entrophy

[inaccensa]

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Entrophy is maximum at equilibrium & entrophy of a reversible reaction is zero. Can anyone explain the difference. Equilibrium is reached when the both the forward and reverse rates are equal. Isn't this contradicting?

 

[Schemp]

First off, just to be clear, it's spelled "entropy." No biggie.

Anyway, as you've probably read, all reactions seek the maximum entropy possible. So, when a reversible reaction proceeds, it will only go far enough to reach maximum entropy and then the forward and reverse reactions will be progressing at the same speed, equilibrium.

However, truly reversible reactions are not possible. You could theoretically have a reaction with zero entropy change, but in real life this doesn't happen. Therefore, a "reversible" reaction in the truest sense is not possible. If a reaction WERE truly reversible, products and reactants would exhibit equal amounts of entropy so they could interchange freely. The statement "entropy of a reversible reaction is zero" is an idealized scenario to illustrate an idea, but doesn't actually happen.

It might be easy to think of it this way: In the case of equilibrium, you're measuring a value of entropy, but in the case of a truly reversible reaction, you're measuring the change in entropy between products and reactants (and every step along the way, I believe. The change would be zero at every point or it would also reach an equilibrium and stop.).

I really feel like I didn't explain that too well, but hopefully it helped in some small way.

 

[inaccensa]

First off, just to be clear, it's spelled "entropy." No biggie.

Anyway, as you've probably read, all reactions seek the maximum entropy possible. So, when a reversible reaction proceeds, it will only go far enough to reach maximum entropy and then the forward and reverse reactions will be progressing at the same speed, equilibrium.

However, truly reversible reactions are not possible. You could theoretically have a reaction with zero entropy change, but in real life this doesn't happen. Therefore, a "reversible" reaction in the truest sense is not possible. If a reaction WERE truly reversible, products and reactants would exhibit equal amounts of entropy so they could interchange freely. The statement "entropy of a reversible reaction is zero" is an idealized scenario to illustrate an idea, but doesn't actually happen.

It might be easy to think of it this way: In the case of equilibrium, you're measuring a value of entropy, but in the case of a truly reversible reaction, you're measuring the change in entropy between products and reactants (and every step along the way, I believe. The change would be zero at every point or it would also reach an equilibrium and stop.).

I really feel like I didn't explain that too well, but hopefully it helped in some small way.

Thanks for pointing out the correct spelling. It does make sense, but if a question on the Mcat was lets say What is the entropy of a reversible system? Will be answer be zero? or should be question be what is the change in entropy of a reversible system? which based on what he said is zero.

What i understood from your explanation is that at equilibrium, the entropy of a system is max, but for a reversible system the change is entropy is zero. However, how does the relationship above relate to the formula S = dQ/T

 

[RogueUnicorn]

First off, just to be clear, it's spelled "entropy." No biggie.

Anyway, as you've probably read, all reactions seek the maximum entropy possible. So, when a reversible reaction proceeds, it will only go far enough to reach maximum entropy and then the forward and reverse reactions will be progressing at the same speed, equilibrium.

However, truly reversible reactions are not possible. You could theoretically have a reaction with zero entropy change, but in real life this doesn't happen. Therefore, a "reversible" reaction in the truest sense is not possible. If a reaction WERE truly reversible, products and reactants would exhibit equal amounts of entropy so they could interchange freely. The statement "entropy of a reversible reaction is zero" is an idealized scenario to illustrate an idea, but doesn't actually happen.

It might be easy to think of it this way: In the case of equilibrium, you're measuring a value of entropy, but in the case of a truly reversible reaction, you're measuring the change in entropy between products and reactants (and every step along the way, I believe. The change would be zero at every point or it would also reach an equilibrium and stop.).

I really feel like I didn't explain that too well, but hopefully it helped in some small way.

this is in fact what happens with reversible reactions.

 

[Schemp]

Thanks for pointing out the correct spelling. It does make sense, but if a question on the Mcat was lets say What is the entropy of a reversible system? Will be answer be zero? or should be question be what is the change in entropy of a reversible system? which based on what he said is zero.

What i understood from your explanation is that at equilibrium, the entropy of a system is max, but for a reversible system the change is entropy is zero. However, how does the relationship above relate to the formula S = dQ/T

I suppose it would have to mean there is no heat exchange between system and surroundings, the numerator would be zero.

And I think if it asked what the entropy change of a reversible system was, then in that case you would say zero. Remember though, the equation is dS = dQ/T. I don't think you can measure entropy as a static number, only the change in it.

 

[RogueUnicorn]

I suppose it would have to mean there is no heat exchange between system and surroundings, the numerator would be zero.

And I think if it asked what the entropy change of a reversible system was, then in that case you would say zero. Remember though, the equation is dS = dQ/T. I don't think you can measure entropy as a static number, only the change in it.

unlike enthalpy, i believe entropy is a measurable quantity in some instances (residual entropy for example)

 

[kentavr]

I didn't get to physics section of MCAT yet, but as far as I remember from the past:
1. Entropy is a measure of disorder (can be calculated, just not in MCAT)
2. Equilibrium is the most random state. Move it to either way and order will increase. (entropy decrease)
3. Far from equilibrium the concentrations and separation of spices has a higher order since they separated(less probability to occur)
4. In equilibrium the direct reaction plus reverse reaction does not produce entropy (we at the maximum) It means that taking separately they either do not change entropy at all or have different sign of production/consumption of entropy.