1. ENERGY OF COMBUSTION BY BOMB CALORIMETRY
OBJECTIVE
Measure the energy of combustion ∆U of stearic acid – our equivalent to camel fat – using a constant
volume (bomb) calorimeter. Determine from this the molar enthalpy of combustion of stearic acid and
thereby estimate the amount of “metabolic” water available to the camel.
INTRODUCTION
Read the Introduction to Calorimetry available as a pdf on the 2PA3 WebCT site before proceeding.
.
THERMODYNAMICS AND CAMELS
Because the camel consumes large quantities of H2O - as much as 100 L - at a time, it was thought that
its hump served as a water reservoir, which the animal could make use of during long treks across a
waterless desert. It is now accepted that in order to facilitate the dissipation of heat through the skin, the
camel has localised its fat in the hump rather than in a layer of subcutaneous tissue, which would impede
the heat flow. In times of need, the oxidation of the fat yields large quantities of energy and more than its
weight of water. One kilogram of fat yields 1.07 kilogram of H2O (10). Thus the hump may also be
considered as a reservoir of “metabolic water”.
The average energy requirement of a camel is about 42,000 kJ/day. If we assume the hump contains 15-
20 kg of fat in the form of hydrated fatty acids, we should be able to estimate the energy stored in the
hump. Estimates of the degree of hydration of the fat vary from as low as 2% to as high as 68% by
weight. 35% hydration seems to be a reasonable compromise and this we will assume in our
calculations.
PROCEDURE
A constant volume calorimeter will be used for the experiment. Also called the bomb calorimeter (Fig. 1),
it consists of a high pressure stainless steel vessel (the bomb), which stands in a can containing 2 L of
H2O. The temperature of the water is measured with a very sensitive precision thermometer. A
mechanical stirrer keeps the system thermally homogeneous.
The steel bomb contains a pan to hold the substance to
be oxidised and a pair of electrodes with a fine wire
connected across them. The combustion of the
substance is initiated by passing an electric current
pulse through this wire. A one-way valve is provided to
allow admission of oxygen gas at high pressure (ca. 25
atmospheres). The system (calorimeter can + water +
bomb and contents) is surrounded by an insulating
jacket to reduce heat conduction to the surroundings
and the calorimeter can is polished to reduce radiation
losses.
Figure 1 Bomb calorimeter

2. CHEM 2PA3, 2004 Experiment 2
CALIBRATION
1. Make a pellet of benzoic acid weighing approximately 0.8 to 1.0 g.
2. Cut a 10 cm length of fuse wire and weigh it. Fuse the pellet onto the wire by heating the wire with an
electric current from a 6 V battery.
3. Weigh the wire and pellet and calculate the exact weight of benzoic acid.
4. Using an Eppendorf pipette, introduce exactly 1.0 ml of pure water into the clean dry bomb.
5. Tie the wire to the electrodes, assemble the bomb and pressurize it with oxygen to approximately 25
atmospheres.
6. Place the can inside the insulating jacket, set the bomb in the can, attach the electric leads and pour in
exactly 2 L of water at a temperature about 1-2 degrees below room temperature and close the cover.
7. Start stirring the water in the calorimeter, and after about 2 minutes start taking temperature readings
at 1 minute intervals for at least 5 minutes.
8. Discharge the capacitor (ignition unit) to initiate combustion exactly 30 sec. after the previous reading
and release the ignition switch when the red pilot light goes out.
9. Record the temperature 30 seconds after ignition and then every 30 seconds while the temperature is
on the rise. After a maximum temperature is observed continue to take temperature readings for a further
5 minutes, reverting to one minute intervals.
10. Open the calorimeter, take out the bomb, release the pressure and open the bomb. Remove the
unburned wire and weigh it. Calculate the amount of wire burnt.
NOTE: Occasionally an incomplete combustion takes place. This is evident by a greasy black
deposit on the walls of the bomb. If this happens, there is no other way but to clean out the bomb
thoroughly and start again.
