How do I determine if a process is reversible without numeric information?

It would be nice to know how to determine whether a process is reversible in order to solve problems LIKE this one:

A weather balloon is filled with Helium gas and released from the ground. It goes up 18km and achieves a diameter of 15m. Determine if the following values are greater than zero, less than zero, or equal to zero: ΔV, ΔP, ΔT, ΔU, ΔH, S_sys, S_surr, S_tot.

Here is my attempt at solving this problem. I believe at least half of my error comes from not knowing how to determine if this process, or any other process merely described, is reversible.

Obviously, ΔV increases because the balloon expands as the diameter gets to be large compared to what it presumably was at the time of release, even though the diameter at that time isn't mentioned. ΔV > 0

ΔP < 0 because pressure decreases as altitude increases.

ΔT < 0 because temperature increases as altitude increases.

ΔU < 0 because ΔT < 0 ΔH < 0 because ΔT < 0

It seems that I have assumed that the external properties of T and P apply to the inside of the balloon, so I guess the system is reversible. Stot = 0

Ssys = -Ssurr

I don't really know where to go from here...

I want to argue that because the balloon actually does manage to overcome gravity, it's change in volume must be pretty substantial (making the w = -PdV be what causes the balloon to go up). Maybe it's more substantial than the change in temperature, giving a large positive term due to volume in the entropy state equation listed above. Ssys > 0

However, if the system is being looked at as the balloon itself rather than the air inside the balloon, then when the balloon isn't inflated, it has more freedom of movement than after it's fully inflated and therefore more entropy, meaning that the final entropy change of the system would be negative.

If I am correct in guessing that this process is reversible AND in guessing that Ssys > 0, then Ssurr < 0.

I would rather not have all of my answers to physical chemistry problems be guesses. How can I solve this problem without guessing? How can I know if my guesses are correct or not? How can I better approach this problem? It's possible that the correct answer to this problem is due to something that I have completely overlooked. How do I not overlook considerations while doing these types of problems?


2 Answers 2


Any process is irreversible if it involves:

  1. Friction
  2. Heat transfer across a finite temperature difference
  3. Expansion when there is a finite pressure difference
  4. Mixing of difference substances under any real circumstances
  5. Anything else where work is not being maximized

So air friction against the balloon rising at a non-zero velocity makes the process irreversible.

Heat transfer from the balloon when the inside temperature is different from the outside temperature makes the process irreversible.

For more information see: http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node34.html


This might help you understand why it is a reversible process http://how-it-looks.blogspot.com/2010/10/reversible-processes.html

and check out this website in order to get an understanding of how hot air balloon works http://socratic.org/questions/how-do-gas-laws-affect-hot-air-balloons Charles Law chemed (.) chem (.) purdue (.) edu/genchem/topicreview/bp/ch4/gaslaws3.html

When thinking of whether or not something is reversible, ask yourself can the system goes back to the way it was before without exerting external work on the system.


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