Page 77 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

unity. In general,  is about 1.4 for diatomic gases such as carbon monoxide (CO),

hydrogen (H2), nitrogen (N2), and oxygen (O2). For monatomic gases such as argon (A),

and helium (He),  is about 1.6, and for triatomic gases such as carbon dioxide (CO2), and

sulphur dioxide (SO2),  is about 1.3. For some hydro-carbons the value of  is quite low

(e.g. for ethane (C2H6),  = 1.22, and for iso-butane (C4H10),  = 1.11.

Some useful relationships between Cp , Cv , R, and  can be derived.

From equation 2.25

Cp - Cv= R

Dividing through by Cv

C R
p −1 =
C v C v

C
Therefore using equation 2.25,  = p , then,
C v

R
 −1 =
C
v

R
C v = (2.26)
( − ) 1

Also from equation 2.25, Cp = Cv hence substituting in equation 2.26,

R
Cp = Cv =
( − ) 1


R
Cp = (2.27)
( − ) 1


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