The above procedure will allow the calorimeter to be calibrated using known heats of combustion (see
Calculations). The procedure is repeated to determine heats of combustion of stearic acid – our
equivalent of camel fat.
UNKNOWN HEATS OF COMBUSTION TO BE MEASURED
The chemical composition of carcass fat of the camel given in Hilditch and Williams corresponds grossly
to oleylpalmitylstearylglycerate. The composition of the fatty acid - stearic acid C17H35COOH -
corresponds closely to oleylpalmitylstearylglycerate. In this experiment we will assume an average
composition of camel’s fat to be that of stearic acid (octadecanoic acid) which oxidises according to:
CH3(CH2) 16COOH(s) + 26O2(g) 18CO2(g) + 18H2O( ) [1]
Because this is an oxidation by molecular oxygen, the enthalpy change of the reaction may be
determined by bomb calorimetry.
CALCULATIONS
For both calibration and stearic acid runs, plot graphs of temperature versus time using an expanded
interrupted temperature scale and extrapolate to determine the temperature change (see Introduction to
Calorimetry). Calculate the heat capacity of the calorimeter using the data from the benzoic acid run and
2

3. CHEM 2PA3, 2004 Experiment 2
the specific energies of combustion of benzoic acid and iron, U (benzoic acid) = -26.421 kJ g-1 and U
(wire) = -6.694 kJ g-1
Deduce the value for the energy of combustion of stearic acid from the temperature change and the
amount of wire burnt in each run. Calculate the molar energy of combustion, and also the molar
enthalpy of combustion H for stearic acid:
H = U + (PV) = U + ng(RT) [2]
where ng is the change in total number of moles of gas species.
Compare your values with those in the literature.
Calculate the standard enthalpy of formation of stearic acid using -393.50 and -285.85 kJ mol-1 for the
standard enthalpies of formation of CO2(g) and H2O(l) respectively at 25oC.
Calculate the total energy change on oxidation of a 20 kg mass of hydrated “fat” stored in the hump
(assume the degree of hydration of the fat to be 35% by weight). Calculate the volume of water - both of
hydration and metabolic - obtained from the hump. It is estimated that the inhalation of oxygen needed
for oxidation of the fat results in loss of H2O in breath equal to 1.70 kg per 42, 000 kJ of energy
generated.
Estimate the net amount of water lost/gained by the camel per day, i.e., per 42,000 kJ of energy
produced by the oxidation of the fat.
DISCUSSION
In the discussion of your errors you should comment on the following:
• The magnitude of the error introduced by a lack of knowledge of the heat capacities of reactants
and/or products of the reaction.
• The magnitude of the error introduced by assuming the ideal gas law in calculating H.
• Does the value of U calculated correspond to that at the initial or final temperature of the
calorimeter?
• Why is it necessary to measure the volume of the water for each run?
REFERENCES
1. International Critical Tables, Vol. V, 162 (1929). (Thode Library Ref. Q199.N27)
2. Selected Values of Chemical Thermodynamic Properties, Nat. Bureau Stand. Circular 500 (1952).
3. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related
Compounds, Am. Inst. Res. Proj. 44 Report (1955).
4. Shoemaker, Garland, Steinfeld and Nibbler, "Experiments in Physical Chemistry", 4th edition,
McGraw-Hill, p. 131 ff.
5. J.H. Noggle, "Physical Chemistry", 3rd ed, Harper Collins, 1996, p 272.
6. Any CRC Handbook of Physics and Chemistry.
7. R.G. Mortimer "Physical Chemistry", Benjamin/Cummings, Redwood City, Calif., 1993, 87-90.
8. R.J. Sime “Physical Chemistry: Methods, techniques and experiments” Saunders (1990)
9. K. Schmidt-Nielson, “Desert Animals” O.U.P (1964) p. 40
3