Gen Chem Phase Diagram Question

[Xo1991]

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http://imgur.com/Rft0rWi

For question 67, I put D. Which corresponds to point Z. The answer Kaplan gives (E) is that condensation can occur at the triple point. I do not see how this can occur according to the phase diagram. Condensation is gas to liquid and at no point can that happen at the triple point. But I do see how it can go through deposition through the triple point. Is that possible?


For question 69, the answer they give is B. I put D. I say D because heat capacity refers to the slope. The first slope is of much longer length for the first line (Solid temperature increasing) than for the second line (liquid temperature increasing).

Why is it B? The heat of vaporization refers to the flat lines of the graph. The heat of fusion line is shorter (less Q/heat) than the heat of vaporization, so how the heck is the heat of vaporization greater than the Heat of fusion?

 

[patricklin27]

cant see the pic

 

[Xo1991]

sorry, here it is

http://imgur.com/Rft0rWi

 

[Illfavor]

The answer to question #67 is always, always, always the triple point. Also, how could ice exists at such a high temperature? The thing is, by definition, where all 3 phases exist.

For #69, no clue what they mean, I would also put D.

 

[Xo1991]

The answer to question #67 is always, always, always the triple point. Also, how could ice exists at such a high temperature? The thing is, by definition, where all 3 phases exist.

For #69, no clue what they mean, I would also put D.

I think you're looking at #66, not #67

 

[kyeongkong]

For #67, I think, condensation can happen at triple point also because at triple point, all 3 phases can exist. So that could mean, any phases can interchange to any other phases.

For #69, I think, since definition of heat capacity is heat needed to increase 1 degree C, so it shouldn't be the length of the graph but the slope of the graph. So compare, if 1 unit of T increase, then how much of Q is going to increase? The one with bigger increase in Q will have bigger heat capacity. So lower heat capacity would shown as steeper slope.

Also, for B, if you just simply look at x-axis, the Q value (heat) increases as you go to right. So that's why heat of vaporization if greater than heat of fusion.

And to my eyes, the slopes look like as if heat capacity of liquid is higher as slope of solid phase is more steeper but I guess since the question doesn't give you exact number for it, I can say B is most correct???

 

[SafetyDog]

Yeah, I'm a bit confused on #69 as well.

All the slopes look similar in terms of steepness so it eliminates the last 3.

But for B, it looks like the lengths between the fusion / vaporization determines how much heat is added.

So less heat is taken to finish the heat of vaporization stage compared the heat of fusion stage since it's shorter.

Wouldn't that make heat of fusion greater than heat of vaporization because it takes more heat to finish the change of state completely?

 

[haiKUHN]

http://imgur.com/Rft0rWi

For question 67, I put D. Which corresponds to point Z. The answer Kaplan gives (E) is that condensation can occur at the triple point. I do not see how this can occur according to the phase diagram. Condensation is gas to liquid and at no point can that happen at the triple point. But I do see how it can go through deposition through the triple point. Is that possible?


For question 69, the answer they give is B. I put D. I say D because heat capacity refers to the slope. The first slope is of much longer length for the first line (Solid temperature increasing) than for the second line (liquid temperature increasing).

Why is it B? The heat of vaporization refers to the flat lines of the graph. The heat of fusion line is shorter (less Q/heat) than the heat of vaporization, so how the heck is the heat of vaporization greater than the Heat of fusion?

As everyone else said regarding #67, at the triple point, all 3 phases are in equilibrium, so if both the gas and liquid phases are present, condensation can occur. Actually, any of the 6 phase changes could occur at the triple point.

 

[Piepiesuperpie]

You can tell that Hv has the larger constant, because the phase transition requires less heat input, which is why you see a shorter flat line.

I thought that if the Hv is a larger constant (given same amount of mols), the phase transition would require MORE heat input, therefore you would see a longer flat line.

 

[haiKUHN]

You're right, pie. I guess I'm just as confused on the book's answer choice as everyone else.

 

[FeralisExtremum]

Yeah, I'm a bit confused on #69 as well.

All the slopes look similar in terms of steepness so it eliminates the last 3.

But for B, it looks like the lengths between the fusion / vaporization determines how much heat is added.

So less heat is taken to finish the heat of vaporization stage compared the heat of fusion stage since it's shorter.

Wouldn't that make heat of fusion greater than heat of vaporization because it takes more heat to finish the change of state completely?

I'm completely with you on this one. Last 3 are invalid since the slopes are the same, and B doesn't make sense since the heat of fusion is greater than the heat of vaporization.

 

[Vistabane]

For question #69, I do not think that anything can really be inferred from the length of the flat lines, at least quantitatively this is not the case. I feel like this also refers to the slopes of the lines as well, they are too close to each other to realistically tell what the heat capacity might be, and I do not think that any quantitative information can be extracted from the slopes, at least I have never been in a situation in which I have been taught that the slopes can directly relate to heat capacity.

That leaves answer (B) as the only possible choice since (A) is definitely wrong. While I think that a better "phrased" answer would have been "The temperature of the heat of vaporization is greater than the temperature of the heat of fusion," I still think that (B) is the best answer from the ones that they give you even if it is not 100% correct.