EK Chemistry 30 Minute Test 4 Question 92

[StretchDoe]

How do you do this problem? It was the last question in the exam and I kind of ran out of time. Either way I don't really know how to approach it.

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[sazerac]

deltaS =
deltaShot + deltaScold =
deltaQ / Thot + deltaQ / Tcold =
-1000J / 200K + +1000K / 100K =
-5 J/K + +10J/K =
+5J/K

Check your work:
The first law of thermodynamics ("1. you can't win") means answer A is wrong.
The second law of thermodynamics ("2. you can't break even") means answer B is wrong.

This question is testing the difference between the simple student (or rushed student) who mistakenly thinks or hopes that that:
dS = dQ / dT (WRONG!)
vs the student who has a gut feeling that multiple denominator math (like resistors in parallel or capacitors in series) is tricky and slows down and does algebra correctly.

Good question!

 

[StretchDoe]

Yes, because that's the only way you reach equilibrium. At equilibrium, the forward and reverse rates are equal. If the forward rate increases and the reverse rate decreases, those rates will never converge. That's like forward rate = 10 and reverse rate = 2 initially and then forward rate = 11, reverse rate = 1, etc. Those rates won't converge. The first series of charts on this page might help you visualize it: http://www.800mainstreet.com/7/0007-008-le_chatelier.html

deltaS =
deltaShot + deltaScold =
deltaQ / Thot + deltaQ / Tcold =
-1000J / 200K + +1000K / 100K =
-5 J/K + +10J/K =
+5J/K

Check your work:
The first law of thermodynamics ("1. you can't win") means answer A is wrong.
The second law of thermodynamics ("2. you can't break even") means answer B is wrong.

This question is testing the difference between the simple student (or rushed student) who mistakenly thinks or hopes that that:
dS = dQ / dT (WRONG!)
vs the student who has a gut feeling that multiple denominator math (like resistors in parallel or capacitors in series) is tricky and slows down and does algebra correctly.

Good question!

Thanks! One issue I had was I did not know the equation DeltaS= Q/T. I've never seen that equation/don't really understand it. Is that somehow derived from the Gibbs free energy equation? And also what do you mean you can't win and you can't break even in regards to the two laws?

This question got me all the way messed up lol. Questioning my understanding of thermodynamics.

 

[sazerac]

Thanks! One issue I had was I did not know the equation DeltaS= Q/T. I've never seen that equation/don't really understand it. Is that somehow derived from the Gibbs free energy equation? And also what do you mean you can't win and you can't break even in regards to the two laws?

This question got me all the way messed up lol. Questioning my understanding of thermodynamics.

Be careful... the equation isn't dS=Q/T, the equation is dS=dQ/T. It's a subtle but important distinction, especially for this question.

The three laws of thermodynamics are (roughly):
1. dS > 0 (the universe gets more random)
2. dS <> 0 (no such thing as perpetual motion)
3. These rules always apply

But I find it much easier to remember them as:
1. You can't get ahead
2. You can't break even
3. You can't get out of the game
It sounds like a depressing summary of casino gambling, yes?

 

[StretchDoe]

Be careful... the equation isn't dS=Q/T, the equation is dS=dQ/T. It's a subtle but important distinction, especially for this question.

The three laws of thermodynamics are (roughly):
1. dS > 0 (the universe gets more random)
2. dS <> 0 (no such thing as perpetual motion)
3. These rules always apply

But I find it much easier to remember them as:
1. You can't get ahead
2. You can't break even
3. You can't get out of the game
It sounds like a depressing summary of casino gambling, yes?

Haha that's a good way to put it. Thank you. Is that equation dS = dQ/T derived from an equation we should know for the mcat or is it just an equation we should know for the mcat on it's own?

 

[aldol16]

Thanks! One issue I had was I did not know the equation DeltaS= Q/T. I've never seen that equation/don't really understand it. Is that somehow derived from the Gibbs free energy equation? And also what do you mean you can't win and you can't break even in regards to the two laws?

Why did you quote my post from a completely different thread?

Also, the three laws of thermodynamics can be summarized in the three statements alluded to earlier (it's a humorous way to remember the three laws): 1) You can't win - energy must be conserved so you can't ever get out of a system more energy than you put into it. In other words, energy cannot be created or destroyed - it can only be transformed. 2) You can't break even - entropy never decreases. Entropy will always get you in the sense that it will always increase (or stay the same). 3) You can't even get out of the game - only a perfect crystal has zero entropy at absolute zero. You can't get to absolute zero, so entropy always exists.

 

[StretchDoe]

Why did you quote my post from a completely different thread?

Also, the three laws of thermodynamics can be summarized in the three statements alluded to earlier (it's a humorous way to remember the three laws): 1) You can't win - energy must be conserved so you can't ever get out of a system more energy than you put into it. In other words, energy cannot be created or destroyed - it can only be transformed. 2) You can't break even - entropy never decreases. Entropy will always get you in the sense that it will always increase (or stay the same). 3) You can't even get out of the game - only a perfect crystal has zero entropy at absolute zero. You can't get to absolute zero, so entropy always exists.

Sorry didn't realize I did that. I think your post was in my quote thing by accident causing a multi quote.

 

[sazerac]

Haha that's a good way to put it. Thank you. Is that equation dS = dQ/T derived from an equation we should know for the mcat or is it just an equation we should know for the mcat on it's own?

It is something you should know. I would go back to your physics book and just see where they introduce the concept of entropy.

I checked Khan Academy and I think he has a really odd way of introducing S=Q/T that is a bit confusing.

The good news is, if you can accept this equation as true, then his Physics -> Thermodynamics -> Entropy Intuition lecture:

https://www.khanacademy.org/science...cs/laws-of-thermodynamics/v/entropy-intuition

will lead you through solving the original question and give you a better gut feeling for entropy. And the MCAT is a lot about gut feeling.