Do Spontaneous Reactions actually need activation energy?

[Redriq1]

I've been researching a lot of chemistry lately and one thing confused me, apparently some spontaneous reactions require activating energy?

At least thats what i found on Google but would it be contradicting the spontaneous part? Like when i think of spontaneous reaction i think like Nitroglycerin, a compound that explodes violently if not taken care properly. So spontaneous should not need any activating energy since its own energy would have enough to start the reaction right??

But then i found out burning wood is also a spontaneous reaction and basically any combustion reaction and also diamonds turning into graphite is also a spontaneous reaction since it get less energy after turning into graphite. All of them need some kind of energy to kickstart the reaction and all of them are spontaneous reactions.

Sooo can anybody explain to me whats going on? Thank you

 

[.Scott]

I think there are to factors here: the QM one and the practical one.
I will let the QM experts (such as @A. Neumaier ) correct me if I'm wrong, but any exothermic reaction, given enough time (even if measured in multiple ages of the universe) will occur through tunneling (if not without it).
So, from a theoretical stand point: No, if you're willing to wwwwwaaaaaaaiiiiiitttttt, you don't need a kickstart.
I'm suspecting the diamond/graphite case may fall into that category.

As a matter of practicality, a vial of Nitroglycerin at the wrong temperature is simply waiting for the right physical conditions to push it over the edge. Will the C-sharp note from the music down the hall set up a standing wave on that liquid surface? Perhaps a slight updraft in that wood pile. Or a moments exposure to direct sunlight. An electrostatic discharge.

In the case of wood, there is a very wide range of temperatures that describe its destruction by heat. Raising it to about 500F will cause destructive distillation, but not necessarily combustion. But if your method of raising its temperature is to place it in an oven with wood that is already burning, you will not only trigger destructive distillation, but the ensuing methane will ignite and contribute to the heating of the entire wood pile. At about 700F, that combustion can occur "spontaneously" when a methane/air mixture is in contact with the charred wood surface. Even then, I have noticed that a steady wind will encourage that spontaneity.

 

[Borek]

There are two kinds of stability - thermodynamic stability and kinetic stability. Something can be ready to react and follow thermodynamics, but unable to do so for kinetic reasons (reaction is way too slow). That's where the activation energy comes into play.

In a way every reaction that follows thermodynamics is "spontaneous" assuming temperature is high enough. Cool it down and kinetics comes into play.

 

[Redriq1]

I think there are to factors here: the QM one and the practical one.
I will let the QM experts (such as @A. Neumaier ) correct me if I'm wrong, but any exothermic reaction, given enough time (even if measured in multiple ages of the universe) will occur through tunneling (if not without it).
So, from a theoretical stand point: No, if you're willing to wwwwwaaaaaaaiiiiiitttttt, you don't need a kickstart.
I'm suspecting the diamond/graphite case may fall into that category.

As a matter of practicality, a vial of Nitroglycerin at the wrong temperature is simply waiting for the right physical conditions to push it over the edge. Will the C-sharp note from the music down the hall set up a standing wave on that liquid surface? Perhaps a slight updraft in that wood pile. Or a moments exposure to direct sunlight. An electrostatic discharge.

In the case of wood, there is a very wide range of temperatures that describe its destruction by heat. Raising it to about 500F will cause destructive distillation, but not necessarily combustion. But if your method of raising its temperature is to place it in an oven with wood that is already burning, you will not only trigger destructive distillation, but the ensuing methane will ignite and contribute to the heating of the entire wood pile. At about 700F, that combustion can occur "spontaneously" when a methane/air mixture is in contact with the charred wood surface. Even then, I have noticed that a steady wind will encourage that spontaneity.

So what you're saying is every spontaneous reaction have some activation barrier they need to overcome and different substance have different activation energy?
Nitroglycerin only need a small amount of energy to start and wood needing high temperatures to start?

 

[Redriq1]

There are two kinds of stability - thermodynamic stability and kinetic stability. Something can be ready to react and follow thermodynamics, but unable to do so for kinetic reasons (reaction is way too slow). That's where the activation energy comes into play.

In a way every reaction that follows thermodynamics is "spontaneous" assuming temperature is high enough. Cool it down and kinetics comes into play.

