Calorimeter

1. Determination of Heats of Combustion with Bomb Calorimeter
By
Wei-Ting Chen and Nathan Turpin
CHEMISTRY 423
Section 1001
EXPERIMENT 1
September 16, 2014
Abstract
Students of physical chemistry often perform experiments with the use of an
adiabatic bomb calorimeter. This helps the students better understand the first law of
thermodynamics regarding the energy of a closed system. Heat of combustion for
naphthalene is determined using a Parr Oxygen Bomb Calorimeter with a stirrer
assuming constant pressure conditions. This can be performed quite safely even though
the sample is burned in an atmosphere of high pressure oxygen at constant volume.
Benzoic acid was used as a known to determine the specific heat of the calorimeter.
Naphthalene was run and its heat of combustion was determined with a certainty of …..
Introduction
The heat of combustion for a sample can be measured with the use of an adiabatic
bomb calorimeter. The sample is burned to completion in a stainless steel container, the
bomb, capable of holding high pressures. The bomb is placed inside of a jacket that is
capable of producing the experiment under near-adiabatic conditions such that very little
heat is lost to the surroundings. The temperature change inside the jacket is recorded
from the initial temperature, the combustion of the sample, and until the final temperature
is reached.
The specific heat of the apparatus can be determined with the use of a sample of
known energy of combustion under the constant volume conditions with the following
equation:
Csys = -ΔEknown / (T2 – T1)
With the specific heat of the system known an unknown sample can be analyzed to
determine its enthalpy change (ΔH) with rearrangement of the previous equation and an
additional equation.
ΔE = - Csys(T2 – T1)

2. ΔH = ΔE + RT Δngas
Where Δngas is the increase of moles of gas in the system from the combustion. This can
be determined by the combustion equation for naphthalene:
C10H8(s) + 12O2(g) = 10CO2(g) + 4H2O(l)
Materials and Methods
For this experiment a Parr Oxygen Bomb Calorimeter with Stirring Motor was
employed. The basics of this apparatus consisted of an insulated container with a lid, a
stainless steel bucket, and sealable stainless steel container (the bomb). The stainless
steel bucket was filled with 2.00 l of deionized water after the bomb was placed in it. A
stirrer attached to the insulated container’s lid sat in the water to enable proper
distribution of the heat from the bomb. A Parr model 6775 digital thermometer was also
inserted through the lid into the water bath. This thermometer was used to take
temperature readings at five second intervals during the experiment.
The bomb had two electrodes hanging from the lid. One had a pan at the bottom
to catch any left over reactants. Samples to be analyzed were pressed into pellets with
the use of a die press. The pellets were trimmed to a mass within 0.900 to 1.05 grams.
Pellets were fused to a Parr Fuse Wire about 10 cm in length. The mass of the wire was
recorded. The wire was connected to a variable DC power supply. The sample pellet
was gently pulled onto the wire as current heated it. After the pellet was properly fused,
the mass of both the wire and pellet were recorded. The difference gave the mass of the
pellet. The wire/pellet was attached between the two electrodes of the bomb’s lid with
the pellet suspended just above the pan connected to the electrode. Care was taken to
ensure that the wire wasn’t touching anything other than the electrodes to prevent a short
circuit. The lid was gently screwed onto the bomb hand-tight. The bomb was filled and
pressurized with ultra high purity oxygen to about 30 atm. The bomb was vented and
refilled to 30 atm with UHP oxygen to evacuate atmospheric contaminants. The bomb
was lowered into the stainless steel pail and two 10 cm leads from a Parr 2901EB ignition
unit were connected to the electrodes of the lid. The pail was then filled with 2.00 l of
deionized water. The insulated container lid was put in place, the stirring rod was
connected to the motor, and the thermometer probe was inserted.
Temperature readings were recorded until the rate of change was negligible.
Sample was ignited by pressing the ignition switch. No observables were present at time
of ignition. After about 10 – 15 seconds the temperature began to rise. Temperature was
recorded at five second intervals until the temperature no longer rose. Graphs of
temperature versus time were plotted for each run. Three runs were performed for
benzoic acid and naphthalene each. The benzoic acid runs were used to determine the
average heat capacity of the system. From that the molar enthalpy change for
naphthalene was derived.

3. Results
Control trials of Benzoic acid tablets used to determine bomb calorimeter’s specific heat.

Tables and Graphs
Discussion
References
Garland, C.W., Nibler, J.W., Shoemaker, D.P. Experiments in Physical Chemistry, 7
th
ed.;
McGraw-Hill, New York, 2003; pp 145–158.