tbr test 3 physical sciences q 42

[2010premed]

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According to Figure 1, which of these statements about heated gases definitely is NOT true?

A. The more atoms there are in a gas molecule, the greater the molar heat capacity of the molecule.
B. Of the nonelemental compounds, one gram of methane requires the most heat to raise its temperature from 25˚C to 30˚C.
C. Molecules with similar molecular shapes have similar molar heat capacities.
D.
As a gas cools from 1000 K, it releases the least heat per ˚C at higher temperatures.

How would you go about this question?

 

[ilovemcat]

I'ma take a shot at answering this.

A. True - q = mc(deltaT) where mc=Heat Capacity. Increasing #atoms = increase in mass.
B. True - It has the largest change of heat capacity (most vertical line) compared to the other non-elemental molecules. Because the Heat Capacity is rising the fastest, it would require the most heat input for a short temperature change.
C. True. Based on the Graph, all Diatomic Elements have similar Heat Capacities, so I'd assume this statement is true.
D. False. If you look at the slopes of H20 and CO2 above they get smaller and smaller as the temperature gets lower. A smaller slope corresponds to a smaller heat capacity which in turn corresponds to a smaller amount of heat released since q=CdeltaT where C is heat capacity.

What was the answer?

 

[Rabolisk]

I'ma take a shot at answering this.

A. True - q = mc(deltaT) where mc=Heat Capacity. Increasing #atoms = increase in mass.
B. True - It has the largest change of heat capacity (most vertical line) compared to the other non-elemental molecules. Because the Heat Capacity is rising the fastest, it would require the most heat input for a short temperature change.
C. True. Based on the Graph, all Diatomic Elements have similar Heat Capacities, so I'd assume this statement is true.
D. False. If you look at the slopes of H20 and CO2 above they get smaller and smaller as the temperature gets lower. A smaller slope corresponds to a smaller heat capacity which in turn corresponds to a smaller amount of heat released since q=CdeltaT where C is heat capacity.

What was the answer?

I agree that the answer is D, because it is the only one that is definitely falsifiable through the graph. I don't exactly agree with the reasoning for A and B, but nevertheless, D is clearly the correct answer.

 

[fizzgig]

prob just missing something, but can someone explain B to me? 25-30degC is roughly 300K, and C2H6 has the highest slope there...

 

[Rabolisk]

prob just missing something, but can someone explain B to me? 25-30degC is roughly 300K, and C2H6 has the highest slope there...

Look closely at the y axis for the graph.

 

[fizzgig]

J/mol K, so C2H6 in j/g K is divided by 2x while CH4 in J/g K is divided by x, so in terms of per gram methane is higher?

 

[Rabolisk]

That's right.

 

[fizzgig]

thanks a lot! devil's in the details. i have done some of the strategy of writing down why i missed questions... the number of times i just had to write down or underline/star the same *#*($@ reason - "READ" - was unbelievable... gotta start catchin these things and snag some points back!

 

[IamSuperDoctor]

its a tricky question, the best approach is to choose the answer thats obviously wrong and move on instead of contemplating.

 

[BerkReviewTeach]

its a tricky question, the best approach is to choose the answer thats obviously wrong and move on instead of contemplating.

Is it really tricky? They give you a graph with all of the information necessary and then make you determine which statement is invalid. This is exactly what the MCAT requires, so I tend to think these are the questions where students need to absorb the thought process more than any other type.

Choice A: "The more atoms there are in a gas molecule, the greater the molar heat capacity of the molecule." This can be read directly from the graph. The monatomic and diatomic species are lower than the polyatomic molecules, so A is true according to the graph.

Choice B: "Of the nonelemental compounds, one gram of methane requires the most heat to raise its temperature from 25?C to 30?C." This is the trickest of the answers I think, only because they give you the temperature in ?C in the answer choice while the graph is in K. If you draw a vertical line (mentally) at the 300 K mark, methane is 38 or so, CO2 is 40 or so, and ethane is 52 or so. I'm ignoring water and N2, because both are heavier than methane and have lower molar heat capacities. 38/16 > 40/44 and 38/16 > 52/30, so B fits with the graph.

Choice C: "Molecules with similar molecular shapes have similar molar heat capacities." I guess so. I don't see enough in the graph to dispute this, so it gets a "sure, why not?" assessment for now. Labeling an answer choice in such an uncertain way used to really bother me, but with more and more practice, I learned to be okay with ignoring it for the moment to see if there was an obviously better answer. This may not seem like much of a test skill, but it was enormous for me.

Choice D: "As a gas cools from 1000 K, it releases the least heat per ?C at higher temperatures." Every line in the graph has a positive slope as you move left to right (increasing temperature), so they have higher heat capacities at 1000K than at a lower temperature of like 298K (for instance). This means that all of the species in the graph absorb more heat per ?C at higher temperatures, which also means that all of the species in the graph release more heat per ?C at higher temperatures. This makes D false. For D to be true, all of the slopes in the graph would have needed to be negative.

After all of that work on B and uncertainty on C, it ends up that D is a clear best choice.

 

[2010premed]

I guess the confusing part for me is the definition of heat capacity. I always thought of heat capacity as the necessary heat a compound must absorb in order to change temperature. The wording of D "releases the least heat per ˚C at higher temperatures" is confusing

 

[BerkReviewTeach]

I guess the confusing part for me is the definition of heat capacity. I always thought of heat capacity as the necessary heat a compound must absorb in order to change temperature. The wording of D "releases the least heat per ?C at higher temperatures" is confusing

Good point. They teach and define heat capacity as the heat absorbed to raise 1g of a material by 1 degree and never emphasize that means that it is also the heat released by 1g of a material when it drops by 1 degree.

It's a simple concept that is so unfamiliar, that it throws people. This is exactly what frustrates people on the MCAT with many questions. Better to get the treatment now, so you'll be ready on test day.

 

[nickelbackfan]

Good point. They teach and define heat capacity as the heat absorbed to raise 1g of a material by 1 degree and never emphasize that means that it is also the heat released by 1g of a material when it drops by 1 degree.

It's a simple concept that is so unfamiliar, that it throws people. This is exactly what frustrates people on the MCAT with many questions. Better to get the treatment now, so you'll be ready on test day.

So I had the same problem with the wording on this question. Just to clarify, Heat capacity is measured in both ways: the amount of heat absorbed while raising temp (absorbed) OR the amount of heat released when decreasing temperature?

 

[sciencebooks]

So I had the same problem with the wording on this question. Just to clarify, Heat capacity is measured in both ways: the amount of heat absorbed while raising temp (absorbed) OR the amount of heat released when decreasing temperature?

Yes. If it absorbs that much one way, it must release that much the opposite way.