# Entropy and Free Energy

I'm having trouble understanding why in a reaction, when the free energy G of the product equal the free energy G of the reactant, the reaction is at equilibrium. Here, as my book say, the system has reached its minimum free energy. I don't really get why... Could someone explain to me...? Thanks!!

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Oh...never mind. It seems like the reaction just like the Delta G, not G, to be negative to be spontaneous, right? Now The only thing I don't get is:
Since:
Gtotal = Greactant + Gproduct
when Greactant decrease, Gproduct increase, so how can it say at equilibrium, "the system has reached minimum free energy"?
Thanks

chemisttree
Homework Helper
Gold Member
... Now The only thing I don't get is:
Since:
Gtotal = Greactant + Gproduct
when Greactant decrease, Gproduct increase, so how can it say at equilibrium, "the system has reached minimum free energy"?
Thanks
Is it correct to say "Gtotal = Greactant + Gproduct"?

Gibbs Free Energy is the maximum amount of useful work you can obtain from a process not an inherent energy present in either the products or reactants. Where have you seen this equation? It's news to me.

Oh, it was in my chemistry book...

Mapes
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Gold Member
Gibbs Free Energy is the maximum amount of useful work you can obtain from a process not an inherent energy present in either the products or reactants. Where have you seen this equation? It's news to me.
The Gibbs free energy G is just the enthalpy minus TS. It's definitely an inherent property (a state function) of a substance.

MonsieurWise, the question is by what magnitude the Gibbs free energies of the reactants and the products increase and decrease. The magnitudes aren't constant; they depend on the amounts that already exist. At equilibrium, $\Delta G_\mathrm{reactant}$ and $\Delta G_\mathrm{product}$ are equal and opposite.

chemisttree
Homework Helper
Gold Member
The Gibbs free energy G is just the enthalpy minus TS. It's definitely an inherent property (a state function) of a substance.
And just how is that calculated? Say I've got 10 grams of hydrogen at STP... What Gibbs free energy is appropriate for that substance?

Mapes
$$(10\,\mathrm{g})\left(\frac{1}{2.016} \frac{\mathrm{mol}}{\mathrm{g}}\right)\left[0\,\frac{\mathrm{J}}{\mathrm{mol}}-\left(298\,\mathrm{K}\right)\left(131\,\frac{\mathrm{J}}{\mathrm{mol}\cdot\mathrm{K}}\right)\right]\approx -190\,\mathrm{kJ}$$