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Given the formula $\Delta G = \Delta H - T\Delta S$. It is commonly written that all spontaneous reactions necessarily have $\Delta G < 0$. I was curious if the converse is true: Are all reactions with $\Delta G < 0$ spontaneous. If not, why?

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    $\begingroup$ Yes they are spontaneous from a thermodynamic point of view. Kinetics is another matter... $\endgroup$ – bon Feb 4 '17 at 11:14
  • $\begingroup$ Will a reaction with ΔG < 0, involving thermodynamically unstable, but kinetically stable reactants be considered spontaneous, then? $\endgroup$ – b3nj4m1n Feb 5 '17 at 10:06
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    $\begingroup$ yes spontaneous. Thermodynamics does not involve time at all, just energy changes and heat will be given out in such a reaction. With a catalyst this may be fast if not can be incredibly slow, eons. Spontaneous should not be confused with, as it were, 'it will happen as we watch it'. Kinetically slow, thermodynamically spontaneous is very common, thankfully; its the state we all exists in in an oxygen atmosphere :) $\endgroup$ – porphyrin Feb 8 '17 at 11:50
  • $\begingroup$ @porphyrin That comment is just good as an answer. Do you please mind writing one? ;) Thanks! $\endgroup$ – Gaurang Tandon Mar 22 '18 at 11:28
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No, Gibbs energy is only the relevant state function in the special case of constant temperature and pressure.

In the wording of IUPAC:

partial derivative of Gibbs energy with respect to extent of reaction at constant pressure and temperature [is negative] for spontaneous reactions

In the special case of constant temperature and volume, Helmholtz energy is the relevant state function.

Most generally, a process is spontaneous if the entropy of the universe (system + surroundings) increases.

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