Reversible and irreversible reactions

[ananthu]

I need some clarification on the following points on the reversible and irreversible reactions.

In the examples cited in textbooks, for reversible reaction, isothermal expansion and compression are given. But the adiabatic exp and comp are not given.

Why is it not included in the list? For example, when a gas is adiabatically expanded, its pressure decreases, volume increases and the temp. falls. But, when the same is adiabatically compressed, again its pressure increases, vol. decreases and temp. increases to their original values in the reverse order. Is it not sufficient to call it reversible change?

Also, another thing I can not understand is the inclusion of electrolysis in the list of reversible process. How does the electrolysis of water fit into the category of reversible process? When electricity is passed into water, the latter splits into hydrogen and oxygen.But how does the reverse happen?

 

[Drakkith]

The hydrogen and oxygen will very easily react with each other to form water. Both reactions will start and end with an equal number of parts. There are no wasted parts or unused parts. Hence, reversible. Look it up here. http://en.wikipedia.org/wiki/Reversible_reaction

Also, I'm not sure, but it doesn't seem right to me that a gas that is expanding or compressing is a reaction at all.

 

[Mapes]

Any process (what you call a reaction) could be assumed to be reversible as long as any gradient (in temperature, pressure, chemical potential, etc.) can be made arbitrarily small. If your adiabatic process meets this requirement, you're free to model it as reversible.

 

[ananthu]

The hydrogen and oxygen will very easily react with each other to form water. Both reactions will start and end with an equal number of parts. There are no wasted parts or unused parts. Hence, reversible.

My doubt is not on that line. What I wanted to clarify is the following: On passing electric current, the water splits into oxygen and hydrogen. Similarly, if the process is reversed the opposite should happen. ie. When electricity is applied to a mixture of oxygen and hydrogen they should form water. Does it actually happen? I think, only when a spark is introduced into the mixture they form water with a "plop".

"Any process (what you call a reaction) could be assumed to be reversible as long as any gradient (in temperature, pressure, chemical potential, etc.) can be made arbitrarily small. If your adiabatic process meets this requirement, you're free to model it as reversible."
(Mapes)
Here also I want to add some more details regarding my doubt.
One of the conditions cited for reversible change is that the process should be slow and the other one is the system shuld be in thermal equilbrium with the surroundings. Though both these conditions fail to apply in the case of adiabatic changes, gas returns to the original conditions when the operations are done in the reverse order. In the carnot's cycle both isothermal and adiabatic expansions and compressions are shown as reversible changes. So. whole theings appear to be contradictory and confusing.

 

[Mapes]

Does it actually happen?

Sure, it's called a hydrogen fuel cell.

 

[Drakkith]

The chemical reaction of hydrogen and oxygen releases energy, which is exactly opposite from what happens when you supply electricity (IE Energy) to split the water molecule up. Like the above post says, in a hydrogen fuel cell you can combine hydrogen and oxygen and form water and electricity.