Entropy and Free Energy

[MonsieurWise]

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!

 

[MonsieurWise]

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:
G_total = G_reactant + G_product
when G_reactant decrease, G_product increase, so how can it say at equilibrium, "the system has reached minimum free energy"?
Thanks

 

[chemisttree]

... Now The only thing I don't get is:
Since:
G_total = G_reactant + G_product
when G_reactant decrease, G_product increase, so how can it say at equilibrium, "the system has reached minimum free energy"?
Thanks

Is it correct to say "G_total = G_reactant + G_product"?

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.

 

[MonsieurWise]

Oh, it was in my chemistry book...

 

[Mapes]

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]

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]

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