  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.
