Law of Mass Action

The Law of Mass Action states that the rate of a chemical reaction at each instant is proportional to the product of the concentration terms of all the reactants, with each concentration raised to a power equal to the number of molecules of that reactant appearing in the balanced chemical equation. For a general reaction $A+B\rightarrow C$, where A and B are reactants (concentrations expressed in mol L$^{-1}$, written in square brackets) and C is the product, the law of mass action gives the proportionality Rate $\propto[A][B]$. Introducing a proportionality constant $k$, called the rate constant of the reaction, turns this into the rate equation, Rate $=k[A][B]$. If a reactant's stoichiometric coefficient is greater than 1, its concentration is raised to that power in the rate expression -- Problem 12.1 works through exactly this for $C(s)+O_2(g)\rightarrow CO_2(g)$ (Rate $=k[C][O_2]$) and $2KClO_3(s)\rightarrow 2KCl(s)+3O_2(g)$ (Rate $=k[KClO_3]^2$, since KClO3 has coefficient 2). In any reversible reaction carried out in a closed system, the forward reaction's rate is high at the start (when only reactants are pre …