2.8 Calculation of Energy Changes of Chemical Reactions 103
d.The fact that reversible processes cannot occur in the
real world makes calculations of reversible processes
useless.
e.Because enthalpy is a convenience variable, it would be
possible to carry out thermodynamic calculations
without using it.
f. ∆Hand∆Uvalues are nearly equal for all processes.
g.∆Hand∆Uvalues are usually roughly equal to each
other for chemical reactions.
h.The difference betweenCPandCVcan be neglected in
gases.
i.The Joule experiment is a useful way of studying gas
nonideality.
j.Calculations of∆Hvalues from bond energies and
from enthalpies of formation are equally accurate.2.72 a.Calculate the enthalpy change of combustion at 25◦C
of 1.000 mol of each of the following: (1) propane,
C 3 H 8 (g); (2) iso-octane (2,2,4-trimethyl pentane),C 8 H 18 (l); (3) hexadecane (cetane), C 16 H 34 (l). Assume
that the combustion is complete, forming only CO 2 and
H 2 O, and that gaseous H 2 O is formed. The enthalpy
change of formation of liquid hexadecane is equal to
−448 kJ mol−^1.
b.Liquefied propane is sold as a fuel for various
purposes. Iso-octane is a component of gasoline and is
the substance used as the reference compound for
“octane numbers.” (Its performance is rated as “100
octane.”) Cetane is a component of diesel fuel and is
the reference compound for diesel fuel. In order to
compare these fuels, compute the enthalpy change of
combustion of 1.000 kg of each substance.
2.73 The decrease in air temperature with increasing altitude is
partially due to cooling of the air as it expands with
decreasing pressure. A typical barometric pressure at the
peak of Mount Everest is 251 torr. Find the temperature of
a sample of air that is reversibly and adiabatically
expanded from a temperature of 298 K and a pressure of
1.00 atm to a pressure of 251 torr. Assume that air is a
single ideal gas withCV, m 5 R/ 2.