<P> The initial state of the body, according to Gibbs, is supposed to be such that "the body can be made to pass from it to states of dissipated energy by reversible processes". In his 1876 magnum opus On the Equilibrium of Heterogeneous Substances, a graphical analysis of multi-phase chemical systems, he engaged his thoughts on chemical free energy in full . </P> <P> According to the second law of thermodynamics, for systems reacting at STP (or any other fixed temperature and pressure), there is a general natural tendency to achieve a minimum of the Gibbs free energy . </P> <P> A quantitative measure of the favorability of a given reaction at constant temperature and pressure is the change ΔG (sometimes written "delta G" or "dG") in Gibbs free energy that is (or would be) caused by the reaction . As a necessary condition for the reaction to occur at constant temperature and pressure, ΔG must be smaller than the non-PV (e.g. electrical) work, which is often equal to zero (hence ΔG must be negative). ΔG equals the maximum amount of non-PV work that can be performed as a result of the chemical reaction for the case of reversible process . If the analysis indicated a positive ΔG for the reaction, then energy--in the form of electrical or other non-PV work--would have to be added to the reacting system for ΔG to be smaller than the non-PV work and make it possible for the reaction to occur . </P> <P> The equation can be also seen from the perspective of the system taken together with its surroundings (the rest of the universe). First, assume that the given reaction at constant temperature and pressure is the only one that is occurring . Then the entropy released or absorbed by the system equals the entropy that the environment must absorb or release, respectively . The reaction will only be allowed if the total entropy change of the universe is zero or positive . This is reflected in a negative ΔG, and the reaction is called exergonic . </P>

What physical quantity is provided directly by the gibbs free energy change
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