Question 5.8: Identifying Oxidizing and Reducing Agents. Hydrogen peroxide......
Identifying Oxidizing and Reducing Agents
Hydrogen peroxide, H_{2}O_{2}, is a versatile chemical. Its uses include bleaching wood pulp and fabrics and substituting for chlorine in water purification. One reason for its versatility is that it can be either an oxidizing or a reducing agent. For the following reactions, identify whether hydrogen peroxide is an oxidizing or reducing agent.
(a) H_{2}O_{2}(aq) + 2 Fe^{2+}(aq) + 2 H^{+}(aq) → 2 H_{2}O(l) + 2 Fe^{3+}(aq)
(b) 5 H_{2}O_{2}(aq) + 2 MnO_{4}^{−}(aq) + 6 H^{+}(aq) → 8 H_{2}O(l) + 2 Mn^{2+}(aq) + 5 O_{2}(g)
Analyze
Before we can identify the oxidizing and reducing agents, we must first assign oxidation states, and then identify which substance is being oxidized and which substance is being reduced. The oxidizing agent causes another substance to be oxidized. The reducing agent causes another substance to be reduced.
Solve
(a) Fe^{2+} is oxidized to Fe^{3+} and because H_{2}O_{2} makes this possible, it is an oxidizing agent. Viewed another way, we see that the oxidation state of oxygen in H_{2}O_{2} is −1. In H_{2}O_{2}, it is −2. Hydrogen peroxide is reduced and thereby acts as an oxidizing agent.
(b) MnO_{4}^{−} is reduced to Mn^{2+}, and H_{2}O_{2} makes this possible. In this situation, hydrogen peroxide is a reducing agent. Or, the oxidation state of oxygen increases from −1 in H_{2}O_{2} to 0 in O_{2}. Hydrogen peroxide is oxidized and thereby acts as a reducing agent.
Assess
The versatility of H_{2}O_{2} lies in its ability to act as an oxidizing agent and a reducing agent. When H_{2}O_{2} acts as an oxidizing agent, it is reduced to H_{2}O, in an acidic solution, as was the case in part (a), or to OH^{−} in basic solution. When it acts as a reducing agent, it is oxidized to O_{2}(g), as was the case in part (b).