CHEM 305 Assignment #3

CHEM 305
1.
Assignment #3
From the following data, determine ΔfH⦵ for diborane, B2H6(g) at 298 K:
1 
2 
3
B2H6 g   3O2 g   B2O3s   3H2O g 
r H   1941kJ mol-1
2Bs   23 O2 g   B2O3s 
r H   2368kJ mol-1
H2 g   21 O2 g   H2O g 
r H   241.8kJ mol-1
2. Given that the standard enthalpy of combustion of graphite is -393.5 kJ mol−1 and that of diamond is
−395.41 kJ mol−1, calculate the standard enthalpy of the graphite-to-diamond transition.
3. For air temperatures near 25°C and pressures in the range 0 to 50 bar, the µJT values are all reasonably
close to 0.2°C/bar. Estimate the final temperature of the gas is 58 g of air at 25°C and 50 bar undergoes
a Joule-Thomson throttling to a final pressure of 1 bar.
4. a.
Derive the following:
 H 

  C P JT
 P T
b. Show that
V 
JT     CV J   P  1 
 CP 
where1κ is
by

Vdefined
  

V  P N ,T
Hint: Start by taking   P T of H = U + PV.
5. Set up a thermodynamic cycle for determining the enthalpy of hydration of Ca2+ ions using the following
data: enthalpy of sublimation of Ca(s), +178.2 kJ mol-1; first and second ionization enthalpies of Ca(g), 589.7
kJ mol-1 and 1145 kJ mol-1; enthalpy of vaporization of bromine, +30.91 kJ mol-1; dissociation enthalpy of
Br2(g), +192.9 kJ mol-1; electron gain enthalpy of Br(g), -331.0 kJ mol-1; enthalpy of solution of CaBr2(s), -103.1
kJ mol-1; enthalpy of hydration of Br(g)−, -337 kJ mol-1; formation enthalpy of CaBr2(s), -682.8 kJ mol-1.
6. Show that the following functions have exact differentials:
a. f  x , y   x 2 y  3y 2
b. f  x , y   x cos xy


c. f  x , y   x x  e y  y
7. Calculate the enthalpy of vaporization of water at 473 K (the normal boiling temperature is 373 K) using
the data for water below. You may assume that the heat capacities are independent of temperature over
this temperature range.

vap H373K
 40.7 kJ mol-1
C P ,liq  75JK 1 mol1
C P ,vap  3.5JK 1 mol1
Copyright © 2014 by Eric B. Brauns
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