hard G chem problem

[FutureDental88]

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A Container of gas is allowed to expand so that its volume triples. What would happen to the temperature of the gas?

a: It stays constant
b: It increases
c: It decreases
d: The direction of the temperature change would depend on the PV work done by the expansion of the gas.




I thought I could use PV= NRT. But nope.

Answer is C

 

[mh0000]

When a gas expands, it does positive work on the environment by displacing atmospheric molecules that provide a counter-pressure. By doing positive work, some of the translational kinetic energy is being transfrered to the environment. Remember, temperature is a measure of the average translational KE of a sample of ideal gas. WHen the sample loses KE, the temperature will decrease. If this gas expanded in an evacuated environment, then no work is done and the temperature will be the same. But I would presume it expands against an atmosphere unless otherwise stated. Also, if the expansion is done reversibly (expanded in infinitely small increments) and the gas is not a closed system, then the temperature should not change either.

 

[indigenoustw]

It sounds like an adiabatic expansion.

Which means expansion without addition of heat into the system.
No addition of heat, then where does the energy coming from to perform the expansion?
Cooling.


Read "Adiabatic cooling" on Wikipedia.

 

[HannibalLecter]

Ehhh this is actually a bad question because no where in the question does it say that pressure stays constant during the reaction. If the volume expands, pressure could go down or it could remain constant.

But I want to bring you back to a Chads video concept that he mentions just for ideas sake:

He states that when he uses an air duster, he sees the gas molecules when coming out of the canister expand which is a cooling effect so your hand feels cold. Is pressure constant in this, no. The canister was small volume and when it is out in the open it increases volume and thus has a lower temperature and feels cold. Hope this helps.

But doesn't erase the fact that this problem is poorly worded.

 

[FutureDental88]

well, what mh0000 said sounded most to their reasoning. The answer states; " The expansion of the gas does work, so the internal energy of the gas decreases. If the internal energy of the gas decreases, the average kinetic energy also decreases. The temperature is a measure of average kinetic energy ".


So basically... even after reading the answer,,, I still dont get it... Maybe cuz I cant visualize it.... weird huh. lol

 

[mh0000]

well, what mh0000 said sounded most to their reasoning. The answer states; " The expansion of the gas does work, so the internal energy of the gas decreases. If the internal energy of the gas decreases, the average kinetic energy also decreases. The temperature is a measure of average kinetic energy ".


So basically... even after reading the answer,,, I still dont get it... Maybe cuz I cant visualize it.... weird huh. lol

don't be too hard on yourself. Where is this question from anyways?

Ehhh this is actually a bad question because no where in the question does it say that pressure stays constant during the reaction. If the volume expands, pressure could go down or it could remain constant.

But I want to bring you back to a Chads video concept that he mentions just for ideas sake:

He states that when he uses an air duster, he sees the gas molecules when coming out of the canister expand which is a cooling effect so your hand feels cold. Is pressure constant in this, no. The canister was small volume and when it is out in the open it increases volume and thus has a lower temperature and feels cold. Hope this helps.

But doesn't erase the fact that this problem is poorly worded.

As related to pressure, it depends on how it expands. If it is done very quickly or as a closed system where it cannot exhange energy with the nevironment, then the pressure will decrease. If it is done isothermally, then the pressure should also be reduced, but to a smaller extent:

PV=nRT
nRT are constants because the expansion is isthermal and the # of moles are the same. THerefore, as V increases, P has to decrease proportionately since the right side of the equation is constant. If it was an isobaric expansion, then the temperature would have to increase to maintain constant pressure. I do agree that the question can be worded more clearly.