Problem: Which atom has a change in oxidation number of –3 in the following redox reaction?K2Cr2O7 + H2O + S → KOH + Cr2O3 + SO2

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We’re being asked to determine the atom that has a change in oxidation number of –3 in the following redox reaction

K2Cr2O7 + H2O + S → KOH + Cr2O3 + SO2


The rules for oxidation states are as follows:

A. General Rules:

1. For an atom in its elemental form (Zn, Cl2, C(graphite), etc.)    O.S. = 0

2. For an ion (Li+, Al3+, etc.)                                                       O.S. = charge


B. Specific Rules:

1. Group 1A              O.S. = +1

2. Group 2A              O.S. = +2

3. Hydrogen              O.S. = +1, with nonmetals

                                  O.S. = –1 with metals and boron

4. Fluorine                 O.S. = –1

5. Oxygen                  O.S. = –1 in peroxides (X2O2, X = Group 1A)

                                  O.S. = –1/2 in superoxides (XO2, X = Group 1A)

                                  O.S. = –2 in other compounds

6. Group 7A              O.S. = –1 (except when bonded to O)


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Problem Details

Which atom has a change in oxidation number of –3 in the following redox reaction?

K2Cr2O7 + H2O + S → KOH + Cr2O3 + SO2

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