equilibrium and entropy

[Oh_Gee]

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From GS-6 PS

can someone explain if A is also a true answer. I understand that D is the BEST answer but I've also been told that entropy is max at equilibrium so doesn't it make sense that delta entropy will also 0? Also what happens to delta H at equilibrium. just curious about that last point

 

[type12]

The question is terribly worded, and wouldn't be seen on the MCAT. You need two states to have a change. What are we comparing the equilibrium state to? In short, if we take this into physics territory, the minimum required is a difference of time, which this question doesn't even provide. And also, the change of what is zero? The universe? The reaction? The environment? Who knows.

Worded as it is, there is no right answer. Or, all of them are correct because there's only one state, so the change is zero.

 

[Oh_Gee]

oh boy i feel like i should know this but what do you mean by "state"?

 

[type12]

oh boy i feel like i should know this but what do you mean by "state"?

Let's try this a different way.

This question:
You are standing. What changed?

Can you answer the above question? Does it make sense? How can there be change if there's one situation.

How MCAT questions are:
You are standing. Now you are sitting. What changed?

Here you have two different states. You can't compare one thing with itself. Here "change" makes sense.

 

[Oh_Gee]

Let's try this a different way.

This question:
You are standing. What changed?

Can you answer the above question? Does it make sense? How can there be change if there's one situation.

How MCAT questions are:
You are standing. Now you are sitting. What changed?

Here you have two different states. You can't compare one thing with itself. Here "change" makes sense.

would a valid re-write of this question be: if the reaction stopped moving in the forward direction and is now at equilibrium, which of the following is true??

 

[MedHopeful20134]

At equilibrium, you can only say that Delta G is zero. You need more information to know what is happening to entropy or enthalpy.

 

[Oh_Gee]

At equilibrium, you can only say that Delta G is zero. You need more information to know what is happening to entropy or enthalpy.

what about if the question was changed to this:

if the reaction stopped moving in the forward direction and is now at equilibrium, which of the following is true?

 

[MedHopeful20134]

Well, if it was moving in a forward direction before reaching equilibrium, we could say that the reaction is spontaneous in the forward direction (delta G is negative).
At equilibrium (dynamic equilibrium) however, delta G is zero.
Key equation is this: Delta G = Delta H - T Delta S

You can't really say anything about entropy (Delta H) or enthalpy (Delta S) without more information.

 

[type12]

No, you really can't say what delta G is (delta refers to change) without having two different states

 

[MedHopeful20134]

@type12

At equilibrium, there would be no net change in free energy, so wouldn't delta G be equal to zero?

 

[type12]

@MedHopeful20134 There's no change in state, so there's no change. delta G doesn't exist. It's like asking. "What's the difference between Tom and Tom at 12:00 AM?" Sure, you can argue semantics, but with your logic, all answer choices are correct.