Does spontaneity really only apply to the net free energy change of a reaction?

I used to be taught that spontaneous reactions could occur on their own BUT without enzymes or a spark they might take thousands of years to occur.

So teachers would say that spontaneity is more describing whether a reaction will occur on its own. Rather than a statement of reaction speed.

Because there is still activation energy to deal with, so technically spontaneous reactions to need an input of energy.

So is a spontaneous reaction more a statement of the reaction energetics?

And could a non-spontaneous reaction occur on its own given enough time?

  • 1
    $\begingroup$ Thermodynamics is not concerned with time only equilibria and that means starting and ending conditions. A huge activation barrier, between reactants and products makes compounds stable on a human time scale, for example carbon reacting with oxygen means that a diamond is stable for millenia at room temperature, but heat it in a flame and the effective activation energy is less and reaction can be completed in short order. Reactions occur whether they are spontaneous or not, its just a matter of how much product vs reactant is present at equilibrium. $\endgroup$
    – porphyrin
    May 26 '18 at 18:57

An analogy may help. Imagine a rock perched at the top of a hill. If given a small push, the rock will roll all the way down the hill without any further input of energy (in fact, releasing energy as it goes). That's what we mean by a "spontaneous" reaction. The reverse (pushing the rock up the hill) is a "nonspontaneous" reaction into which you have to continuously put energy for it to proceed.

But now let's consider that small initial push. Just how small is it? If the rock is small and perched on a very sharp hilltop, maybe a strong breeze or a squirrel jumping on it is enough. But if the rock is big and perched on a rather broad hilltop, it may need an earthquake or big tree falling on it to get moving. Of course, earthquakes and big trees falling are much rarer events than a squirrel jumping around or high wind. So if the rock needs a tiny push, we expect it to come flying down the hill quite soon after it's put up there, and if it needs a big push, it will be quite a long time before it gets rolling.

This is what we mean by the kinetics of the reaction -- how fast it goes. A fast reaction needs hardly any push to get started, a slow reaction needs a big push, and small pushes are much more common than big pushes. (The "push" for a real chemical reaction is some fluctuation in energy in the environment, like the coincidence of a bunch of simultaneous high-speed collisions of molecules.)

Hopefully the analogy makes clear that the thermodynamics of a reaction (whether it is spontaneous or not) is completely separate from the kinetics (whether it is fast or not).

Finally, use caution when everyday words (like "spontaneous") are used in scientific language. The implications may not be the same! In everyday speech "spontaneous" often has the additional implication of "quick." If I said "I spontaneously break into song during meetings" you would find it odd if I only did it every 25 years. There's an implication, in everyday speech, of speed as well as no need for a driving force. But that implication does not carry over to the scientific use of the word!


Spontaneity has no relation to speed or rate of reaction. A reaction might be spontaneous and at the same time it might take a thousand years to complete. And it is true that spontaneous reactions occur on their own without the input of external energy.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.