Page 117 - Essentials of physical chemistry

The First Law of Thermodynamics 79

For heats of reaction for T > 298.158K:

ð T
4
2
0 0 (Da þ DbT þ DcT þ DdT þ DeT )dT:
3
rxn
rxn
DH (T) ¼ DH (298:15) þ
298:15
PROBLEMS
4.1 Calculate q, w, and DU for the reversible, isothermal compression of 10 mol of ideal gas from
1 to 10 atm at a constant temperature of 08C.
4.2 Calculate the temperature of air compressed adiabatically in a one-cylinder diesel engine from
1060 cc at 258C to 60 cc. Given that C V ¼ (5=2)R, compute Q, W, DH, and DU for this
compression.
4.3 Use heats of combustion to calculate the DH 0 for 3HC CH ! C 6 H 6 . Although this
298:15
reaction seems like an improbable stereospecific termolecular collision is required in the gas
phase, it does occur on surfaces so much so that high-pressure tanks of acetylene have added
impurities to inhibit the formation of benzene.
4.4 Given that 0.500 g of n-heptane (C 7 H 16 ) burned in a constant volume combustion calorimeter
with C V ¼ 1954 cal=8C causes DT ¼ 2:934 C at T ave ¼ 258C, calculate the molar DH 0 for

comb
n ¼ heptane.
4.5 A little known fact is that most gasoline engines will run on ‘‘wood smoke’’ from a smoldering
fire of burning wood, paper, mulch, almost any cellulose material in a limited amount of O 2 .
Although there is an added safety concern regarding having a stove on the same vehicle as an
alternate tank of liquid fuel, and the overall power is less than with gasoline, the main reaction
is the further combustion of CO. Thus, calculate the heat of the reaction COþ1=2O 2 ! CO 2 at
0
10008K to estimate the molar heat of the reaction in the combustion chamber, DH (1000 K).

rxn
TESTING, GRADING, AND LEARNING?
Throughout this text we will include actual tests that have been given by the author either in a one
semester course (CHEM 305) for forensic majors, CHEM 303–304 for chemistry majors at Virginia
Commonwealth University, or CHEM 311–312 at Randolph Macon College. Copies of these tests
are provided to all the students along with the answer keys and they are encouraged to add material
to their quiz answers that may not be asked (at lower point value). In some courses, only a few
students have access to old tests but here we provide old tests to the whole class to give them an
‘‘equal study opportunity.’’ Why not, if they actually learn from these old tests?
Physical chemistry 305 Fall 2005 Midterm examination D. Shillady, Professor, VCU
(Points) (Attempt all problems) 90 min*

(10) 1. Compute DH 0 for the reaction: C 6 H 6 þ 3H 2(g) ! C 6 H 12 , given DH comb (C 6 H 6 ) ¼ 782:3
298
kcal=mol, DH comb (C 6 H 12 ) ¼ 937:8 kcal=mol,and DH comb (H 2 ) ¼ 68:3 kcal=mol.
(Ans. DH 0 ¼ 49:4 kcal)
298
(15) 2. Given C P (CO 2 ) ¼ 8.87, C P (CO) ¼ 6.97, and C P (O 2 ) ¼ 7.02 cal=mol 8K and DH
298 ¼
67:700 kcal for CO (g) þ 1=2O 2(g) ! CO 29(g) , estimate the DH 0 value at 3508K assuming
350
the C P values remain constant. (Ans. DH 0 ¼ 67:784 kcal)
350
* Note DS finished this in 61 min. Some of these questions require ideas treated in Chapters 5 and 6. We put this test here
because most of it relates to thermochemistry as given in this chapter. The time limit was supposed to be 55 min but since
the room was empty after this class, the time limit was extended to 90 min for the students.