If a reaction has negative $\Delta G$, can we then say with certainty that the reaction will be spontaneous?
Is negative $\Delta G$ the sole determinant for spontaneity?
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$\begingroup$ please ignore kinetics of reaction in above quo $\endgroup$– Crescent TethysMay 20 '17 at 7:21
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$\begingroup$ What do you mean by reaction? Are we talking about whole conversion of product to reactants? Or are we talking about whether one molecule of reactant converts at all to product? $\endgroup$– ZheMay 20 '17 at 15:09
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$\begingroup$ @Zhe. I would like to know how does that matter? Well,take my previous comment casually: so that some spontaneous reaction taking too long may not become point of argument. $\endgroup$– Crescent TethysMay 22 '17 at 13:52
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$\begingroup$ Because the microscopic view is governed by likelihood. Likelihood compounds in samples with large numbers. If you're talking about 1 molecule, maybe it's not completely unlikely. If you're talking about a reaction, it's quite possible that given the numbers, you'll never observe an appreciable amount of the product in the sense. Though spontaneity has a very specific definition, so I guess you're right that my previous comment is moot. $\endgroup$– ZheMay 22 '17 at 19:37
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For a spontaneous process at constant temperature and pressure, $\Delta G$ must (emphasis on must) be negative.
Spontaneity of a process , explicitly, depends on the enthalpy change and the entropy change. The Gibbs free energy change includes both these factors to predict the spontaneity of a process.
So, yes it can be said that Gibbs free energy change is the main factor that decides the spontaneity of a process.
Putting into better words, $\Delta G$ being negative is a consequence of a process being spontaneous.
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1$\begingroup$ Technically, $\Delta G <0$ isn't really necessary. It's just most probable. But for any macroscopic sample the huge number of atoms/molecules stacks the odds very heavily. If this were sports, I would all $\Delta G <0$ a prohibitive favorite. $\endgroup$ May 20 '17 at 12:14
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$\begingroup$ To add to what @OscarLanzi said, the requirement that $\Delta G$ be negative for spontaneity is just a Rule of Thumb. $\endgroup$ May 20 '17 at 12:38
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4$\begingroup$ Then again, you could just argue that "spontaneity" itself is a probabilistic label. $\endgroup$– orthocresol ♦May 20 '17 at 14:58
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1$\begingroup$ @orthocresol , you are welcome to edit my answer. $\endgroup$– MitchellMay 20 '17 at 15:17
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2$\begingroup$ You might want to mention that different state variables are used to determine spontaneity when "at constant temperature and pressure" does not hold; for instance, Helmholtz free energy for reactions taking place at constant temperature and volume but not constant pressure. $\endgroup$– zwolMay 20 '17 at 15:48