Gibbs Free Energy, Enthalpy, and Energy

If one already knew the change in enthalpy of a system (in Gibbs change in free energy equation), wouldn't he automatically be able to tell whether the reaction is exergonic or endergonic without knowing the exact value of the temperature multiplied by the change in entropy portion of the equation? Given this, what is the purpose of the Gibbs free energy equation?
- Nithya (age 14)
San Jose, CA

Sure, the enthalpy does tell you whether heat will be released or soaked up as the reaction occurs. There's something else important to know also: which way the reaction goes and how far! For that, you need to know the Gibbs free energy which is minimized in equilibrium at constant pressure and temperature.

The basic underlying principle is that the total entropy of the system and its environment gets maximized in equilibrium. If you tried to figure out which way the reaction went just from the system enthalpy (which affects the environment entropy), you'd be leaving out the system entropy.

Mike W.

(published on 11/16/2015)