compound. Keep in mind that oxidation states are not “real” charges and carbon
dioxide is not an ionic substance. In this case, the oxidation state of +4 for carbon
indicates that for electron bookkeeping purposes, carbon will not be responsible
for the electrons it is sharing in the bond between the carbon and oxygen atoms
and the responsibility will lie with the oxygen. When CO 2 is involved in a
chemical process and carbon’s responsibility for electrons changes (i.e., its
oxidation state changes), a redox reaction will be recognized.
Example 3
In the polyatomic ion dichromate (Cr 2 O 7 2−), what is the oxidation state of
chromium?
In a polyatomic ion, the algebraic sum of the positive and negative oxidation
states of all the elements must equal the charge on the ion.
Since the sum of these values must equal −2 (the charge on the polyatomic
ion)
The oxidation state of chromium in dichromate is +6.Using Oxidation States to Recognize Redox Reactions
Once oxidation states can be assigned to elements in the substances involved in a
chemical reaction, recognition of the process as being redox or not is
straightforward. If the oxidation states change, then a transfer of electrons is
taking place. If the oxidation states remain the same, then a redox reaction is not
occurring.
Consider these two reactions:
The oxidation states of each element in all the substances can be determined
as shown above each of them here: