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According to the assumptions of Constant P and T and no non mechanical work, we get

ΔG=ΔH-TΔS

But there is another form of the Gibbs free energy equation which takes Reversible process and no non mechanical work as assumption

dG=Vdp-SdT

If I Use the assumptions constant Pressure and Temperature, according to the above equation we get dG=0,which contradicts the first equation.

What is the mistake I am making? Also, when ΔGsystem=-TΔStotal

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    $\begingroup$ For T const, p const, zero nonmechanical work and for reversible system, Delta H = Q = T Delta S, so delta G = 0. $\endgroup$
    – Poutnik
    Feb 7 at 17:31
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    $\begingroup$ You omitted the effect on G of changes in the number of moles of the reacting chemical species. $\endgroup$ Feb 7 at 19:10
  • $\begingroup$ @Chet Miller, Can u elaborate a bit $\endgroup$ Feb 8 at 1:50
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    $\begingroup$ See my answer at chemistry.stackexchange.com/questions/138563/… for the restrictions on $dG=Vdp-SdT$. $\endgroup$
    – theorist
    Feb 8 at 5:34
  • $\begingroup$ The differential you took as formula for Gibbs free energy is merely that for a compression/expansion, there is no reaction involved. A compression at constant T and P is a non-process. $\endgroup$
    – Alchimista
    Feb 8 at 8:48
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The Gibbs free energy of a multicomponent mixture is a function not only of temperature and pressure, but also of the number of moles of the various species in the mixture: $$G=G(T, P, n_1, n_2. m_3. ...)$$So, $$dG=\frac{\partial G}{\partial T}dT+\frac{\partial G}{\partial P}dP+\frac{\partial G}{\partial n_1}dn_1+\frac{\partial G}{\partial n_2}dn_2+\frac{\partial G}{\partial n_3}dn_3+...$$or $$dG=-SdT+VdP+\mu_1dn_1+\mu_2dn_2+\mu_3dn_3+...$$ where the $\mu's$ are the partial derivatives of the Gibbs free energy with respect to the number of moles of each of the species, and are referred to as the so-called Chemical Potentials of the various species in the solution. The numbers of moles of the species in the mixture change as the reaction proceeds, and that causes the Gibbs free energy to change.

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  • $\begingroup$ Im sorry, the chemical potential is not yet introduced to us (Im just a high school student). Is it a part of non PV Work? $\endgroup$ Feb 8 at 5:05
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    $\begingroup$ It is contribution to the system G by chemical compounds, describing how much G increases by addition of the unit amount of compound. It is kind of measure of compound energy usable potentially for work. Note that the value changes with concentration, i.e. is incremental ( differential ) $\endgroup$
    – Poutnik
    Feb 8 at 7:08
  • $\begingroup$ E.g., the same galvanic cell,but with different concentration of electrolytes, is able to provide different maximal electromotoric work per the same amount of spent active components. $\endgroup$
    – Poutnik
    Feb 8 at 7:33
  • $\begingroup$ What I'm saying is that, in addition to temperature and pressure, the Gibbs free energy of a solution/mixture depends on the proportions of the chemical species that make up the solution (or gas mixture).. Is that so hard to accept? $\endgroup$ Feb 8 at 12:14

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