http://www.ck12.org Chapter 22. Oxidation Reduction Reactions
Example 22.1
Assign oxidation numbers to each atom in the compound KMnO 4.
Answer:
Potassium is an alkali metal, so we would expect it to have an oxidation state of +1, and we can assume that oxygen
has an oxidation state of -2. Overall, this is a neutral compound, so the sum of all the oxidation states must equal
zero. Therefore:
K + Mn + 4(O) = 0
+1 + Mn + 4(-2) = 0
Mn - 7 = 0
Mn = +7In this compound, manganese has an oxidation state of +7. Note that we must include an oxidation state of -2 for
eachof the four oxygen atoms. Let’s look at another example.
Example 22.2
What is the oxidation state of iron in Fe 2 O 3? Does it have a different oxidation state in FeCl 42 −?
Answer:
In the first compound, we can assume that oxygen has an oxidation state of -2, since it is bonded to a metal, which
would be significantly less electronegative.
2(Fe) + 3(O) = 0
2(Fe) + 3(-2) = 0
2(Fe) - 6 = 0
2(Fe) = +6
Fe = +3In iron(III) oxide, iron has an oxidation state of +3. Note that this is the same as its charge when we dissociate this
compound into its ions. Because the polyatomic ion has an overall charge, the oxidation states in FeCl 42 −will add
up to -2, not zero. We can assume that chlorine has an oxidation state of -1, because it is bonded to a metal and not
a very electronegative nonmetal.
Fe + 4(Cl) = -2
Fe + 4(-1) = -2
Fe - 4 = -2
Fe = +2In this ion, iron has an oxidation state of +2. Both of these are common oxidation states for iron, so we must look at
the surrounding atoms to determine which is present in a given compound.
Identifying and Analyzing Redox Reactions
The system of assigning oxidation numbers allows for a more general definition of redox reactions. A redox process
is one in which the oxidation numbers of one or more atoms change over the course of the reaction. Any atom
whose oxidation number becomes more positive (or less negative) is being oxidized, and an atom whose oxidation
number becomes more negative (or less positive) is being reduced. By assigning oxidation numbers to each atom in
a reaction, we can determine whether or not it is a redox reaction.