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Free energy changes determine if a
reaction is endothermic or exothermic.
Processes in nature are driven in two directions: toward least
MAIN IDEA
enthalpy and toward greatest entropy. When these two
oppose each other, the dominant factor determines th
direction of change. As a way to predict which factor will
dominate fora given system, a function has been defined to
relate the enthalpy and entropy factors at a given temperature
tropy
and constant pressure. This combined enthalpy-entropy function is callepd t
free energy, G, of the system; it is also called Gibbs free energy. This function
simultaneously assesses the tendencies for enthalpy and entropy to
change. Natural processes proceed in the direction that lowersthefree
energy of a system.
Only the change in free energy can be measured. It can be defined in
terms of changes in enthalpy and entropy. At a constant pressure and
temperature, the free-energy change, AG, of a system is defined as the
difference between the change in enthalpy, AH, and the product of the Kelvin
temperature and the entropy change, which is defined as TAS.
Free Energy Change
AGO= AH°- TASO
Note that this expression is for substances in their standard states.
The product TAS and the quantities AG and AHhave the same umor
usually kJ. The units of AS for use in this equation are usually N
If AG<0, the reaction is spontaneous.
AH and AS in the free-energy equation can have positive or negative
values. This leads to four possible combinations of terms.
