Editing Chemical thermodynamics (section)

===System constraints===

In this regard, it is crucial to understand the role of walls and other ''constraints'', and the distinction between ''independent'' processes and ''coupling''. Contrary to the clear implications of many reference sources, the previous analysis is not restricted to [[wiktionary:homogeneous|homogeneous]], [[isotropy|isotropic]] bulk systems which can deliver only ''P''d''V'' work to the outside world, but applies even to the most structured systems. There are complex systems with many chemical "reactions" going on at the same time, some of which are really only parts of the same, overall process. An ''independent'' process is one that ''could'' proceed even if all others were unaccountably stopped in their tracks. Understanding this is perhaps a "[[thought experiment]]" in [[chemical kinetics]], but actual examples exist.

A gas-phase reaction at constant temperature and pressure which results in an increase in the number of molecules will lead to an increase in volume. Inside a cylinder closed with a piston, it can proceed only by doing work on the piston. The extent variable for the reaction can increase only if the piston moves out, and conversely if the piston is pushed inward, the reaction is driven backwards.

Similarly, a [[redox]] reaction might occur in an [[electrochemistry|electrochemical]] cell with the passage of [[electric current|current]] through a [[wire]] connecting the [[electrode]]s. The half-cell reactions at the [[electrode]]s are constrained if no current is allowed to flow. The current might be dissipated as [[Joule heating]], or it might in turn run an electrical device like a [[electric motor|motor]] doing [[mechanical work]]. An [[automobile]] [[lead]]-[[acid]] [[battery (electricity)|battery]] can be recharged, driving the chemical reaction backwards. In this case as well, the reaction is not an independent process. Some, perhaps most, of the Gibbs free energy of reaction may be delivered as external work.

The [[hydrolysis]] of [[Adenosine triphosphate|ATP]] to [[adenosine diphosphate|ADP]] and [[phosphate]] can drive the [[force]]-times-[[distance]] work delivered by living [[muscle]]s, and synthesis of ATP is in turn driven by a redox chain in [[Mitochondrion|mitochondria]] and [[chloroplast]]s, which involves the transport of [[ion]]s across the membranes of these [[cell (biology)|cell]]ular [[organelle]]s. The coupling of processes here, and in the previous examples, is often not complete. Gas can leak slowly past a piston, just as it can slowly leak out of a [[rubber]] [[balloon]]. Some reaction may occur in a battery even if no external current is flowing. There is usually a coupling [[coefficient]], which may depend on relative rates, which determines what percentage of the driving free energy is turned into external work, or captured as "chemical work", a misnomer for the free energy of another chemical process.