So spontaneous reactions can happen without activating anergy it just way to slow it doesn't even happen in the first place and activating energy helps that by speeding up the molecules encouraging collisions and making the reactions happen? Is that what you're saying?

Im sorry if im just repeating what you're saying i just want to be clear and really understand what you just said

 

[Mayhem]

Where E_A is the activation energy. If you spend some time in synthetic chemistry, you will find that some reactions just stands still and still and still until the hotplate reaches the right temperature and reaction is instant (not always this ideal).

I have always interpreted this as practical demonstration of the above.

 

[Borek]

So spontaneous reactions can happen without activating anergy

No, activation energy is needed always, see the plot Mayhem posted. It may just happen that if the temperature is high enough enough molecules already have thermal energy high enough to overcome the energy barrier (average energy per molecule is proportional to kT - the higher the temperature, the higher the energy).

 

[snorkack]

No, activation energy is needed always, see the plot Mayhem posted.

Funnily enough, I found an example of a reaction that actually has negative activation energy:
https://www.ispc-conference.org/ispcdocs/ispc6/content/6/06-0588.pdf
A different source suggesting low or no activation energy:
https://www.researchgate.net/publication/289539333_The_Reactivity_of_Fluorine

 

[Redriq1]

Funnily enough, I found an example of a reaction that actually has negative activation energy:
https://www.ispc-conference.org/ispcdocs/ispc6/content/6/06-0588.pdf
A different source suggesting low or no activation energy:
https://www.researchgate.net/publication/289539333_The_Reactivity_of_Fluorine

Chemistry is so confusing man but interesting at same time.. i like that

 

[A. Neumaier]

I think there are to factors here: the QM one and the practical one.
I will let the QM experts (such as @A. Neumaier ) correct me if I'm wrong, but any exothermic reaction, given enough time (even if measured in multiple ages of the universe) will occur through tunneling (if not without it).

??? At these astromonical time scales the systems involved are likely to no longer exist for other reasons....

No chemical company will build a reactor for a reaction whose spontaneous initialization takes more than a tiny time. The usual way to proceed is to use catalyzers that remove the energy barrier of an exothermic reaction - then it happens spontaneously and quickly.

 

[snorkack]

No chemical company will build a reactor for a reaction whose intialization takes more than a tiny time.

Plenty of chemical companies build bombs. The point of a useful explosive is that until the explosion is intentionally initiated (normally by supplying heat or impact), it will not explode spontaneously, and also the exothermic reaction will not happen spontaneously molecule by molecule, dissipating the heat without exploding, because that would mean the explosive degrading.

The usual way to proceed is to use catalyzers that remove the energy barrier of an exothermic reaction - then it happens spontaneously and quickly.

Or provide the energy to cross the barrier, in form of heat.

 

[A. Neumaier]

Plenty of chemical companies build bombs. The point of a useful explosive is that until the explosion is intentionally initiated (normally by supplying heat or impact), it will not explode spontaneously,

Of course. I had meant spontaneous initialization - solely through tunnneling or thermal barrier crossing. A bomb would be very dangerous for its creator if the latter would have an appreciable probability....

 

[Mayhem]

This kind of yapping about quantum tunneling and the heat death of the universe is exactly why chemists avoid physics rigor in their work!

 

[.Scott]

??? At these astromonical time scales the systems involved are likely to no longer exist for other reasons....

No chemical company will build a reactor for a reaction whose spontaneous initialization takes more than a tiny time. The usual way to proceed is to use catalyzers that remove the energy barrier of an exothermic reaction - then it happens spontaneously and quickly.

Yes. As I said, there's the practical answer and the technically accurate one.

 

[ketoenol]

Funnily enough, I found an example of a reaction that actually has negative activation energy:
https://www.ispc-conference.org/ispcdocs/ispc6/content/6/06-0588.pdf
A different source suggesting low or no activation energy:
https://www.researchgate.net/publication/289539333_The_Reactivity_of_Fluorine

The usual phrase to look for in the literature is "barrier-less reactions". For example imagine 2 hydrogen atoms each with one electron approaching each other from infinity: there is a constant decrease in energy from the free atoms to the bonded molecule. Lots of radical reactions are essentially barrier-less (although you have to form the free radical somehow, which certainly has an energy barrier)