson 1.5 Oxidation Numbers and Redox Reactions

Oxidation numbers can be used to differentiate redox reactions from non-redox reactions.

To identify a redox reaction:

Assign oxidation numbers to all entities in the reaction.
Identify substances that change in oxidation number.

If the oxidation number of an atom or ion changes during a chemical reaction, the reaction is a redox reaction. A reaction in which all oxidation numbers remain the same is not a redox reaction.

Return to the reaction involving copper coil and aqueous silver nitrate.

\( \mathrm { Cu(s) + 2 AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2 Ag(s) } \)

Oxidation numbers can be used to

prove this is a redox reaction
identify which atom/ion is oxidized and which atom/ion is reduced
identify the oxidizing agent (OA) and the reducing agent (RA)

First, assign oxidation numbers to all atoms in the balanced equation.

Fig. 1

Next, determine whether the oxidation number of an atom/ion has changed because of the reaction.

If the oxidation number of an atom/ion has increased (that is, become more positive), the atom/ion has undergone oxidation and is, therefore, the reducing agent in the reaction.

If the oxidation number of an atom/ion has decreased (that is, becomes less positive), the atom/ion has undergone reduction and is, therefore, the oxidizing agent in the reaction.

Notice how the copper atom and the silver atom have both changed in oxidation number. The changes in oxidation number confirms that the reaction of copper solid with silver nitrate solution is a redox reaction.

Fig. 2

The copper atom lost electrons in the above reaction. We know this because its oxidation number increased (that is, changed to a more positive value on the number line).

The silver ion gained electrons in the above reaction because its oxidation number decreased (that is, changed to a more negative value on the number line).

Fig. 3

Is the reaction below a redox reaction? Use oxidation numbers to find out.

Fig. 4

Because no atom changed in oxidation number, this is NOT a redox reaction.

To summarize, oxidation numbers can be used to identify the oxidized and reduced species in chemical reactions. Therefore, they can be used to differentiate redox reactions from non-redox reactions.

Read "Oxidation Numbers and Redox Reactions" on pages 585 to 588 of the textbook.

Complete Practice Questions 6 to 8 on pages 588 and 589 of the textbook. Click on the banner below to check your work.

Self-Check Answers

Page 588 Practice Question 6

Carbon is oxidized from -2 in methanol to 0 in methanal.
Manganese is reduced from +7 in permanganate ion to +2 in manganese (II) ion.

CH₃OH(l) is the reducing agent.

MnO₄^- is the oxidizing agent.

Page 588 Practice Question 7

This is a redox reaction.

Cu(s) is the reducing agent.

AgNO₃(aq) is the oxidizing agent
This is not a redox reaction.
This is a redox reaction.

KI(aq) is the reducing agent.

Cl₂(aq) is the oxidizing agent.
This is a redox reaction.

NaCl(l) is the reducing agent.

NaCl(l) is the oxidizing agent.

However, note that this is not a disproportionation reaction.
This is not a redox reaction.
This is a redox reaction: Al(s) is the RA, Cl₂(g) is the OA.
This is a redox reaction.

O₂(g) is the OA.

C₄H₁₀(g) is the RA.
This is a redox reaction.

H₂O₂(l) is the OA.

H₂O₂(l) is the RA.

Note that this is disproportionation because O is undergoing both oxidation and reduction.