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Paul Matsumoto was absolutely correct in writing “The Reaction Quotient Is Unnecessary To Solve Equilibrium Problems” (1). As he pointed out, algorithms that solve for changes in reactant or product molarity (x) using the equilibrium constant expression, will tell you from the sign of x whether the reaction goes forward or backward to reach equilibrium. Thus calculating the reaction quotient is unnecessary if you plan to solve the problem either by approximation or by algorithm (exactly). Two points are worth considering here however, one regarding the real advantage of the reaction quotient, and one regarding the advisability of using algorithms and Keq to solve for “exact” equilibrium concentrations. The reaction quotient (Q) has two major advantages in characterizing reaction systems away from equilibrium. First, by calculating Q and comparing it to Keq, one can make an accurate qualitative prediction as to which direction the reaction will proceed in order to reach equilibrium. Second, the maximal free energy available to do work as this system proceeds toward equilibrium can be calculated from the simple equation ∆G = ∆G° + RTlnQ = RTln (Q/Keq) These two uses of Q generate quick and useful predictions for chemists dealing with real, nonstandard-state, nonequilibrium systems; this is where Q serves its most useful purpose. Furthermore, these conclusions do not require any complicated algorithmic solution regarding the eventual equilibrium situation. This brings me to my second point. It has been stated many times, in this Journal and elsewhere, that asking students to use complex algorithms to solve for “exact” equilibrium concentrations is not a good use of classroom time (2–6). Owing to the effects of both non-ideality (influence of pressure, temperature, and ionic strength on activity coefficients) and competing side reactions, “exact” calculations based on tabulated equilibrium constants may yield concentrations that differ from experimental results by an order of magnitude or more (2–7). These “exact” mathematical predictions regarding equilibrium are inaccurate, and thus not very useful from a chemical standpoint. Class time is better spent on the underlying chemistry, for example, a more rigorous qualitative discussion of equilibrium and non-equilibrium systems. Literature Cited- Matsumoto, P. S. J. Chem. Educ. 2005, 82, 406–407.
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