Topscore Test2 GC 62

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ET87

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Which of the following reactions has the greatest negative change in entropy?

C) 2 NO (g) + O2 (g) --> NO2 (g)
D) H2O (g) --> H2O (l)

There is a typo in this question, i feel like it should be 2NO2 (g) for C. but does anyone know discretely how to really evaluate these kinds of questions, comparing phase changes with mole changes?

Like let's say you had
A) 3X (g) --> 1Y (g)
B) 3Y (g) --> 1Z (g)

the entropy change could be A > B, B > A, or A = B depending on the molecules, right? So basically a question like that will never be seen?

Now let's say you had
A) 1X (g) --> 1X (l)
B) 1X (g) --> 1X (s)

obviously greatest negative entropy change Must occur in B, right?

Now let's say you had
A) 2X (g) --> 1Y (g)
B) 1X (g) --> 1X (l)
C) 100000000X (g) --> 1Y (g)

Then C has a greater negative entropy change than A, but is there a point at which something like C (molar change) becomes a greater negative entropy change than a phase change that occurs that happens in B?

Thanks for the thoughts!

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Haha, I just finished this test about an hour ago.

Which of the following reactions has the greatest negative change in entropy?

C) 2 NO (g) + O2 (g) --> NO2 (g)
D) H2O (g) --> H2O (l)
Okay, in answer choice D, we have a gas that is condensing into a liquid. Gasses have much more disorder than liquid.

A negative change in enthalpy will surely be seen when going from...
gas to liquid
gas to solid
liquid to solid

Like let's say you had
A) 3X (g) --> 1Y (g)
B) 3Y (g) --> 1Z (g)

the entropy change could be A > B, B > A, or A = B depending on the molecules, right? So basically a question like that will never be seen?
It definitely depends on the molecules, so I doubt we would get anything like that.

Now let's say you had
A) 1X (g) --> 1X (l)
B) 1X (g) --> 1X (s)

obviously greatest negative entropy change Must occur in B, right?
Correct.

Think about this..
1X (g) --> 1X (l) Let's say delta S = -90J/K
1X (l) --> 1X (s) Let's say delta S = -50J/K


From that, we can determine the change in entropy for the deposition of X.
1X (g) --> (s)
delta S = -90 - 50 = -140

Now let's say you had
A) 2X (g) --> 1Y (g)
B) 1X (g) --> 1X (l)
C) 100000000X (g) --> 1Y (g)

Then C has a greater negative entropy change than A, but is there a point at which something like C (molar change) becomes a greater negative entropy change than a phase change that occurs that happens in B?
Hmmm... This one would be a tough one, because we're going from a kajilionbillion molecules to a single molecule vs gas to liquid. I don't think this type of situation would be very clear cut.. Considering that C) starts with a bunch of stuff.. I would say that has a greater change in entropy, but something like this is pretty much impossible.

What are the chances of 1000000000 things colliding with the correct orientation? Probably not going to happen..
 
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Hmmm... This one would be a tough one, because we're going from a kajilionbillion molecules to a single molecule vs gas to liquid. I don't think this type of situation would be very clear cut.. Considering that C) starts with a bunch of stuff.. I would say that has a greater change in entropy, but something like this is pretty much impossible.

What are the chances of 1000000000 things colliding with the correct orientation? Probably not going to happen..

lol yea that's why i was looking to see if there is a discrete way, numerically, to compare these two situations. I guess i'll just assume for any reasonable change in moles is not as great as a change in phase unless anyone else has something to say?
 
Some things to keep in mind. A "negative" entropy implies a decrease in delta(S)....which means "more order". A "positive" entropy elicts an increase in delta(S) which means "less order, more random". Going from a liquid to gas there is more randomness, so one would see a positive delta(S). For rxn equations that you can't tell just by looking at the phase of the reactants and products........then use the coefficients to determine the change in S. These are just simple rules.......the questions are just testing these....try not to get mixed up in the tricks.
 
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Now let's say you had
A) 2X (g) --> 1Y (g)
B) 1X (g) --> 1X (l)
C) 100000000X (g) --> 1Y (g)

Then C has a greater negative entropy change than A, but is there a point at which something like C (molar change) becomes a greater negative entropy change than a phase change that occurs that happens in B?

Thanks for the thoughts!

yes. You can find the exact value for an entropy change from gas to liquid for most substances. There is no universal value. That value for the state change is precisely the value you seek in the bolded statement (although you may not realize it until you read this message).
 
Some things to keep in mind. A "negative" entropy implies a decrease in delta(S)....which means "more order". A "positive" entropy elicts an increase in delta(S) which means "less order, more random". Going from a liquid to gas there is more randomness, so one would see a positive delta(S). For rxn equations that you can't tell just by looking at the phase of the reactants and products........then use the coefficients to determine the change in S. These are just simple rules.......the questions are just testing these....try not to get mixed up in the tricks.

This question tests to see if you know which produces the most order, not whether you can tell the sign delta(S). In this case, they want to see if you know whether phase changes or changes in the # of moles of a reaction produces greater order.

Majik, I see your point! So the question now is, in general (or in most practical situations) phase changes produce more order than a decrease in the # of moles of gas?
 
Majik, I see your point! So the question now is, in general (or in most practical situations) phase changes produce more order than a decrease in the # of moles of gas?

Yes, phase changes result in a greater decrease in entropy then #moles of gas because it's not likely that you will see 1000000 particles (or any huge number) reacting to become just a few molecules.
 
This question tests to see if you know which produces the most order, not whether you can tell the sign delta(S). In this case, they want to see if you know whether phase changes or changes in the # of moles of a reaction produces greater order.

Majik, I see your point! So the question now is, in general (or in most practical situations) phase changes produce more order than a decrease in the # of moles of gas?

My bad! I was reading quickly before. Yes, changes in # of moles and phase changes can also be added to the fundamental lists of rules I mentioned before. All are good aspects to know and are frequently tested. sorry about that. :thumbup:
 
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