Redox Chemistry Review
Oxidation State or Number
or number of a compound gives a
measure of how
(electron-rich) a compound is. The number is relative because it
is only meaningful when compared to the number for another compound, to determine which
one is more oxidized or more reduced.
Rules for calculating the oxidation state of an element in a molecule (from Brock,
Ed. Appendix A-1).
The oxidation state of an element in an elementary substance (
) is zero.
The oxidation state of the ion of an element is equal to its charge (
= +1, O
The sum of the oxidation numbers of all atoms in a neutral molecule is zero. Thus, H
O is neutral
because it has two H at +1 each and one O at –2.
In an ion, the sum of the oxidation numbers of all atoms is equal to the charge on that ion. Thus,
in the OH
ion, O(-2) + H(+1) = -1
In compounds, the oxidation state of O is virtually always –2, and that of H is +1 (this gets more
complicated in some organic compounds).
In simple carbon compounds, the oxidation state of C can be calculated by adding up the H and O
atoms present and using the oxidation states of these elements as given in #5, because in a
neutral compound, the sum of the oxidation numbers must be 0. Thus, the oxidation state of
carbon in methane, CH
, is –4 (4 H at +1 = +4).
In organic compounds with more than one C atom, it may not be possible to assign a specific
oxidation number to each C atom, but it is still useful to calculate the oxidation state of the
compound as a whole. Thus, the oxidation state of carbon in glucose, C
, is zero and the
oxidation state of carbon in ethanol, C
O, is –2.
i. Calculate the oxidation state of each element in the following compounds (answers at end).
sulfate – SO
B. hydrogen sulfide – H
ammonia – NH
D. nitrite – NO
E. nitrate – NO
G. iron hydroxide – Fe(OH)
Reduction and oxidation reactions
The oxidation states you just calculated above provide information about how
(electron-poor) each element in a compound is. The smaller (more
negative) the oxidation state, the more reduced a compound is; conversely, larger (more
positive) oxidation states are associated with more oxidized compounds. Hence, Fe
(oxidation state +3) is more oxidized than elemental Fe (oxidation state 0).