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The
Enthalpy of Combustion of Various Alcohols |
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This experiment will ask students to investigate
the enthalpy of a chemical reaction, namely the combustion of various alcohols
(SCSCPS, p 33). Introduction Determining the heat yielded by a chemical
reaction is an important part of determining the economical efficacy of
different fuels. In this experiment we
are going to determine the enthalpy of several alcohols (methanol, ethanol,
and propanol) in a calorimeter. Combustion Calorimetry The caloric content of chemicals is often
determined by burning it in a closed metal calorimeter pressurized with
oxygen and completely surrounded by water. The calorimeter can be calibrated by burning a
substance whose heat of combustion has been accurately determined. The method
we will employ to measure the heat (enthalpy) of combustion of various
alcohols is similar in principle, but the calorimeter we will use is a crude
version of the combustion calorimeter used for accurate scientific work. We
will construct a lamp that burns alcohol. The heat from the burning alcohol
will be used to heat water and the temperature increase of the water will be
a measure of the energy produced in the combustion. The enthalpy change for a
sample of water that is heated from temperature T1 to temperature
T2 is given by ∆H = m cp ∆T where ∆H
is the enthalpy change in calories (heat absorbed by the water in this
example), m is the mass of water in grams, cp is the heat capacity
(at constant pressure) in J/g·K, and ∆T is
the temperature change (∆T = T2 - T1) in Celsius
degrees. Not all of the energy released in burning the
alcohol finds its way into the water heated in the calorimeter, so it is
necessary to determine the efficiency of the calorimeter (the fraction of
energy that is captured) by burning a substance whose heat of combustion is
known. We will use paraffin, whose enthalpy of combustion has been determined
to be 41.382 J/g, to calibrate the calorimeter by burning a known mass of
paraffin under the same conditions used to burn a known mass of alcohol. The
measured enthalpy change for the alcohol will then be divided by the
efficiency to get a corrected value that should be close to the true value
for the enthalpy of combustion of the alcohol under test. ∆Hcombustion = ∆H
measured / efficiency Materials &
Methods Materials: paraffin,
alcohols (methanol, ethanol, propanol and butanol.). Equipment: 12-oz aluminum beverage can thermometer (0 to 110°C) 100-mL graduated cylinder ring stand and supports for can and thermometer Procedure: A. Setup
of equipment 1. Mount
the can vertically with a three-finger clamp held on a ring stand if not
already done. Suspend thermometer so
that thermometer bulb is not touching but is about 0.5 cm from bottom of
can. 2. Light
both the paraffin candle and alcohol lamps to see that they burn in a
satisfactory manner. The end of the wick must not be too broad, otherwise a
bushy, sooty flame will be obtained and combustion will be incomplete. If
necessary, use scissors to trim the wick. The size of the flame is controlled
by the height of the wick above the surface of the alcohol lamp. The bottom
of the can should be positioned about 1 cm above the tip of the flame. Blow
out both flames. B. Calibration
of the Calorimeter and Determination of Efficiency 1. Weigh
the paraffin candle as accurately as possible on the analytical balance and
record the mass. Measure and record the room temperature. 2. Prepare
some water that is about 10C° cooler than room temperature by mixing chilled
water (from a refrigerated drinking fountain) or crushed ice with tap water.
If you use ice, fill a 400-mL beaker about one-third full of crushed ice.
Fill a 600-mL beaker about half full of tap water. Mix the contents of the
two beakers back and forth until the ice is melted. Measure the temperature
of the cold water, which should be about 10 C° lower than room temperature.
Using your 100-mL graduated cylinder, measure out exactly 100 mL of the cold water and pour it into the aluminum can,
being very careful not to allow any water to spill down the outside of the
can. Stir the water thoroughly with a thermometer, then read the temperature
to the nearest 0.2 C°, and record it. (There must be no ice in the water
added to the can, otherwise energy would go into
melting the ice rather than heating the water.) 3. Quickly
relight the weighed paraffin candle and place it under the can of water, carefully
centering the flame under the bottom of the can. Stir the water with a
thermometer and when the temperature of the water in the calorimeter has
risen about as many degrees above room temperature as the initial temperature
was below, remove the paraffin candle and blow out the flame. Keep stirring
the water in the calorimeter, recording the highest temperature attained to
the nearest 0.2 C°. 4. Reweigh
the paraffin candle and record the mass. You now have the necessary data to
calibrate the calorimeter, using the known heat of combustion of paraffin
(41.382 J/g) and the known heat capacity of water. 5. At
the end of the experiment, pour out the water and look at the bottom of the can.
A light coating of soot is normal. If the coat is heavy, the wick should be
trimmed and the measurement repeated. Between each measurement, remove the
soot from the bottom of the can by scouring it with steel wool. If your
instructor so directs, repeat the measurement. With care, the temperature
change of the water divided by the mass change of the lamp should agree
within 10% for the two trials. C. Enthalpy
of Combustion of Alcohols 1. Record
in your report the type of alcohol used. Weigh and record the mass of the
lamp containing alcolhol. Using exactly the same
procedure as for the paraffin, prepare a fresh portion of cold water, put
exactly 100 mL of the cold water in the calorimeter
can, and carry out the temperature measurements, heating, and weighing as
before. Record the initial and final temperatures of the water and the mass
of the alcohol lamp after it has cooled to room temperature. 2. By
assuming that the fraction of the heat captured by the calorimeter is the
same for the alcohol lamp as for the paraffin, you now have the necessary
data to calculate the heat of combustion of the alcohols. If your instructor
so directs, repeat the measurements to obtain a duplicate set of values.
Remember to keep the wick trimmed and to scour the bottom of the can to
remove soot if necessary after each measurement. 3. Calculate
the efficiency of the calorimeter from the data obtained with the paraffin
and the enthalpy of combustion of the alcohol from the measured efficiency
and the data obtained with the alcohol. 4. Compare the experimentally determined enthalpy for each of
the alcohols, with a calculated enthalpy based upon published enthalpies for
each of the reactants. Remember to
convert to moles as published enthalpies are in kJ/mol. These published values should form the
basis of your hypothesis. Safety: This procedure involves the use
of open flames. Wear goggles at all
times. Dispose of chemicals properly. Discussion/Conclusions: In your discussion/conclusion
section of your report discuss specifically how the results relate to your
hypothesis. Be sure to discuss
potential sources of error in your experiment and how the experiment might be
improved or modified to obtain better results. Questions 1. Calculate
the expected amount of temperature change in 100 mL
of water from burning 0.58 g of hexane? 2. Why
is the calorimeter cooled to below room temperature and then heated to above
room temperature during the experiment? 3. If
you assume that animal fat has the same chemical composition as paraffin,
.how many food calories (C) would be present in a pound of fat (1 cal = 4.18
J. 1000 cal = 1 C)? 4. If
a 100 lb person burns 192 Calories per hour in walking at 17 min/mi, how far must
this person walk to burn one pound of fat? 5. Why
would a bomb calorimeter be more efficient than the device used in this
experiment? |
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