Question from TPR test 2015-Thermochem

[howlcat]

Hey guys, I have a question about this question I encountered on a TPR practice test..

The passage stated this: Automobile air-conditioning (AC) systems act like heat engines in reverse. Air moves over the fins of the evaporator, heating the refrigerant and converting it from a liquid into a gas. The refrigerant flows into the condenser where it is changed back into a liquid. The air which was cooled when heating the fins of the evaporator flows back into the car’s interior. See Figure 1.

The question was:
The amount of heat removed from the car depends primarily on which of the following characteristics of the refrigerant?

A)Boiling point

B) Specific heat

C) Heat of vaporization

D) Strength of the intramolecular bond

I was able to cross out B and D because of lack of relevance, and I assumed it was A because the boiling point of a fluid is an important characteristic. Apparently the answer was C-heat of vapourization. While I know C is related to the answer, how is it a characteristic of the refrigerant? More importantly, how is it a better answer choice than A? I couldn't find a solution anywhere so I decided to ask.

Thanks guys.

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[exposedbrickcampaign]

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Thats just for visual reference.
So you have a loop which is filled with a fluid that evaporates and condenses. At one end of the loop in the condenser and at the other is the evaporator. Lets begin at a point where we are leaving the condenser. Fluid leaves the condenser and as stated in a liquid. Being that it is liquid we can assume it has low temp (and high pressure). As it continues around the loop passing through the evaporator the temp of the fluid increases (and pressure on the fluid drops) and the fluid evaporates into a gas. The fluid leaves the evaporator as a gas and continues toward the condenser. Entering the condenser, the fluid is cooled and a higher pressure is applied causing the gas to evaporate. The cycle continues.
So two key points, the condenser, here, the fluid is condensed from gas to liquid. In order to do so, the temp has to decrease and the applied pressure to the gas increases, increasing the intermolecular forces, causing the gas to condense into a liquid.
At the evaporator, the applied pressure decreases and the temp increases, lessening the intermolecular forces, allowing the liquid to become a gas.
At the the evaporator, the liquid in the loop is at is cool. We dont know how cold, but it is cool nonetheless. With the liquid inside the evaporator, ambient air outside the loop passes over the fins of the evaporator, giving off the thermal energy (a transfer of heat) to the evaporator. This heat from the ambient air to the evaporator allows the evaporator to heat and evaporate the liquid inside it. The ambient air goes from warm to cool as it passes over the fins of the evaporator due to it transferring it heat to the evaporator and ultimately gets blown into the cabin of the car.
So to the crux of this post, the amount of heat removed from the ambient air and transferred to the evaporator is a direct result of the amount of heat needed to evaporate the liquid inside the evaporator.

 

[Dr. Trenb]

Few hints you should pay attention to:

1) The question is regarding "the amount of heat removed from the car depends..." Therefore it is a quantity, amount of something rather than a point/mark of a temperature.
2) The question has the word "heat." It might sound silly and too simple but some of the MCAT choices can be narrowed to the ones closest to what the question is asking in terms of key words. Although this is not always the case (such as choice B), you already ruled that out and so choice C is left. This is not to say you should match words, but staying close to the question.
3) The definition for heat of vaporization is - Amount of heat required to convert liquid into a gas. Boiling point, is the point which a substance boils. Now, which one sounds close to the question.

 

[edgerock24]

Few hints you should pay attention to:

1) The question is regarding "the amount of heat removed from the car depends..." Therefore it is a quantity, amount of something rather than a point/mark of a temperature.
2) The question has the word "heat." It might sound silly and too simple but some of the MCAT choices can be narrowed to the ones closest to what the question is asking in terms of key words. Although this is not always the case (such as choice B), you already ruled that out and so choice C is left. This is not to say you should match words, but staying close to the question.
3) The definition for heat of vaporization is - Amount of heat required to convert liquid into a gas. Boiling point, is the point which a substance boils. Now, which one sounds close to the question.

This x100.

 

[howlcat]

thanks, that does help. I went with the word "characteristic", and didn't remember heat of vapourization being a characteristic of an fluid, but remembered boiling point being one..but the explaination makes sense..thanks!