Spontaneous reactions always; A) go to completion. B) are fast. C) involve phase changes. D) release energy and/or show an increase in the system's entropy.
The most important criterion for spontaneity of a chemical reaction is a negative value of free energy change, i.e ` ` `delta G = (-)ve.`
Since,` delta G = delta H - T delta S` ` ` , for the release of energy enthalpy or `delta H` is negative and for increase in entropy `delta S` is positive. Hence, `delta G` in this case is always negative.
Again, these reactions are usually fast and go to near completion but not always. Also, spontaneity has nothing to do with phase change.
Hence, the correct answer is option D) only.