1. In the following reactions, identify which reactants are the oxidizing agent and which are the reducing agents.
a. 2S2O32- (aq) + I2 (s) �¨ 2I- (aq) + S4O62- (aq)
b. NO3- (aq) + Zn (s) �¨ Zn2+ (aq) + NO2 (g)
Oxidation is the loss of electrons; that substance is the reducing agent.
Reduction is the gain of electrons; that substance is the oxidizing agent.
To identify the oxidizing agent and reducing agent in a reaction, you need to determine the changes in oxidation states of the elements involved.
a. In the reaction 2S2O32- (aq) + I2 (s) → 2I- (aq) + S4O62- (aq), the oxidation state of S in S2O32- is +2 and changes to +6 in S4O62-. This means S2O32- is oxidized. The oxidation state of I2 is 0 and changes to -1 in I-. This means I2 is reduced.
So, in this reaction:
- The oxidizing agent is I2 because it causes the oxidation of S2O32-.
- The reducing agent is S2O32- because it causes the reduction of I2.
b. In the reaction NO3- (aq) + Zn (s) → Zn2+ (aq) + NO2 (g), the oxidation state of Zn in Zn is 0 and changes to +2 in Zn2+. This means Zn is oxidized. The oxidation state of N in NO3- is +5 and changes to +4 in NO2. This means NO3- is reduced.
So, in this reaction:
- The oxidizing agent is NO3- because it causes the oxidation of Zn.
- The reducing agent is Zn because it causes the reduction of NO3-.