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why do we say that reversible processes are slow whereas irreversible processes are fast? Rusting is irreversible yet slow

Qmechanic
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Prathit Prasad
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  • Related https://physics.stackexchange.com/q/488619/226902 and https://physics.stackexchange.com/q/409327/226902 – Quillo Jan 27 '23 at 02:53

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The terms fast and slow with regard to irreversible vs reversible processes apply only to transport mechanisms such as fluid deformation (viscous flow), heat conduction, and molecular diffusion. For these aspects of the process to be reversible, they must occur slowly (i.e., close to thermodynamic equilibrium). On the other hand, if a chemical reaction is occurring far from equilibrium, even if its kinetics are slow, it will still be irreversible. Thus the criterion for reversibility of all of these is not really fast vs slow but instead close to- vs far removed from thermodynamic equilibrium.

Chet Miller
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What are the properties of reversible and irreversible thermodynamic processes?

Reversible thermodynamic processes are those in which the system and its surroundings can return to their original states without any change in the system's surroundings. These processes are characterized by infinitesimal changes in the system's state variables, such as temperature, pressure, and volume. They are also identified by the absence of any dissipative forces, such as friction or heat transfer across a finite temperature difference.

Irreversible thermodynamic processes, on the other hand, are those in which the system and its surroundings cannot be returned to their original states. These processes are characterized by finite changes in the system's state variables and the presence of dissipative forces.

Why do we say that reversible processes are slow whereas irreversible processes are fast? Rusting is irreversible yet slow.

Reversible processes are slow because they occur at an infinitesimal rate and involve a very small change in the system's state variables. Whereas, irreversible processes are fast occurring at a finite rate and involve a large change in the system's state variables. This is due to the presence of dissipative forces, which cause the system to lose energy and increase the rate of change of the state variables.

Rusting is an irreversible process that is slow. Rusting is a chemical reaction between iron and oxygen that is exothermic and releases heat, but it takes time to occur.

The terms "fast" and "slow" in this context are relative, they are comparing the speed of the process relative to other processes that also involve changes in entropy, not in an absolute sense.

Facebook
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    You didn’t address the OP’s example of rusting being slow yet irreversible – Bob D Jan 27 '23 at 03:12
  • @BobD Better now? – Facebook Jan 27 '23 at 03:34
  • A bit. But it still doesn't explain why rusting is an exception to the commonly stated rule that a non quasi static process is normally considered irreversible. I think the distinction has to do with physical vs chemical processes. Whereas the former doesn't alter matter, the latter (e.g. rusting) does making the latter irreversible even though quasi-static. – Bob D Jan 27 '23 at 13:14
  • The important point is in Chet MIller's answer above: not "slow" VS "fast" or "chemical" VS "physical" (that is not a very clear distinction IMO) but rather "close" VS "far" from equilibrium. Most slow processes are always "close to equilibrium", hence reversible (reversible = practically always at equilibrium)... but not always! But yes @Facebook, with the edit better (I like the part: "fast" and "slow" in this context are relative, they are comparing the speed of the process relative to other processes that also involve changes in entropy, not in an absolute sense.). – Quillo Jan 27 '23 at 15:27