TPRH SW Gen Chem Passage 54 Q 3

[jeffs office]

3. During each trial, the temperature of the vessel's contents increased by several degrees as the water was added to the sample of gas. This is because:

A. The dissolution of gas in water is an endothermic process.
B. The ionization of a weak acid is an exothermic process.
C. The compression of a gas releases heat.
D. Friction converts the kinetic energy of flowing water into potential energy.

The answer is C. I chose answer B. I think I'm getting confused about Work and Pressure and I guess specific terms. I thought that gas releases heat when it expands, resulting in a cooler gas temperature. I thought answer C was the opposite of that, so wouldn't that make C partially wrong? I can see compression increasing heat, if expanding decreases heat, but i guess I'm stuck on this 'release heat' phrase. I understand why answer B is wrong and why the other 2 are wrong, but I need some help with my logic. Can someone explain to me what's wrong with it? Thanks!

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[Jepstein30]

3. During each trial, the temperature of the vessel's contents increased by several degrees as the water was added to the sample of gas. This is because:

A. The dissolution of gas in water is an endothermic process.
B. The ionization of a weak acid is an exothermic process.
C. The compression of a gas releases heat.
D. Friction converts the kinetic energy of flowing water into potential energy.

The answer is C. I chose answer B. I think I'm getting confused about Work and Pressure and I guess specific terms. I thought that gas releases heat when it expands, resulting in a cooler gas temperature. I thought answer C was the opposite of that, so wouldn't that make C partially wrong? I can see compression increasing heat, if expanding decreases heat, but i guess I'm stuck on this 'release heat' phrase. I understand why answer B is wrong and why the other 2 are wrong, but I need some help with my logic. Can someone explain to me what's wrong with it? Thanks!

Just did this last night. Wasn't sure about it at all but used POE.

A) can't be right, would have to be exothermic
B) it says during "EACH" trial but we know that N2 is NOT a weak acid (there was a question about this earlier), therefore that cannot explain the increase in temperature in the N2 trials
D) makes no sense.. potential energy isn't temperature..

so not sure why C is correct but that is why B is wrong.

 

[sazerac]

3. During each trial, the temperature of the vessel's contents increased by several degrees as the water was added to the sample of gas. This is because:

A. The dissolution of gas in water is an endothermic process.
B. The ionization of a weak acid is an exothermic process.
C. The compression of a gas releases heat.
D. Friction converts the kinetic energy of flowing water into potential energy.

The answer is C. I chose answer B. I think I'm getting confused about Work and Pressure and I guess specific terms. I thought that gas releases heat when it expands, resulting in a cooler gas temperature. I thought answer C was the opposite of that, so wouldn't that make C partially wrong? I can see compression increasing heat, if expanding decreases heat, but i guess I'm stuck on this 'release heat' phrase. I understand why answer B is wrong and why the other 2 are wrong, but I need some help with my logic. Can someone explain to me what's wrong with it? Thanks!

An expanding gas absorbs heat; that's why it feels cold. Without getting into the thermodymanics of it, here are some examples:

If you fill a helium balloon (or suck on compressed nitrous like from Redi-Whip) it is unusually cold. Opening a really cold bottle of soda will sometimes freeze a little of the soda inside. If you've ever filled a scuba tank, they get really hot. A refrigerator runs a fluid in a cycle - inside the icebox the fluid expands and absorbs the heat from the food, but outside the icebox is a compressor that recompresses the fluid, generating a lot of heat (released from the fluid itself).

 

[jeffs office]

Just did this last night. Wasn't sure about it at all but used POE.

A) can't be right, would have to be exothermic
B) it says during "EACH" trial but we know that N2 is NOT a weak acid (there was a question about this earlier), therefore that cannot explain the increase in temperature in the N2 trials
D) makes no sense.. potential energy isn't temperature..

so not sure why C is correct but that is why B is wrong.

So I just spent a ridiculous amount of time trying to figure this out. The reason C is right is because the heat added TO the system is equal to the heat done BY the system. So when the gas is expanding, the system is doing work, so heat must be added. The opposite is true for compressing. When the gas is being compressed, work is being done to the system, so that means that the gas is giving off heat. I missed that small concept. I thought work done by the system was the same as heat lost from the system. Completely wrong.

Another concept that ties into this is how fluids boil at lower boiling points at high altitudes (lower pressure), so when the pressure is being lowered, a fluid is more likely to boil, turning into gas and expanding, and this phase change requires heat, so it takes heat from the surroundings, cooling the environment. An example is an aerosol can that cools in your hand when you release the fluid.

I hope this makes sense. Thanks for the explanations!

An expanding gas absorbs heat; that's why it feels cold. Without getting into the thermodymanics of it, here are some examples:

If you fill a helium balloon (or suck on compressed nitrous like from Redi-Whip) it is unusually cold. Opening a really cold bottle of soda will sometimes freeze a little of the soda inside. If you've ever filled a scuba tank, they get really hot. A refrigerator runs a fluid in a cycle - inside the icebox the fluid expands and absorbs the heat from the food, but outside the icebox is a compressor that recompresses the fluid, generating a lot of heat (released from the fluid itself).

Thanks so much! I was confusing the relationship between work and heat.

 

[mehc012]

Another way to think about it is on the molecular level of the gas...heat is the movement of the molecules of the gas. As you compress it, the molecules are bouncing around in a smaller space, hitting each other and the walls more frequently (which is why the pressure goes up) and making the amount of molecular movement higher in a given volume than before compression.