CK-12-Chemistry Intermediate

22.2. Oxidation Numbers http://www.ck12.org

MEDIA

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What types of reactions are not redox reactions? Double-replacement reactions such as the one below are not redox
reactions because ions are simply trading partners without any additional transfer of electrons.

+ 1

Na 2

+ 6

S

− 2

O 4 (aq)+

+ 2

Ba(

+ 5

N

− 2

O 3 ) 2 (aq)→ 2

+ 1

Na

+ 5

N

− 2

O 3 (aq)+

+ 2

Ba

+ 6

S

− 2

O 4 (s)

Note that the oxidation numbers for each element remain unchanged in the reaction.

Acid-base reactions involve a transfer of a hydrogen ion instead of an electron. Acid-base reactions, like the one
below, are also not redox reactions.

+ 1

H

− 1

F(aq)+

− 3

N

+ 1

H 3 (aq)→

− 3

N

+ 1

H+ 4 (aq)+

− 1

F−(aq)

Again, the transfer of an H+ion leaves the oxidation numbers unaffected. In summary, redox reactions can always
be recognized by a change in oxidation number for at least two of the atoms in the reaction. Any reaction in which
no oxidation numbers change is not a redox reaction.

Lesson Summary

• The oxidation number of an atom is the charge that atom would have in a molecule if all of the bonding
  electrons were assigned to the more electronegative element.


• Oxidation results in an increase in the oxidation number of an atom. Reduction results in a decrease in the
  oxidation number of an atom.


• Redox reactions can be analyzed by oxidation number changes in order to determine what is being oxidized
  and what is being reduced.


• Combination, decomposition, single-replacement, and combustion reactions are generally redox reactions.
  Double-replacement reactions, including acid-base reactions, are generally not redox reactions.

Lesson Review Questions

Reviewing Concepts

1. What is a free element, and what is the oxidation number for atoms that exist as a free element?


2. Why is the oxidation number of fluorine always−1, while the oxidation number of chlorine can range from
   −1 to +7?


3. What is the highest oxidation number that sulfur can have? The lowest?


4. What is the oxidation number of the alkali metals when they are present in compounds? The alkaline earth
   metals?