Heats of Combustion



 
 
 
 

Oct. 4, 1999
Rana Salameh
Lab Partners: Nichole Miller & Eric Dooyema
 
 

Abstract

In this experiment a bomb calorimeter was used to determine the heat of combustion of naphthalene, C10H8. Our experimental value of D H= -6105.7kJ/mol for naphthalene. The calculated literature value of the heat of combustion of naphthalene, D H= -5156.95kJ/mol.

Introduction:

The purpose of this experiment is to determine the heat of combustion of an organic substance like naphthalene. A bomb calorimeter was used for that. The combustion reaction is:

                                            C10H8(s) + 12O2(g) = 10CO2(g) + 4 H2O(l)                                                                 (1)

Experimental Method:

The experimental method was similar to that used in the textbook (Shoemaker, Garland, Nibler, 6th ed. Exp. 6, p. 152). The experiment was run twice, for benzoic acid and for naphthalene. The bomb calorimeter used is similar to that shown in figure 1 (Atkins, 6th ed. P. 153)

The bomb sits in a calorimeter pail containing about 2L of water. Into the pail, there is a stirrer and a precision thermometer.

Some of the precautions used to insure that the experiment is successful:
The bomb was cleaned and dried, we made sure that no bits of iron were left on the terminals. And the electrical terminals were polished with sandpaper.

Filling the bomb:

The pellet and the wire were installed into the bomb, the wire touching the terminals. See table1 below for the weights of the iron wires and pellets. The bomb was assembled carefully and the cap was screwed hand-tight. Then the bomb was attached to the oxygen filling apparatus at 30 atm.

Assembly of calorimeter:

The dried bomb was placed in the dry pail. Then the pail was set in the calorimeter. We added about 2L water into the pail carefully avoiding splashing. See table 1 below for the weights of the 2000 ml flasks and the water added to the pail.
 
Weight of Fe wire Weight of pellet Weight of empty 2000ml flask Weight of 2000 ml flask w/left over water Water added
Benzoic acid 8.9049g 1.0578g 399.61g 400.85g 1.24g
Naphthalene 8.9049g 1.1438g 403.00g 404.25g 1.25g
Table 1

Results:

Making the run:

We began time-temperature readings, reading the precision thermometer every 30 seconds and recording the time and temperature. See table 2. The ignition switch was turned on and then off immediately after a steady rate had persisted for at least 5 min. The time of the ignition* was recorded.
 
Naphthalene
Benzoic acid
Time(s) Temperature (F) Temperature (F)
0 75.60 74.99
30 75.60 74.98
60 75.61 74.98
90 75.61 74.98
120 75.61 74.97
150 75.60 74.97
180 75.60 74.97
210 75.60 74.98
240 75.60 74.98
270 75.60 74.98
300 * 75.60=24.22C=ti 74.98
330 76.59 74.98
360 79.78 * 74.98=23.88C=ti
390 81.72 77.55
420 82.64 78.51
450 83.09 79.12
480 83.35 79.46
510 83.50 79.63
540 83.59 79.73
570 83.64 79.80
600 83.67 79.85
630 83.69 79.87
660 83.69 79.89
690 83.69 79.89
720 * 83.69=28.72C=tf 79.89
750 83.68 79.90
780 83.68 79.90
810 83.67 79.89
840 83.66 79.89
870 83.64 * 79.89=26.61C=tf
900 83.63 79.88
930 83.62 79.88
960 83.61 79.87
990 83.60 79.86
1020 83.60 79.85
1050 83.59 79.84
1080 83.58 79.84
1110 83.56 79.83
1140 83.55 79.83
1170 83.55 79.82
1200 83.54 79.81
1230 83.52 79.81
1260 83.51 79.81
1290 83.50 79.80
1320 83.50 79.80
1350 83.49 79.79
1380 - 79.79
1410 - 79.79
Table 2

The temperature in Celsius was calculated by the following equation:
C=(F-32)*(5/9)
For the plots we used the temperature is Celsius.

