Chemistry 231
Weekly Quiz (?) #2
G. Marangoni
October 2, 2007.
Answer the following questions in the time allotted. There are no tricks on this quiz, so don’t
look for any. Marks are indicated in the [
]
Name:
ID# :
1. 1.00 mole of a diatomic ideal gas (Cv,m = 5/2 R) is expanded from 30.0 L and 300 K to 90.0
L. Calculate final temperature and pressure of the gas and the values of U, H and q and w
if the expansion is carried out [10 marks]
a) isothermally and reversibly;
b) adiabatically and reversibly.
A.
U = H = 0
Isothermal so T2 = 300 K
And P2 = 1/3 P1 = 0.831 bar /3 = 0.277 bar
w = -nRT ln (V2/V1)
= -1.00 mole x 8.314 J/ (K mole) x 300 K x ln (90.00 L / 30.00 L)
= -2740 J = -2.74 kJ
 q = +2.74 kJ
B.
Adiabatic and Reversible
Q = 0 and U = w
Need to calculate the final temperature
V
T2  T1 x  1
 V2



R/C v, m
 30.00 
T2  300 K x 

 90.00 
= 193 K
2/5
 U = 5/2 x 8.314 J/(K mol) (-107 K)
= -2224 J
= -2.22 kJ
 w = -2.22 kJ
H = 7/2 x 8.314 J/(K mol) (-107 K)
= -3114 J
= -3.11 kJ
P2 
Lbar
x193 K
nRT2 1.00 mol x 0.08314
K mol

V2
90.00 L
 0.178 bar