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Entropy related questions - Reversible or Irreversible?

  1. Nov 11, 2011 #1
    Entropy related questions -- Reversible or Irreversible?

    1. The problem statement, all variables and given/known data
    A 15.0kg block of ice at 0C melts to liquid water at 0C inside a large room that has a temperature of 20C. Treat the ice and the room as an isolated system, assume that the room is large enough for its temperature change to be ignored. a) Is the melting of the ice reversible or irreversible? Explain, using simple physical reasoning without resorting to any equations.
    b) Find the net entropy change of the system during this process. Explain whether or not this result is consistent with your answer to part a.

    3. The attempt at a solution
    I initially said the process is reversible because it stays in thermal equilibrium (0C), but the answer key says it's irreversible. As for the entropy I got 1253J/K which is the right answer.

    Could someone please explain to me the theory behind entropy and reversible/irreversible processes?

  2. jcsd
  3. Nov 11, 2011 #2

    Andrew Mason

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    Re: Entropy related questions -- Reversible or Irreversible?

    A reversible process is one in which the direction of the process can be reversed by an infinitessimal change in conditions.

    If the room was kept at temperature 0C+δT/2 where δT was an arbitrarily small temperature difference, a change in temperature of δT to 0C - δT/2 would cause the ice to freeze instead of melt. So in that case, the melting of the ice would be reversible (but it would also take an infinitely long time to occur).

    But here the room is at 20C. An infinitessimal change in room temperature would not put it below 0C so it would not cause the water to freeze - the ice would still melt. So it is not reversible.

  4. Nov 11, 2011 #3
    Re: Entropy related questions -- Reversible or Irreversible?

    Ah I see, so the temperature of the room must equal the ice to be in thermal equilibrium and be reversible.

    Edit: Okay what about this question, it is isothermal (same temperature) so I don't see how it is irreversible?

    Also A 10L gas tank containing 3.2 moles of ideal He gas at 20C is placed inside a completely evacuated, insulated bell jar of volume 35L. A small hole in the tank allows the He to leak out into the jar until the gas reaches a final equilibrium state with no more leakage. A) what is the change in entropy of the system due to the leaking of the gas? b) is the process reversible or irresversible? How do you know?

    ^ The ans to the change in entropy is 33.3J/K, which is what I got but how do you know if the process is reversible or not with relation to entropy? The temperature stays in the gas and surroundings are the same; the work the gas does is the same as the work done on the surroundings. So wouldn't this mean the process is irreversible?
    Last edited: Nov 12, 2011
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