How to Determine Which Element Is in an Oxidation Reaction
- 1). Copy down the chemical equation for the reaction. If you are burning propene in oxygen, for example, the chemical equation will be as follows:
C3H6 + 5 O2 ---> 3 CO2 + 4 H2O - 2). Note the following rules for determining oxidation numbers:
-- An atom in its elemental state has oxidation number 0. O2 or Cl2, for example, both have oxidation number 0.
-- A monatomic ion has an oxidation number equal to its charge. Na+ has oxidation number +1 for example.
-- Oxidation numbers for all the atoms in a species must always add up to the net charge on the species. If the compound has no net charge, the oxidation numbers must add up to zero. In CO2, for example, the oxidation numbers must add up to 0.
-- The oxidation number of hydrogen is +1 when it's in a compound with a nonmetal and -1 when it's in a compound with a metal such as sodium, e.g., sodium hydride.
-- The oxidation number of oxygen in a compound is -2, unless it is in a peroxide, e.g., hydrogen peroxide, in which case it has -1; or in a compound with fluorine, in which case it has +2.
-- The oxidation number of a halogen in a compound, e.g., chlorine, is -1. - 3). Determine the oxidation number of each atom in the reactants and products using the rules in Step 2.
Example:
Propene (C3H6) has 6 hydrogens in a compound with a nonmetal, so each hydrogen has an oxidation number of +1. The sum of all oxidation numbers in the compound must be 0 since there is no net charge, so the oxidation number on carbon is -2.
O2 has an oxidation number of 0.
In CO2, each oxygen atom has -2 since it is not combined with fluorine; since CO2 has no net charge, carbon must have an oxidation number of +4.
In H2O, each hydrogen atom has an oxidation number of +1, while oxygen has an oxidation number of -2. - 4). Look for species containing elements whose oxidation number increased, e.g., became more positive. These species were oxidized. Species containing elements whose oxidation number decreased were reduced.
Example: The oxidation number on carbon has increased, so it was oxidized. Oxygen, by contrast, was reduced.
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