Replacement and the Equating of Volume and Weight

Abstract

A method is presented for writing molecularly, volumetrically, and electrically balanced equations for replacement reactions. Two general situations obtain: (A) enough is known of the chemistry of the particular reaction to write a complete equation, or (B) only enough is known to permit the setting-down of the essential facts of the reaction in skeleton form. In the first case a simple, molecularly balanced equation for the reaction in question will show that there are three different possibilities: (1) that too much guest is furnished for the volume left by the departing host; (2) that too little guest is produced to fill the volume of host removed; and (3) that the volumes involved are approximately equal. In the second case the balanced skeleton equation will show that (1) there is a deficiency in the host of the large ions (sulfur, oxygen) needed in an equal volume of the guest; (2) there is an excess of these large ions in the host over the amount that can be used in an equal amount of guest; and (3) the number of large ions in equal volumes of host and guest are the same. In both cases the third category is relatively rare. Methods by which the additional material needed can be provided or the extra material can be removed are suggested. In supergene sulfide replacements the main problems in achieving complete balance are (a) a mechanism for the oxidation of excess $$S^{-2}$$, (b) a source of hydrogen sulfide, and (c) a mechanism for the reduction of $$S^{+6}$$ in solution to provide additional $$S^{-2}$$. A naturally present reducing agent which can do the required work is not immediately apparent; the most likely oxidizing agent is $$Fe^{+3}$$; $$H_2S$$ probably derives from the action of $$H_2SO_4$$ on certain sulfides. Hypogene sulfide and sulfosalt metasomatic reactions cannot be written in complete form except in a few cases. If such replacements require that sulfur be removed, it is considered to be oxidized to the sulfate ion. If additional sulfur is needed, it is provided directly from the ore-bearing solutions. No attempt is made to show how it got to the scene of deposition. Other replacements than those of sulfides and sulfosalts are produced in equational form, and similar conclusions are reached as to the requirements for putting them in complete balance.