CHEM 205
-lecture 12” PS2 #9, See notes in notebook
Lecture 13: Explanation of what Gibbs Free Energy really is
• For equation ΔG®=-RTlnK, units end up as J/mol
o Can modify this to just J by using ΔG®=-RTlnK *1mol
• Things to remember:
o Cv=3/2RT=dU/dT
• Definition of Gibbs Free Energy: max amount of energy that can be used to do useful
work that is NOT Expansion work.
o Achieved only when all 3 conditions are met: Constant T, P of system, and
REVERSIBILITY
(max W, q always achieved from reversible conditions)
o Thus, when ΔG=0, the ability for useful work is 0, example would be a dead
battery
• Gibbs equation can be used to find the temperature at which a reaction changes from
spontaneous to nonspontaneous and vice versa:
o ΔG= ΔH-T ΔS…Thus, T= ΔH/ ΔS at which equilibrium temperature is achieved
This makes sense since at this temperature, ΔG=0, rearranging the initial
equation gives that.
• Derivation of Free Energy and Work: (aka, how did we get that definition of Gibbs free
energy from above?)
o dG=dH-TdS-SdT
to make it easier for us, let us define T=constant, this simplifies equation
to:
o dG=dH-TdS
we know U=q+w and thus H=U-w=U+PVwork, so we can write equation
as:
o dG=dU+PdV + VdP – TdS to simplify, let us define P=constant for the system (which must also equal
outside P. This sim

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