1
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
Introduction
The purpose of this experiment is to determine the standard heat of formation for pure
naphthalene (C10H8) using an oxygen bomb calorimeter. The heat of formation is the enthalpy
change associated with the formation of products within the reaction of a specific substance. The
actual heats of formation cannot be calculated directly since formation reactions are near
impossible to guarantee just by combining the individual reactants, so bomb calorimetry is used
to determine the heat of combustion of naphthalene, which is then used to determine its standard
heat of formation. The unknown formation reaction and the known combustion reaction can be
seen as follows:
Unknown : 10C (Graphite )+ 4 H 2 ( g) →C 10 H 8 (s )( Naphtalene )
(1)
Known : C10 H 8 ( s )( Naphtalene )+12O2 ( g) →10 CO 2 (g )+4 H 2
O (l )
(2)
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
, 2
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
For this information to be applicable to other scientific uses, it needs to be standardized under a
specific set of conditions. This experiment is not controlled under the standard conditions (1 bar
pressure, 298k, ideal gas behavior), so its calculated heat of formation needs to be
adjusted/corrected using Hess’s Law and correction factors. Hess’s law states that the net
enthalpy change (ΔH) of a reaction is equal to the sum of the heats of reactions of its
intermediates / coupled reactions. The combustion won’t follow ideal gas behavior, and instead
follows the following thermodynamic cycle
1. ¿ C10 H8 ( s )+12 2O ( pressure=P) →10 CO
2 ( pressure=P')+4 H O (l )
2
2. ¿12 O2 ( pressure=0) →12 O2 ( pressure=P )
3. ¿ 10 CO2 ( pressure=P ' )→ 10C O2 ( pressure=0)
4.
¿ C ¿10 H 8( s )+12 O2( pressure=0) →10 CO 2 ( pressure=0)+ 4 H
2 O ( l)
(3)
Where P is the mean pressure in the bomb before combustion, P’ is the mean pressure in the
bomb after combustion, and 0 corresponds to ideal gas interaction. The first reaction corresponds
to the combustion in the bomb, the second and third reactions are summed to give the ΔH3
correction factor to account for the nonideality of those gaseous molecules, and the fourth
reaction represents the reaction under ideal gas circumstances.
When Hess’s Law is applied to the heat of formation,
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
, 3
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
Δ f H [C 10 H 8 ( s) ] =10 Δf H [ C O 2 (g ) ] +4 Δ f H [ H 2 O (l ) ]− Δc H
(4)
where Δc H is the enthalpy of combustion / heat of reaction of naphthalene and Δf H is the
heat of formation of the corresponding molecule. The heat of formation can then be determined
by calculating the difference between tabulated values and the calculated enthalpy of
combustion. The specifics of these calculations can be found in the results section of this report.
There are additional correction factors that can be observed, namely ΔH1 and ΔH2, which are the
corrections for the temperature (actual temperatures vs 298.15k) and for the pressure (~30atm to
ensure a complete reaction vs the ideal 1 bar) respectively. However, these values can be
neglected if experimental conditions minimize their effect. The amount of reactant (1g) was
chosen so the temperature change did not raise about 2-3 degrees Celsius, and it was ran around
room temperature, so ΔH1 could be disregarded. The naphthalene was compressed into a solid
pellet so it could be assumed to be incompressible, thus making ΔH2 small enough to be
disregarded. The correction factor for the gas ideality, ΔH3, could not be experimentally
controlled for, so it was calculated and used for the conversion of heat of formation to standard
heat of formation.
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
Introduction
The purpose of this experiment is to determine the standard heat of formation for pure
naphthalene (C10H8) using an oxygen bomb calorimeter. The heat of formation is the enthalpy
change associated with the formation of products within the reaction of a specific substance. The
actual heats of formation cannot be calculated directly since formation reactions are near
impossible to guarantee just by combining the individual reactants, so bomb calorimetry is used
to determine the heat of combustion of naphthalene, which is then used to determine its standard
heat of formation. The unknown formation reaction and the known combustion reaction can be
seen as follows:
Unknown : 10C (Graphite )+ 4 H 2 ( g) →C 10 H 8 (s )( Naphtalene )
(1)
Known : C10 H 8 ( s )( Naphtalene )+12O2 ( g) →10 CO 2 (g )+4 H 2
O (l )
(2)
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
, 2
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
For this information to be applicable to other scientific uses, it needs to be standardized under a
specific set of conditions. This experiment is not controlled under the standard conditions (1 bar
pressure, 298k, ideal gas behavior), so its calculated heat of formation needs to be
adjusted/corrected using Hess’s Law and correction factors. Hess’s law states that the net
enthalpy change (ΔH) of a reaction is equal to the sum of the heats of reactions of its
intermediates / coupled reactions. The combustion won’t follow ideal gas behavior, and instead
follows the following thermodynamic cycle
1. ¿ C10 H8 ( s )+12 2O ( pressure=P) →10 CO
2 ( pressure=P')+4 H O (l )
2
2. ¿12 O2 ( pressure=0) →12 O2 ( pressure=P )
3. ¿ 10 CO2 ( pressure=P ' )→ 10C O2 ( pressure=0)
4.
¿ C ¿10 H 8( s )+12 O2( pressure=0) →10 CO 2 ( pressure=0)+ 4 H
2 O ( l)
(3)
Where P is the mean pressure in the bomb before combustion, P’ is the mean pressure in the
bomb after combustion, and 0 corresponds to ideal gas interaction. The first reaction corresponds
to the combustion in the bomb, the second and third reactions are summed to give the ΔH3
correction factor to account for the nonideality of those gaseous molecules, and the fourth
reaction represents the reaction under ideal gas circumstances.
When Hess’s Law is applied to the heat of formation,
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
, 3
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
Δ f H [C 10 H 8 ( s) ] =10 Δf H [ C O 2 (g ) ] +4 Δ f H [ H 2 O (l ) ]− Δc H
(4)
where Δc H is the enthalpy of combustion / heat of reaction of naphthalene and Δf H is the
heat of formation of the corresponding molecule. The heat of formation can then be determined
by calculating the difference between tabulated values and the calculated enthalpy of
combustion. The specifics of these calculations can be found in the results section of this report.
There are additional correction factors that can be observed, namely ΔH1 and ΔH2, which are the
corrections for the temperature (actual temperatures vs 298.15k) and for the pressure (~30atm to
ensure a complete reaction vs the ideal 1 bar) respectively. However, these values can be
neglected if experimental conditions minimize their effect. The amount of reactant (1g) was
chosen so the temperature change did not raise about 2-3 degrees Celsius, and it was ran around
room temperature, so ΔH1 could be disregarded. The naphthalene was compressed into a solid
pellet so it could be assumed to be incompressible, thus making ΔH2 small enough to be
disregarded. The correction factor for the gas ideality, ΔH3, could not be experimentally
controlled for, so it was calculated and used for the conversion of heat of formation to standard
heat of formation.
CHEM|Bomb Calorimetry: Heat of Formation of
Naphthalene UPDATED STUDY GUIDE
