Global selection rule in chemical coupling

Abstract

Coupling and decoupling of chemical reactions are explored through a modified heat balance equation. Reaction enthalpies are found to play crucial role; the sign of their product for a pair of consecutive chemical reactions determine whether they couple or not. The possibility of a coupling-uncoupling transition for such reactions is thus introduced for the first time. The present work resolves a paradox concerning negative efficiency of coupled consecutive chemical reactions. Enthalpy is also shown to be a "constant of motion" along the reaction coordinate as long as the mass action ratio varies little with temperature. The present analysis puts the observed difference between calorimetric and van't Hoff enthalpies on a quantitative basis. A case study is presented with a third order reaction where the enthalpic criterion is useful in selecting the more probable of the alternative mechanisms.