Question 5.4: Identifying Oxidation–Reduction Reactions. Indicate whether ......
Identifying Oxidation–Reduction Reactions
Indicate whether each of the following is an oxidation–reduction reaction.
(a) MnO_{2}(s) + 4 H^{+}(aq) + 2 Cl^{−}(aq) → Mn^{2+}(aq) + 2 H_{2}O(l) + Cl_{2}(g)
(b) H_{2}PO_{4}^{−}(aq) + OH^{−}(aq) → HPO_{4}^{2−}(aq) + H_{2}O(l)
Analyze
In each case, indicate the oxidation states of the elements on both sides of the equation, and look for changes.
Solve
(a) The O.S. of Mn decreases from +4 in MnO_{2} to +2 in Mn^{2+}. MnO_{2} is reduced to Mn^{2+}. The O.S. of O remains at −2 throughout the reaction, and that of H, at +1. The O.S. of Cl increases from −1 in Cl^{−} to 0 in Cl_{2}. Cl^{−} is oxidized to Cl_{2}. The reaction is an oxidation–reduction reaction.
(b) The O.S. of H is +1 on both sides of the equation. Oxygen remains at O.S. −2 throughout. The O.S. of phosphorus is +5 in H_{2}PO_{4}^{−} and also +5 in HPO_{4}^{2−}. There are no changes in O.S. This is not an oxidation–reduction reaction. (It is, in fact, an acid–base reaction.)
Assess
Because many redox reactions involve H^{+}, OH^{−} , or insoluble ionic compounds, it is easy to confuse a redox reaction with an acid–base or a precipitation reaction. It is important that you remember the defining features of each type of reaction. Precipitation reactions involve the combination of ions in solution to produce an insoluble precipitate, acid–base reactions involve proton (H^{+}) transfer, and redox reactions involve changes in oxidation states.