# chemistry lecture 3 30 minute exam

Discussion in 'MCAT Study Question Q&A' started by masterMood, Jun 20, 2008.

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Could someone explain to me numbers 68 and 69 of the lecture 3 30 minute quiz for general chemistry?

68) A metal rod is in thermal contact with two heat reservoirs both at constant temperature, one at 100 K and the other at 200 K. The rod conducts 1000 J of heat from the warmer tot he colder reservoir. If no energy is exchanged with the surroundings, what is the total change of entropy?

A. -5 J/K
B. 0 J/K
C. 5 J/K
D. 10 J/K

I understand that the way to calculate this is that entropy is in J/K, therefore we have (-1000 J/200 K) or (-5 J/K) [a decrease in entropy for the higher temperature point]

and 1000 J/100 K or an increase in entropy of 10 J/K for the lower temperature point.

My question(s) is this: 1) Why is the total entropy change of the system 5 J/K. The math behind it was taht they ADDED 10 J/K and and -5 J/K. Wouldn't we be finding the difference between the two instead and thus have 15 J/K?

Also, the answer explanation says that you must have at least C or D as the answer because the change in entropy of any isolated system must be positive for any irreversible process.

I'm confused by this statement. Why can't the change in entropy be negative for an irreversible process?

69) Two ideal gases, A and B, are at the same temperature, volume, and pressure. Gas A is reversibly expanded at constant temperature to a volume V. Gas B is allowed to expand into an evacuated chamber until it also has a total volume V, but without exchanging heat with its surroundings. Which of the following most accurately describes the two gases?

Answer is C) Gas A and B have equal temperatures and enthalpies.

for 69) its obvious that the temperatures are gonna be the same because it does no work or and there is no exchange of heat.

But why is i that enhtalphy is the same between both of them?

Enthalphy is PV + U. P and V are the same for both gases (because it stated it in the question).

But why does U (internal energy) stay the same for both? Internal energy is the energy within the molecules like the vibrations, the rotating of them. etc. so the sum of all that is the same between both gases due to temperature?

2. ### engineeredout 10+ Year Member

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#68: By definition, the reason the process is irreversible is because entropy is positive. Total entropy cannot go in the negative direction. The best possible situation is for there to be a delta-S of 0 in which case the process would be reversible.

3. ### engineeredout 10+ Year Member

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#69: First law: U = q + w. No heat transfer, no work, no change in U, everyone is happy.

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thanks again engineered that makes sense.

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The big bang was a low entropy state, and everything since consists of an increase in entropy, on a broad scale.

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LMFAO THAT WAS HILARIOUS.

Is that authentic or edited?

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