Reducing Agents If one reagent in a reaction removes oxygen, contributes hydrogen, or contributes electrons, it is said to be a reducing agent. Of course, it is oxidized in the process. Since oxidation and reduction are symmetric processes, always occurring together, there is always an oxidizing agent and a reducing agent in the reaction. Since both are always present, why bother with this terminology? If reduction of some substance is the desired end, then it is useful to find an agent which will readily accomplish the reduction. Hydrogen gas is a very useful reducing agent, used widely in the isolation of pure metals by reduction. For example, copper metal can be obtained by reduction in the reaction CuO + H2 -> Cu + H2O The fact that the hydrogen is oxidized in the process is incidental - the hydrogen has performed a very valuable task as a reducing agent. Important tasks are accomplished by reducing agents as photographic developers and antioxidants. And in photosynthesis, the reducing agents accompish a task which is essential to our survival. The relative strengths of reducing agents can be inferred from their standard electrode potentials. The strongest reducing agents are shown from the standard electrode table. Cathode (Reduction) Half-Reaction Standard Potential E° (volts) Li+(aq) + e- -> Li(s) -3.04 K+(aq) + e- -> K(s) -2.92 Ca2+(aq) + 2e- -> Ca(s) -2.76 Na+(aq) + e- -> Na(s) -2.71 Mg2+(aq) + 2e- -> Mg(s) -2.38 Lithium, having the largest negative value of electrode potential, is the strongest reducing agent. By convention, the standard electrode potentials are reduction potentials, or the tendency to be reduced. By that convention, the strongest reducing agents will have large negative potentials.

Index Oxidation/ Reduction concepts Reference Hill & Kolb Ch 8