Don't forget that there are 2 chromium atoms present. Unfortunately, it isn't always possible to work out oxidation states by a simple use of the rules above.
The problem in this case is that the compound contains two elements the copper and the sulfur with variable oxidation states. In cases like these, some chemical intuition is useful. Here are two ways of approaching this problem:. This can also be extended to negative ions. The sulfate ion is SO 4 2 -. The sulfite ion is SO 3 2 -. This ion is more properly named the sulfate IV ion. The - ate ending indicates that the sulfur is in a negative ion.
Because of the potential for confusion in these names, the older names of sulfate and sulfite are more commonly used in introductory chemistry courses.
In each of the following examples, we have to decide whether the reaction is a redox reaction, and if so, which species have been oxidized and which have been reduced. Assign each element its oxidation state to determine if any change states over the course of the reaction:.
The oxidation state of hydrogen has decreased—hydrogen has been reduced. The chlorine is in the same oxidation state on both sides of the equation—it has not been oxidized or reduced.
It is probable that the elemental chlorine has changed oxidation state because it has formed two ionic compounds. Checking all the oxidation states verifies this:. Chlorine is the only element to have changed oxidation state. However, its transition is more complicated than previously-discussed examples: it is both oxidized and reduced.
This type of reaction, in which a single substance is both oxidized and reduced, is called a disproportionation reaction. Oxidation states can be useful in working out the stoichiometry for titration reactions when there is insufficient information to work out the complete ionic equation. Each time an oxidation state changes by one unit, one electron has been transferred.
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