Temperature and Energy

[Schwann]

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Hi,
On one of the TPR tests it says " However, in the adiabatic expansion of an ideal gas into an evacuated region, there is no change in the temperature of the gas." Does that mean that the total internal energy of the gas stays the same? Can someone provide link that explains the differences between isobaric, isochoric, isothernam, and adiabatic? Those 4 sometimes confuse me. Thanks.

 

[spyderracing32]

Adiabatic simply means that there is no exchange of heat between the system and the surroundings. dU = q + w, and since there is no heat exchange the equation is simply dU = w. In the adiabatic expansion of a gas into an evacuated region there is no work done (and W = PdV = nRdT) so the internal energy of the system and the temperature stay the same.

Isochoric is essentially the opposite of adiabatic. The system is at constant volume and thus work cannot be done by the system and the only energy exchange between the system and its surroundings is in the form of heat energy, so dU = q.

Isothermal means at constant temperature so the internal energy of the system does not change. This means that as work is done by the system an equal amount of heat must flow into the system. This also means that as work is done on the system an equal amount of heat must flow out of the system. Thus dU is constant and -q = w.

Isobaric just means at constant pressure, so as heat is added to the system it does work on its surroundings (to increase vol. so pressure can stay constant), but the internal energy of the system is changed.

 

[BerkReviewTeach]

Hi,
On one of the TPR tests it says " However, in the adiabatic expansion of an ideal gas into an evacuated region, there is no change in the temperature of the gas." Does that mean that the total internal energy of the gas stays the same? Can someone provide link that explains the differences between isobaric, isochoric, isothernam, and adiabatic? Those 4 sometimes confuse me. Thanks.

This is known as the Joule-Thompson experiment. I have attached a link:

Joule-Thompson experiment

As you will quickly see, the details are beyond the scope of the MCAT. However, the terminology is critical. Spyderracing32 did an excellent job explaning the terminology, so I shall bow out here.

 

[spyderracing32]

This is known as the Joule-Thompson experiment. I have attached a link:

Joule-Thompson experiment

As you will quickly see, the details are beyond the scope of the MCAT. However, the terminology is critical. Spyderracing32 did an excellent job explaning the terminology, so I shall bow out here.

Woohoo, I'm glad to hear that because thermo is by far my worst topic in gen chem .

 

[Schwann]

why is there no work done in the adiabatic expansion of gas into vacuum?

 

[spyderracing32]

Because it's not exerting any force in simply diffusing into a vacuum. Work is done when a gas expands in say a piston chamber because it pushes (read: exerts a force) against the piston at the interface between the system and the surroundings.

 

[Schwann]

thanks a lot spyderracing!