Is every reaction reversible?

[Jerryt12]

This question came into my mind while I was studying chem. It would be helpful if anyone know. Thank you:-)

[Icefire]

possible, but some reversions would require so much energy that doing so would be unrealistic.

[Jerryt12]

Thanks. Can you give me an example?

[Mr Skeptic]

Yes, all reactions are reversible but the reversal can be unlikely due to a high activation energy, ending up at a higher energy level, or requiring an unlikely collision.

 

Burning is a fairly good example of a mostly irreversible reaction, especially if what you are burning is a complex chemical.

[Jerryt12]

But if a catalyst is used, we can lower the high activation energy right?

[Icefire]

Thanks. Can you give me an example?

 

Hydrogen easily reacts with oxygen to make water, but it is very difficult to split water back into hydrogen and oxygen.

[Mr Skeptic]

But if a catalyst is used, we can lower the high activation energy right?

 

Yes. For example, hydrogen and oxygen at room temperature will not combine into water, even though that reaction is energetically favored (a lot! -- see rocket exhaust for example). In the presence of a catalyst, hydrogen and oxygen can combine at room temperature to form water. Or, water can be split apart into hydrogen and oxygen. However, one of these reactions is by far the more energetically favored. There will still be some equilibrium with a minuscule amount of hydrogen and oxygen, but the vast majority of it will be as water at equilibrium. See equilibrium constants. A fuel cell is actually a nice example, because it is a setup that allows the reaction in one way to produce electricity, and can be done in reverse by inputting electricity. If you play your cards right, you can drive a reaction in a direction that is not energetically favorable by inputting energy. In fact your body does that all the time when it creates ATP.

[Radical Edward]

possible, but some reversions would require so much energy that doing so would be unrealistic.

 

some are also hideously complicated to reverse, like denaturing proteins.