596 MAN IS MASTERING HIS MATERIAL WORLD power, their valence is said to be zero. rewritten to show valence changes: The above reaction may now be Valence 2(+ 2 - 2) 2 Ha + O2 >- 2H2O In this case hydrogen, H2, has changed in valence from zero to 2 X (+ 1). It has therefore increased in valence. Inasmuch as hydrogen is oxidized in the above reaction, an increase in valence must therefore be oxidation. By the same reasoning the loss in valence of oxygen must be reduction. We now have a new and more general definition of oxidation and reduction. Oxidation is the increase in the positive valence, while reduction is a decrease in positive valence. Thus many reactions which involve neither oxygen nor hydrogen are still oxidation-reduction reactions because the valences of elements are changed in these reactions; in fact every reaction in which there is a change of valence is an oxidation-reduction reaction. The term oxidation-reduction reaction is used because in every reaction which involves oxidation there is always a loss in positive valence by one atom which exactly equals the gain in positive valence of another. An example of an oxidation-reduction reaction which involves no oxygen or hydrogen is given as follows: when a globule of mercury is rolled over a sheet of paper covered with mercuric chloride, HgCh, it leaves a fluorescent path when exposed to ultraviolet light because the mercury forms mercurous chloride, HgCl, which, unlike mercuric chloride, HgCb, is fluorescent. Valence + 2-2 2(+ 1 - 1) HgCl2 + Hg —3- 2 HgCl mercuric chloride mercury mercurous chloride In this case the mercuric mercury is changed to mercurous mercury; when metals have two valences, the lower valence is indicated by the suffix 0W5 as in mercnrous or cuprous, and the higher valence is indicated by the sufftx ic as in mercuric and cuprzc. This reaction represents a decrease in positive valence and is therefore reduction. The free mercury increases in positive valence and is therefore oxidized. According to the electron theory, changes in valence are the result of a loss or gain of electrons; thus oxidation and reduction fundamentally involve a gain or loss of electrons by atoms or ions. Inasmuch as an increase in positive valence is oxidation and inasmuch as this change is accomplished by the loss of electrons, oxidation is the loss of electrons or de-electronization, as the process is sometimes called. Reduction must therefore be a gain of electrons, or electronization.
i gain — ELECTRON TRANSFER 597 Oxidizing and Reducing Agents Are Substances Which Gain and Lose Electrons. The separation of metals from their compounds in metallurgy is essentially a process of reduction, because electrons must be added to the metal in the compound to set it free. Coal is used to reduce iron ore: gain of electrons I i 2(+3-3) ^0 3(+4-4) 2Fe203 + 3C —>- 4Fe + 3 CO2 iron oxide carbon (coal) iron carbon dioxide I loss of electrons J An electric current is a stream of electrons and therefore serves as a reducing agent. Sodium, aluminum, magnesium, and other metals which have a relatively poor attraction for electrons are best separated from their compounds by electrolysis. These metals separate at the cathode because electrons are given off at this pole. Nonmetals have a relatively high attraction for electrons, but these elements may be separated from their compounds at the anode by electrolysis because electrons are removed from substances at the anode. Chlorine, bromine, iodine, and oxygen may be prepared by electrolysis. Oxygen has an attraction for electrons which is greater than that of chlorine, and oxygen may therefore be used to liberate chlorine from its compounds. Chlorine, in turn, has an attraction for electrons which is greater than that of iodine, and chlorine will therefore liberate iodine from its compounds. of electrons 5- +1-1 +1-1 CI + Nal —>- I + NaCl chlorine sodium iodide iodine sodium chloride I loss of electrons f In electrolysis, reduction takes place at the cathode and oxidation takes place at the anode. In the reaction I loss of electrons ^ 2( + 2-2) 2(H2) + O2 —>- 2(H20) I gain of electrons ^ hydrogen loses electrons, and oxygen gains electrons. Similarly, in the reaction of sodium with water, sodium goes into solution to form sodium ions, Na"^, losing electrons in the process, while hydronium ions, (HaO)^, accept electrons and form water and hydrogen gas. ["O loss of electrons T 2 Na + 2(0H)-, 2(H30)+ ^2 Na+ 2(0H)- + H2 + 2 H2O I gain of electrons J