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9.7 and 9.11 formula differences

Posted: Sat Jan 27, 2018 5:13 pm
by Emily Glaser 1F
9.7 uses the formula, deltaS = n C ln(T2/T1)
9.11 uses the formula, deltaS = n R ln (T2/T1)
What's the difference? Why

Re: 9.7 and 9.11 formula differences

Posted: Sat Jan 27, 2018 5:32 pm
by Andres Reynoso 1J
The difference lies in what the question is asking.

For 9.7 part a, the question asks for the entropy change at a constant pressure with a change in temperature. For this we use the known equation where Cp is the specific heat of a gas at constant pressure.

For 9.11, the question asks for the entropy change at a constant temperature with a change in pressure. For this, we can use where R is the gas constant. As the solution manual states, we can rewrite that equation since P is the inverse of V for ideal gases.

To summarize, it all really depends on what the question asks. Hope I helped!

Re: 9.7 and 9.11 formula differences

Posted: Sat Jan 27, 2018 9:19 pm
by Timothy Kim 1B
In the first question, we use the first formula because it is asking for entropy at a constant pressure. when you derive the equation, it comes out as that.

For the second problem, the conditions are different because it is isothermal. Thus, you end up using a different equation that keeps into account that the internal energy is zero, which leads to the second equation.

Re: 9.7 and 9.11 formula differences

Posted: Sun Jan 28, 2018 6:12 pm
by Rakhi Ratanjee 1D
For 9.11, the solutions manual states that "since P is the inverse of V for ideal gases, we can write: delta S = n*R*ln(P1/P2).
How can you derive this?

Re: 9.7 and 9.11 formula differences

Posted: Thu Feb 01, 2018 9:57 am
by zanekoch1A
For 9.7 I understand why you use deltas=nCpln(t2/t1) but what do you use for Cp? Is there a way to solve it without knowing Cp because I don't think its given.