In a redox reaction, what do oxidation and reduction mean?
1 Answer
TWO WAYS TO DEFINE OXIDATION
The intuitive definition of oxidation is:
The increase in the number of oxygens, or the decrease in the number of hydrogens on a species.
An alternative definition is:
When an element's oxidation state becomes more positive, or less negative.
Example: (half-reaction)
+2Mn2++2H2O→+4MnO2+4H++2e−
- The number of oxygens in the
Mn -containing species increased, indicating oxidation via the first definition. - The oxidation state of
Mn increased from+2 to+4 , thereby indicating oxidation via the second definition.
Example (full redox reaction):
H3C(HC−OH)CH3+H2CrO4→H3C(C=O)CH3+HCrO−3+H3O+
- The number of hydrogens in the alcohol decreased, indicating oxidation via the first definition on its half-reactoin.
- The oxidation state of
Cr decreased from+6 to+4 , indicating reduction ofCr , therefore showing thatH2CrO4 is an oxidizing agent, causing the alcohol to oxidize into the ketone, in accordance with the second definition.
TWO WAYS TO DEFINE REDUCTION
Reduction is simply the opposite:
The decrease in the number of oxygens, or the increase in the number of hydrogens on a species.
An alternative definition is:
When an element's oxidation state becomes more negative, or less positive.
Example (half-reaction):
0P+3H++3e−→−3PH3
- The number of hydrogens in the
P -containing species increased, indicating reduction via the first definition. - The oxidation state of
P decreased from0 to−3 , thereby indicating reduction via the second definition.
Example (half-reaction):
- See the chromium-based part of the reaction above. Chromium was reduced to a less positive oxidation state, and one oxygen was lost from
H2CrO4 , following both definitions in its half-reaction.
This was that half-reaction:
H2+6CrO4+2H++2e−→H+4CrO−3+H3O+
