QWAQWA CAMPUS / QWAQWA KAMPUS
CEM 214
UNIVERSITEIT VAN DIE VRYSTAAT
UNIVERSITY OF THE FREE STATE
SEMESTER TEST 1
Sat, 19 March 2011
ASSESSOR /
ASSESSOR:
Mrs MA Mokoena
MODERATOR:/
MODERATOR:
Prof AS Luyt
TYD /TIME : 1UUR/1 Hour
PUNTE / MARKS: 50
ANSWER ALL QUESTIONS AND SHOW ALL CALCULATIONS WITH AN INK PEN -- NO PENCILS OR
"TIPPEX" ALLOWED / PAPER CONSIST OF 2 PAGE(S) AND 4 QUESTIONS.
Question 1
1. 2 mol of nitrogen gas is heated at constant pressure from 298 K to 373 K. Calculate S,
given the temperature variation of Cp,m of N2 as :
Cp,m /J K-1 mol-1= 27.296 + 5.23 x 10-3 (T/K) -0.042 x 10-7 (T/K) 2
2. Estimate the molar combustion enthalpy of ethanol CH3OH
(l),
[10]
by using the appropriate average
bond energy data. Assume that the standard enthalpy of vaporization of methanol 35.3 kJmol-1
and that of liquid water 44 kJmol-1 the work done, H, U and the amount of heat transferred kJ
mol-1. Use T = 298 K and show all steps.
[7]
3. A system of one mol of Ar is expanded reversibly and isothermally at 0 oC from 22.4 L to
44.8 L. Calculate the work done, H, U and the amount of heat transferred.
[7]
4. Two moles of ideal gas is reversibly cooled down at constant volume from 200K to 100K;
then it expands isothermally from 10 L to 25 L. Calculate the change in pressure P
during the process.
[6]
Bindingsenergieë/ kJ mol-1 /
Bond energies /kJ mol-1
C-C
343 C=O
C=C
CC
C-H
C-O
615
812
416
351
724
C=O (in CO2)
O-H
O-O
O=O (in O2)
799
464
144
498
MEMORUNDUM
S 
 vap
373 C p
n 
dT
298 T
T
373  27.296  5.23 x 103 (T / K )  0.042 x 107 (T / K )2 
dT
S  n  
T
298 



T
1.  2  27.296 ln 2  5.239 x10 3 (T  T )  1 / 2(0.042)(T 2  T 2 ) 
2 1
2
1 

T1




373
 2  27.296 ln
 5.239 x10 3 (373  298)  1 / 2(0.042)(3732  298 2 ) 


2981


 _______________
2. CH3OH(g) + 3/2O2(g)  CO2(g) + 2H2O(g)
∆Ug  [3xBE(C-H) + BE(C-O) + BE(O-H) + 3/2xBE(O=O)] – [2xBE(C=O) + 4xBE(H-O)]
∆Ug  [3x416 + 351 + 464 + 3/2x498] kJ mol-1 - [2x799 + 4x464] kJ mol-1
∆Ug  [________+ ________ ] kJ mol-1 = ______ kJ mol-1
∆Hg = ∆Ug + ∆(PV)

waar/where ∆(PV) klein is/is small
∆Hg  ∆Ug = ________ kJ mol-1
CH3CH2OH(g) + 3O2(g)  2CO2(g) + 3H2O(g)
CH3OH(l)  CH3CH2OH(g)
_______ kJ mol-1
35.3 kJ mol-1 

2H2O(g)  2H2O(l)
- 88 kJ mol-1
CH3CH2OH(l) + 3O2(g)  2CO2(g) + 3H2O(l)
_____ kJ mol-1 

3. A system of one mol of Ar is expanded reversibly and isothermally at 0 oC from 22.4 L to 44.8
L. Calculate the work done, H, U and the amount of heat transferred.
Work done = wrev = 1 mol Ar :
Thus
[7]
Vm, 2
 P dV
Vm,1
1
PVm = RT;
Wrev = -
Vm, 2

Vm,1
= -RT ln
RT
dV = -RT ln V│Vm.1Vm,2
V
𝑉𝑚,2
⁄𝑉 = RT ln
𝑚,1
𝑉𝑚,1
⁄𝑉 = 8.3145 x 273.15 ln 22.4 ⁄44.8
𝑚,2
= _________J
∆H = Cp,m (T2-T1) = 0
∆U = CV,m (T2-T1) = 0
So qrev = ∆U – w rev = 0 – RTln
𝑉𝑚,2
⁄𝑉 = __________J
𝑚,1
1. Two moles of ideal gas is reversibly cooled down at constant volume from 200K to 100K;
then it expands isothermally from 10 L to 25 L. Calculate the change in pressure P
during the process.
[6]
4.
 P 
 P 
 P 
dP  
 dV    dn  
 dT
 V  n ,T
 n V ,T
 T  n ,V
  nRT
 P 

V
 dP1  
 dT  
 T
 T  n ,V


 dT

 n ,V



 8.314 (100  200) Pa  _________ Pa

 P1   nR dT  nR (T2  T1 )  2[
V
V
0.02
T2
T1
  nRT 
 P 
 dP2  
 dV 

 dV
V  V  n ,T
 V  n ,T
V2

 P2  nRT  V  2 dV  nRT (
V1

1
1
1
1
 )
 [8.314 x100 K (

)]Pa  82309 Pa
V2 V1
0.01 0.02
Ptot  P1  P2  40739 Pa 