Using the standard reduction potentials shown in the table, determine which of the following ions is the strongest oxidizing agent in acidic aqueous solutions. A) Titanium dioxide, which contains titanium(IV), B) dichromate ions, which contain chromium(VI). C) permanganate ions, which contain manganese(VII), D) manganese(II) ions, or E) chromium (III) ions.
An oxidizing agent is something that oxidizes something else. An oxidizing agent that oxidizes something else successfully will in turn be reduced. The strength of an oxidizing agent is indicated by whether it can overpower another oxidizing agent to oxidize it instead of being oxidized itself. We’ve been given the standard electrode potentials for three half-equations. These standard electrode potentials represent the cell potential we would achieve if we paired each of these half-equations against the same process for hydrogen.
H⁺ is not considered a particularly strong oxidizing agent. When H⁺ reacts with electrons, it forms H₂ gas. And in the standard hydrogen electrode, it is given a voltage of zero for its standard electrode potential. This fixed standard is chosen because it isn’t possible to measure half-equation standard electrode potentials in isolation. We always need a reference electrode.
A positive voltage for a standard electrode potential indicates that the chemical agent isn’t more powerful oxidizing agent than H⁺. Meanwhile, a negative value for the standard electrode potential indicates an oxidizing agent less powerful than H⁺. So we have a general principle that the more positive the standard electrode potential, the stronger the oxidizing agent.
Standard electrode potential half-equations are always written with the oxidizing agent on the left, receiving electrons, and the equivalent reducing agent on the right. Remember that reduction is defined as the gaining of electrons. And the oxidizing agent is the chemical that receives electrons and is itself reduced. So let’s go through the candidate ions one by one and see which one wins out.
Option A, titanium dioxide has a standard electrode potential of negative 0.50 volts. This is the lowest of the three standard electrode potentials given. So titanium dioxide is definitely not the strongest oxidizing agent. Dichromate ions, Cr₂O₇²⁻, contain chromium six and have a standard electrode potential of positive 1.33 volts. This is a healthy standard electrode potential for an oxidizing agent, indicating a decent strength. However, permanganate ions, MnO₄⁻, which contain manganese seven have a standard electrode potential of positive 1.51 volts. This indicates that the permanganate ion is a stronger oxidizing agent than the dichromate ion. This follows the general rule of thumb that the higher the oxidation state is of the metal in a metal oxide, the stronger an oxidizing agent that will be.
Now, what about the manganese(II) and chromium(III) ions? The standard electrode potentials for these species are not given. They appear on the wrong side of the half-equations. And so, we don’t know how they would respond if given more electrons. But we can be certain that neither of these will be stronger oxidizing agents than the oxidizing agents that generated them in the first place.
So of the five ions given, the one that is the strongest oxidizing agent in acidic aqueous solutions is the permanganate ion, which contains manganese seven.