## 9.7

$\Delta S = \frac{q_{rev}}{T}$

Esin Gumustekin 2J
Posts: 57
Joined: Thu Jul 27, 2017 3:01 am

### 9.7

Does anyone understand the equation Cp=5/2R? It is used in this problem and I don't understand what this equation is and why we need to use it for this problem.

Thanks!

Hubert Tang-1H
Posts: 44
Joined: Thu Jul 27, 2017 3:00 am

### Re: 9.7

Cp means the heat capacity of a gas at constant pressure. We use this equation as part of the formula for calculating entropy in the problem, along with C_v, which is heat capacity for constant volume.

Posts: 32
Joined: Thu Jul 13, 2017 3:00 am

### Re: 9.7

This refers to page 281 in the textbook. The Cp=5/2 R refers to molar heat capacity at a constant pressure for atoms. This comes from Cp = Cv + R, as Cv for atoms is 3/2R. We can use the molar heat capacity for atoms of ideal gas in this problem as it plugs into ΔS = n*Cp*ln(T2/T1). This formula derived by differentiation from dS = dq/T, where we plug in q=n*Cp*ΔT.

skalvakota2H
Posts: 52
Joined: Sat Jul 22, 2017 3:01 am

### Re: 9.7

The change in molar internal energy of a monatomic ideal gas at a temperature T is (3/2)*deltaT. So, when calculating the molar heat capacity at constant volume, which is deltaU/deltaT, the quotient is (3/2)*T.

Since Cv is (3/2)*T, and Cp=Cv+R, Cp=(5/2)*T.

We substitute this equation in for C in the entropy calculation since the questions asks for the entropy of ideal gas atoms, which is monatomic.