The plots of temperature vs. time (using expanded, interrupted temperature scale) for both runs are shown below:
 


 Fig. 1


Fig. 2
From the graphs we determined the initial and final drift rates as follows:
(dT/dt)i =0, (dT/dt)f =0

The overall graph of T vs. t for Naphthalene is the same as Fig. 2.  See table 2 for ti and tf used.
 
 

Calculation of D T:

                                                                         D T=Tf -Ti-(dT/dt)i(td-ti)-(dT/dt)f(tf-ti)                                               (2)

For benzoic acid,

D T= 26.61C-23.88C-0-0
      = 2.73C

For naphthalene,

D T= 28.72C-24.22C-0-0
      = 4.50C

Cv(H2O)=Cp(H2O)-R
              =75.291J/Kmol-8.314J/Kmol
              = 66.977J/Kmol

The heat capacity, C(S) was obtained by determining the adiabatic temperature rise (Tí2-Tí1), using the following equation:

                                                                                               C(S)= -D Eknown/( Tí2-Tí1)                                            (3)

D E(BA)=-26.41kJ/g, D E(Fe)=-6.68kJ/g (SGN, 6th ed. p.157)

C(S)= -D Eknown/( Tí2-Tí1 ), V=constant
       = -(-26.41kJ/g-6.68kJ/g)/(2.73C)
       =12.12kJ/gC

D E for the combustion of naphthalene was obtained by from the rise of temperature and the heat capacity C(S) by using the following equation:

                                                                                              D E=-C(S)(T2-T1)                                                             (4)

D E= -12.12kJ/gC*(28.72C-24.22C)
      = -54.54kJ/g

Now, D E=D E(naph)+D E(Fe)
-54.54kJ/g==D E(naph)+(-6.68kJ/g)

D E(naph)= -47.86kJ/g
               = (-47.86kJ/g)*(1.1438g)
               = -54.54kJ

The molar energy change D Em was obtained from the number of moles of naphthalene present.

M(C10H8)=128g/mol

Moles of  C10H8 used=(1.1438g)(1mol/128g)
                                 = .00894mol

D Em=D E(naph)/mol(naph)

D Em=(-54.54kJ)/(.00894mol)
        = -6100.67kJ/mol

From equation (1), the number of moles of gas in the system was calculated as follows:

Moles of O2=(.00894molC10H8 )*(12molO2/1molC10H8)= .1073mol O2

Moles of CO2=(.00894molC10H8)*(10molCO2/1molC10H8)= .0894mol CO2

D ngas= .0894 - .1073= -0.0179mol

The molar enthalpy, D Hm was obtained using the following equation:

                                                                                D H=D E+RTDngas                                    (5)

Tf=301.87K is used here,

D H=(-6100.67kJ/mol)(.00894mol)+(8.314J/Kmol)(301.87K)(-0.0179mol)
      = -54.585kJ

D Hm=D H/m
        =(-54.585kJ)/(.00894mol)
        = -6105.7kJ/mol

Calculation of the theoretical D H of C10H8:

The values of D Hf used are from Atkins table 2.6

From eq (1),

D H(rxn)= D Hf(CO2,g)+ D Hf(H2O, l)- D Hf(O2, g)- D Hf(C10H8, s)

             = (10*(-393.51kJ/mol)+4*(-285.83kJ/mol))-(0+78.53kJ/mol)

            = - 5156.95kJ/mol

Discussion:

In this experiment we were able to determine the enthalpy change for Naphthalene. Our calculated D H=-6105.7kJ/mol and the calculated theoretical D H=-5451.95kJ/mol. The % error is 10.7%. Errors to account for in this experiment were due to the accuracy of weighing the substances, reading the thermometer, the measurement of water. The final temperature was used when calculating D H using eq(5).
Equation (5) was used instead of DH=DE+pV because pV is too small compared to DH and DE. Therefore, the perfect gas equation was employed and rewritten as eq(5).