Oxidation and Reduction Occuring Simultaneously


When a substance is oxidized, another should be reduced as illustrated below:
Acid and Bases

Question: How do we determine whether a chemical reaction is an oxidation-reduction reaction?

By following the changes in the oxidation numbers of each element in the reaction.


Zn(s) + 2HCl(l) → ZnCl2(aq) + H2(g)
Here, during oxidation, Zn(s) lost 2e to become Zn2+ whereas H+ gained 2e to become H2(g)
Zn (ON is zero) is oxidized to Zn2+ (ON is +2 ) , while H(ON is +1 ) is reduced to H2 (ON is zero).

H caused Zn to be oxidized → H is an "oxidixing agent"
Zn caused H to be reduced → Zn is an "reducing agent"

Balancing Equations


  1. Cr2O72−(aq) + CH3OH(aq) → Cr3+(aq) + HCO2H(aq)
  2. NO2(aq) + Cr2O72−(aq) → Cr3+(aq) + NO3(aq)

Balancing Equations in a Basic Solution

Here OH and H2O are used instead of H+ and H2O
The same method is used, but here, OH is added to "neutralize" the H+ used, forming H2O


  1. MnO4(aq) + Br(aq) → MnO2(s) + BrO3(aq)
  2. Cr(OH)3(s) + ClO(aq) → CrO42−(aq) + Cl2(g)

For more details, please contact me here.
Date of last modification: Summer , 2019