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.
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)
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|
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.
|Time(s)||Temperature (F)||Temperature (F)|
The temperature in Celsius was calculated by the following equation:
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:
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
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
D T= 28.72C-24.22C-0-0
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
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)
Now, D E=D E(naph)+D
D E(naph)= -47.86kJ/g
The molar energy change D Em was obtained from the number of moles of naphthalene present.
Moles of C10H8 used=(1.1438g)(1mol/128g)
D Em=D E(naph)/mol(naph)
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 Hm=D H/m
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)
= - 5156.95kJ/mol
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).