What exactly is the definition of "reversible"?
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What exactly is the definition of "reversible"?
In the terms reversible and irreversible expansion, what does "reversible" mean? And why are all real processes reversible?
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Re: What exactly is the definition of "reversible"?
Reversible is when there is no net force pushing the change in one direction or the other.
Re: What exactly is the definition of "reversible"?
Reversible is the process where the expansion/compression is done in such small amounts that the overall change for each step is ~ 0 so it's deemed "reversible"
Re: What exactly is the definition of "reversible"?
Reversible processes occur under ideal, frictionless conditions and on slow enough time scales to allow for the most work to be done by a system; no energy is lost as heat, and as such, the entropy of the universe stays constant.
I'm not sure what you mean at the end; on a macroscopic scale, no real processes are reversible.
I'm not sure what you mean at the end; on a macroscopic scale, no real processes are reversible.
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Re: What exactly is the definition of "reversible"?
We didn't touch on reversibility of processes much in lecture, so it completely makes sense to be wondering what exactly this term refers to. Since I'm taking Physics 5B at the same time as Chem 14B, I've been using the physics explanation to put pieces together for thermodynamics. In physics, I learned that most processes in everyday life---those at the macroscopic level---are irreversible. Here is my favorite example from my physics textbook: "An ice cube left on the counter turns into a puddle of water, but a puddle of water on the counter never turns into an ice cube" (Mastering Physics, Chapter 14.1).
Even though a puddle of water can freeze, it would never spontaneously reshape itself into that of an ice cube. And yet, when we look at the microscopic level, many processes are in fact reversible. Think of a single molecule of gas at point A, moving to a surface and bouncing off of it at point B, and ending up at point C. Now think of that same process going in reverse. The gas particle is capable of moving from point C to point B to point A, which is why the process is reversible.
What is the point of this? Essentially, this ties into the second law of thermodynamics, which describes entropy. Once again, taken from my physics textbook: "The individual collisions [of particles] may be reversible, but the dispersal of energy is an irreversible process" (Mastering Physics, Chapter 14.1.1). Think of an untied balloon full of helium. If you let go of the balloon, all of the helium gas will disperse and the balloon will fly around and make a funny noise. This is a spontaneous process---it happens without any external input of energy. But you will never see helium gas spontaneously fly into an empty balloon and fill it up. That is because energy/matter doesn't concentrate itself spontaneously, but it always disperses spontaneously. That is what entropy is. The measure of the dispersal of energy.
Even though a puddle of water can freeze, it would never spontaneously reshape itself into that of an ice cube. And yet, when we look at the microscopic level, many processes are in fact reversible. Think of a single molecule of gas at point A, moving to a surface and bouncing off of it at point B, and ending up at point C. Now think of that same process going in reverse. The gas particle is capable of moving from point C to point B to point A, which is why the process is reversible.
What is the point of this? Essentially, this ties into the second law of thermodynamics, which describes entropy. Once again, taken from my physics textbook: "The individual collisions [of particles] may be reversible, but the dispersal of energy is an irreversible process" (Mastering Physics, Chapter 14.1.1). Think of an untied balloon full of helium. If you let go of the balloon, all of the helium gas will disperse and the balloon will fly around and make a funny noise. This is a spontaneous process---it happens without any external input of energy. But you will never see helium gas spontaneously fly into an empty balloon and fill it up. That is because energy/matter doesn't concentrate itself spontaneously, but it always disperses spontaneously. That is what entropy is. The measure of the dispersal of energy.
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Re: What exactly is the definition of "reversible"?
Reversible is when the reactants being turned into products happens at the same time as the products being turned into reactants
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Re: What exactly is the definition of "reversible"?
a process is reversible because it is done in small enough intervals that no energy is gained or lost by the system and therefore the system and its surroundings remain in equilibrium and therefore there is no net force pushing the reaction in one direction or another. this allows the reaction to be "reversed" because the end state of the energy of the reaction is the same as the beginning energy and therefore it can occur again.
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