The Paradox of Thermodynamic Instability

The constraints on the Gibbs free energy equation required to intercompare the stabilities of chemical species are reviewed, and the concept of thermodynamically unstable but kinetically stable compounds is defined. A method for synthesizing these compounds is then discussed based on a rule first stated by the French chemist, Pierre Macquer, in 1749, and its modern application illustrated using several concrete examples. A simple graphical method for visualizing trends in thermodynamically stable versus thermodynamically unstable compounds is then introduced and illustrated with example plots. The paper concludes with a brief note on terminology.