Common Reducing Agents
A good reducing agentA chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized. must be able to donate electrons readily. This means that it must not have very much attraction for electrons. Among the elements, low electronegativity is characteristic of good reducing agents. Molecules and ions which contain relatively electropositive elements which have low oxidation numbers are also good reducing agents. Oxidizing agents, on the other hand, must be able to accept electrons readily. Highly electronegative elements can do this, as can molecules or ions which contain relatively electronegative elements and even some metals which have high oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. numbers. Bear these general rules in mind as we examine examples of common reducing agents in the following paragraphs.
Metals All metals have low ionization energies and are relatively electropositive, and so they can lose electrons fairly easily. Metals on the left of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. exhibit this property to the greatest extent, and some of them, such as Li or Na, can even reduce H2O:
2Li(s) + 2H2O(l) → 2Li+(aq) + 2OH–(aq) + H2(g)
Zn(s) + 2H3O+(aq) → Zn2+(aq) + 2H2O(l) + H2(g)
This is one of the characteristic reactions of acids. There are a few metals that will not dissolve in just any acid but instead require an acid like HNO3 whose anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell. is a good oxidizing agent. Cu and Hg are examples:
3Hg(s) + 8 H3O+(aq) + 2NO3–(aq) → 3Hg2+(aq) + NO(g) + 12H2O
Finally, a few metals, such as Au and Pt, are such poor reducing agents that even an oxidizing acid like HNO3 will not dissolve them. This is the origin of the phrase “the acid test.” If a sample of an unknown yellow metal can be dissolved in the metal is not gold. Kings who collected tax payments in gold kept a supply of HNO3 available to make sure they were not being cheated.