### Pizza Rolls #5

Posted:

**Wed Feb 12, 2020 2:03 pm**Does anyone understand how to do parts A and B of number 5 on the midterm practice?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=133&t=59291

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Posted: **Wed Feb 12, 2020 2:03 pm**

Does anyone understand how to do parts A and B of number 5 on the midterm practice?

Posted: **Wed Feb 12, 2020 2:46 pm**

For Part A, you have to add up three different delta S values to find the total change of entropy in the system. This includes deltaS for H2 container expansion, deltaS of Krypton gas for container expansion, and deltaS based on temperature change. Since entropy is a state function, this allows us to add these values to find the total entropy change.

For Part B, you can use PV=nRT by plugging in the given values in the prompt to find the final pressure.

For Part B, you can use PV=nRT by plugging in the given values in the prompt to find the final pressure.

Posted: **Wed Feb 12, 2020 3:05 pm**

Rachel Yu 1G wrote:For Part A, you have to add up three different delta S values to find the total change of entropy in the system. This includes deltaS for H2 container expansion, deltaS of Krypton gas for container expansion, and deltaS based on temperature change. Since entropy is a state function, this allows us to add these values to find the total entropy change.

For Part B, you can use PV=nRT by plugging in the given values in the prompt to find the final pressure.

Which specific heat value should we use to calculate the entropy due to temperature change since there are two different gases?

Posted: **Wed Feb 12, 2020 3:09 pm**

Use the Cv of the ideal gas for both because they are ideal gases. Ideal gas, Cv = (3/2) R. and plug into the equation to calculate the change of entropy of change of volume.

Posted: **Wed Feb 12, 2020 3:47 pm**

Rachel Yu 1G wrote:For Part A, you have to add up three different delta S values to find the total change of entropy in the system. This includes deltaS for H2 container expansion, deltaS of Krypton gas for container expansion, and deltaS based on temperature change. Since entropy is a state function, this allows us to add these values to find the total entropy change.

For Part B, you can use PV=nRT by plugging in the given values in the prompt to find the final pressure.

But doesn't this only work for isothermal changes? Since there is a temperature change, how can we use the change in entropy for an isothermal expansion?

Posted: **Sun Feb 16, 2020 7:02 pm**

005290099 wrote:Rachel Yu 1G wrote:For Part A, you have to add up three different delta S values to find the total change of entropy in the system. This includes deltaS for H2 container expansion, deltaS of Krypton gas for container expansion, and deltaS based on temperature change. Since entropy is a state function, this allows us to add these values to find the total entropy change.

For Part B, you can use PV=nRT by plugging in the given values in the prompt to find the final pressure.

But doesn't this only work for isothermal changes? Since there is a temperature change, how can we use the change in entropy for an isothermal expansion?

There are three changes that could lead to change in entropy, if we separate the overall change into three components. When we consider the factors leading to change in entropy separately, we could consider the two changes in volume as isothermal and then use corresponding equation in the formula sheet, whereas still consider the change in temperature as NOT isothermal. Actually, we have to consider the changes separately, because we haven't learned one way to calculate change in entropy that combines volume & temperature changes.