• Support PF! Buy your school textbooks, materials and every day products Here!

Entropy change explanation

  • Thread starter noob314
  • Start date
  • #1
18
0
Can someone explain to me why Cv is used in this example?

Determine delta S for the conversion of a monatomic ideal gas from state 1 to state 2.
Cv = (3/2)R, Cp = (5/2)R
State 1: V1 = 22.0L, P1 = 1.25atm, T1 = 308K, n=0.917mol
State 2: V2 = 34.0L, P2 = 0.700atm, T2=325K, n=0.917mol


delta S = nR(V2/V1) + nCvln(T2/T1)

From what I can see, the process doesn't look like it's carried out in either constant pressure nor constant volume. If that answer is correct, then why is Cp not used?
 

Answers and Replies

  • #2
Mapes
Science Advisor
Homework Helper
Gold Member
2,593
17
The entropy change comes from the differential energy equation

[tex]dU=T\,dS-P\,dV\Rightarrow dS=\frac{dU}{T}+\frac{P\,dV}{T}[/tex]

the ideal gas equation

[tex]PV=nRT[/tex]

and the equation for ideal gas energy

[tex]U=U_0+nc_VT\Rightarrow dU=nc_V\,dT=n(c_P-R)dT[/tex]

[itex]c_V=c_P-R[/itex] is just a constant here. It doesn't imply that any process occurs at constant volume or pressure.
 

Related Threads for: Entropy change explanation

  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
3
Views
10K
  • Last Post
Replies
7
Views
985
  • Last Post
Replies
0
Views
1K
  • Last Post
Replies
11
Views
2K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
3
Views
3K
Top