Chemical Equilibrium: The Law of Mass Action

James Richard Fromm

Active mass, or reactive mass, or as it is more commonly called chemical activity, is the amount of substance which is reactive where the reaction is occurring. It has the units of mol/m3 in the SI, simply because space is three-dimensional, if a reaction takes place within a volume. Some reactions take place only on surfaces, as in electrochemistry and industrial catalysis, and when they do so then active mass has the units of mol/m2 in the SI because the reaction occurs over an area, which is two-dimensional. However, these units are often inconvenient and other units are used for measurement of active mass. Most of them are the common units in which concentration or pressure is measured. Chemical activity, symbolized by a, has been used with many different nominal units. The units are nominal, rather than real, because chemical activity is formally defined as the ratio of the actual chemical activity of a substance to its chemical activity under some defined standard conditions, and ratios have no units because the units divide out. However, we will use nominal units in order to ensure that our calculations involving chemical activity follow the quantity calculus.

A dynamic equilibrium requires that both the forward and reverse processes be considered, and so it is usually indicated by a double arrow or a double-headed arrow (larrow.GIF (55 bytes)rarrow.gif (63 bytes)) rather than by the single arrow used for a chemical reaction proceeding in one direction. Even so, the equilibrium is written with the reactants on the left and products on the right in the same manner as an ordinary chemical reaction. In the equilibrium

CaCO3(s) larrow.GIF (55 bytes)rarrow.gif (63 bytes) CaO(s) + CO2(g),

calcium carbonate is considered to be the reactant while calcium oxide and carbon dioxide are considered to be the products.

Copyright 1997 James R. Fromm