[Bored chemist]

The freezing and thawing of water is not a naturally reversible process.

Utter bollocks.
You don't seem to know the meaning of the word.
https://en.wikipedia.org/wiki/Reversible_process_(thermodynamics)

Well if you want to be all statistical about it ... I am talking natural and not statistical. I remember being a kid and taking out the half frozen ice cubes from the freezer. You could crack them open and drink out the cold water inside.

It fascinated me that the same ice cube would thaw from the outside when it had initially frozen from the outside. This wasn't reversing the process, since it couldn't. You couldn't magically introduce heat into the centre of the ice cube. Certainly nature couldn't do that.

Heat transfer itself is not naturally reversible so how could freezing and thawing be a naturally reversible process? Statistics can hide a multitude of sins.

Given that it's not really to do with statistics, am I allowed to say "Be quiet; the grown ups are talking" ?

[jeffreyH]

The freezing and thawing of water is not a naturally reversible process.

Utter bollocks.
You don't seem to know the meaning of the word.
https://en.wikipedia.org/wiki/Reversible_process_(thermodynamics)

Well if you want to be all statistical about it ... I am talking natural and not statistical. I remember being a kid and taking out the half frozen ice cubes from the freezer. You could crack them open and drink out the cold water inside.

It fascinated me that the same ice cube would thaw from the outside when it had initially frozen from the outside. This wasn't reversing the process, since it couldn't. You couldn't magically introduce heat into the centre of the ice cube. Certainly nature couldn't do that.

Heat transfer itself is not naturally reversible so how could freezing and thawing be a naturally reversible process? Statistics can hide a multitude of sins.

Given that it's not really to do with statistics, am I allowed to say "Be quiet; the grown ups are talking" ?

Well if you can't grasp the subtlety of the point I am making I can't make up for your deficiency of understanding. Maybe go and have a lie down. Sounds like you need the rest. All that thinking appears to have made you tired.

[jeffreyH]

Precisely!!! in principle these things are reversible, but somehow, in reality, they aren't... I wonder why... it's almost as if the world is not made of ideal systems that readily achieve equilibrium, and that machines we make (macroscopic or molecular) can be inefficient at doing things, but more inefficient one way than in the reverse (think of pushing or pulling a mass with a string). LEDs make lousy photodiodes and PV cells make lousy LEDs, because there are ways of inherently tipping where the inefficiencies lie.

It may be less proper to talk about catalysts in systems that cannot achieve equilibrium, but I would rather be developing catalysts that can be useful in non-equilibrium systems that surround us. I should be more careful in defining what is "in" the systems that I work with, but functionally and practically, it still makes sense to me to talk about irreversible catalytic systems.

Single uncharged particles produce a reversible process via elastic scattering. Reversible in time. No entropy. Introduce charges of different polarity, electron and proton, and hey presto you now have irreversible processes and entropy. Once you get molecules you then break the symmetry of the individual particle. Molecules has shapes that are not uniform. They can be ionic. The now don't interact in necessarily time reversible ways.

If this is the case then larger systems have no chance of being symmetrically reversible.

[Bored chemist]

so how could freezing and thawing be a naturally reversible process?

OK, here's what reversible means
"In thermodynamics, a reversible process is a process whose direction can be returned to its original position by inducing infinitesimal changes to some property of the system via its surroundings."
From wiki
https://en.wikipedia.org/wiki/Reversible_process_(thermodynamics)

Now, imagine you have some ice at 0C and you warm it by an infinitesimal amount- to "plus very little"C

What happens?
And, if, as it's melting you change the temperature to minus very little, what happens.



You can also get the system to freeze or thaw completely, buy an infinitesimal change in pressure.
Melting and freezing is the quintessential reversible process.

Single uncharged particles produce a reversible process via elastic scattering. Reversible in time. No entropy. Introduce charges of different polarity, electron and proton, and hey presto you now have irreversible processes and entropy. Once you get molecules you then break the symmetry of the individual particle. Molecules has shapes that are not uniform. They can be ionic. The now don't interact in necessarily time reversible ways.

If this is the case then larger systems have no chance of being symmetrically reversible.

So, you still don't know what reversible means.

[jeffreyH]

Start with show crystals. Apply your infinitesimal change in pressure to thaw the snow. Then reverse the infinitesimal change in pressure. Do you still have the same snow crystals? Considering they are each unique in shape.

ChiralSPO said ...

Precisely!!! in principle these things are reversible, but somehow, in reality, they aren't... I wonder why... it's almost as if the world is not made of ideal systems that readily achieve equilibrium, and that machines we make (macroscopic or molecular) can be inefficient at doing things, but more inefficient one way than in the reverse (think of pushing or pulling a mass with a string).

[Bored chemist]

Do you still have the same snow crystals?

For an infinitesimally small change, yes you do- obviously.

Did you think you had a point there?

[scientizscht (OP)]

Do you still have the same snow crystals?

For an infinitesimally small change, yes you do- obviously.

Did you think you had a point there?

So you are saying that everything is reversible if the reversal is done in infinitesimally small steps? Even the burnt paper can turn back to paper?

[jeffreyH]

Do you still have the same snow crystals?

For an infinitesimally small change, yes you do- obviously.

Did you think you had a point there?

So tell me, how exactly do you determine the infinitesimal amount by which to change pressure. What scale do you use. Maybe we can replicate your procedure. I love practical work. I'm all ears.

[jeffreyH]

Wait. ... Wait ... Just let me get settled with a cuppa and a rich tea.

[Bored chemist]

So tell me, how exactly do you determine the infinitesimal amount by which to change pressure.

By interpolation.


Do you accept that you utterly failed to recognise that the word "reversible" has a technical meaning here?

[Bored chemist]

So you are saying that everything is reversible if the reversal is done in infinitesimally small steps?

I'm not the one who says it. Science does.
https://en.wikipedia.org/wiki/Microscopic_reversibility

But, obviously, if "everything" was reversible then we wouldn't need a word for it, so your assertions'a bit silly anyway, isn't it?

[scientizscht (OP)]

When we say that the reaction A + B <-> C + D is at equilibrium, I suppose we mean that the concentrations of all the substances are not changing with time.

When, we have an increased concentration of B, what happens? Does it mean that the rate of the conversion of B is increasing and thus the concentration of B is decreasing until it reaches again the equilibrium concentraations? What happens if there is not enough A?

[evan_au]

When, we have an increased concentration of B

If there is suddenly more of substance B, then the system is clearly no longer in the original equilibrium.

It's not clear from your description are the conditions that would result in having more of substance B.
1. Did you just pour in more B (input from outside the original system)?
- In this case, I would expect that you would consume more of A, end up with more of C & D.
- Unless you "ran out of" A, which would limit how much C & D can be made.
- There would still be a small amount of A present, as C & D are continually breaking down into A & B, as part of the new equilibrium.

2. Did you change the conditions (eg pressure or temperature), so that B would be more common in the new equilibrium?
- In this case, I would expect there to be less of C & D, and more of A
- In this scenario, you would not run out of A

[Bored chemist]

When we say that the reaction A + B <-> C + D is at equilibrium, I suppose we mean that the concentrations of all the substances are not changing with time.

When, we have an increased concentration of B, what happens? Does it mean that the rate of the conversion of B is increasing and thus the concentration of B is decreasing until it reaches again the equilibrium concentraations? What happens if there is not enough A?

I take it you didn't bother to read this
https://en.wikipedia.org/wiki/Equilibrium_constant