expt11_2006

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Experiment 11

Bomb Calorimetry

A. Purpose

In this experiment, you will measure the standard enthalpy of combustion of benzoic acid anduse its known heat of combustion to determine the heat capacity of a bomb calorimeter. Once the

calorimeter has been calibrated, the enthalpy of combustion will be measured for an unknown sample,

sucrose, and compared to the literature value for this compound.

B. Background and Theory

In your Physical Chemistry coursework you have learned that the enthalpies of reactions can aid

in predicting the likelihood of a chemical reaction occurring. In this lab, you will use an instrument

called a “bomb calorimeter” to measure the heat evolved during the combustion of a sample at a

constant volume. Calorimetry is an integral category of physical and analytical chemical techniques,

and is especially important to the study of fuels and food chemistry – yes, those “calories” listed on your

candy bar wrapper are measured by cooking the candy bar in a calorimeter.

The bomb calorimeter is shown schematically in Figure 1. The calorimeter consists of a metal

reaction chamber that is immersed in a water bath with a known volume of water. The metal reaction

chamber, or “bomb cell”, maintains a

constant volume and allows the heat

generated in its interior to be transferred

efficiently to the surrounding bath. Inside

this chamber, the sample is ignited by

passing electrical current through a “fuse”

wire. In the combustion process, some,

but not all, of the fuse wire is also

consumed. The interior of the reactionchamber is pressurized with oxygen to

ensure efficient combustion of the material

of interest. The water bath is insulated

from the outside environment to prevent

transfer of heat beyond the water bath.

Figure 1.

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Therefore the bomb calorimeter is an adiabatic system.

From the First Law of Thermodynamics, we know that the change in internal energy ( ! U ) in a

system is given by the sum of the work done on the system ( w) and the energy transferred to the system

as heat ( q).

" U = q + w (1)

For example, if 12 kJ of work is done on a system in the form of mechanical compression, and 6 kJ of

energy escapes from the system into the surrounding environment (not a closed system), then the change

in internal energy ! U is 12 kJ – 6 kJ = 6 kJ. The bomb calorimeter is a unique instrument because it

provides a closed system (or nearly so for our purposes), which does not allow heat to escape into the

surrounding environment ( q = 0). In addition, the interior of the bomb is very rigid and able to

withstand large expansion pressures (even explosions, hence the “bomb” part of its name) without

changing its volume ( dV = 0). Upon ignition, the heat released by combustion of the sample is

equilibrated through the walls of the bomb cell into the surrounding water bath where a temperature

increase is recorded as a function of time. The temperature increase of the system is proportional to the

heat of combustion of the sample, and they are related through a constant of proportionality. For the

chemical reaction occurring inside the bomb cell (constant volume), the change in internal energy is

equal to the product of the heat capacity of the sample ( C V ) with the change in temperature ( dT ) inside

the bomb cell:

" U = C V • " T (2)

However, in this experiment, the temperature change is measured in the water bath, and the observed

temperature increase must then be related to the heat released inside the bomb cell. Therefore, we must

determine the heat capacity of the calorimeter ( C cal) by combusting a sample with a known mass and

heat of combustion. This is a calibration procedure that will determine the accuracy of your measuredheat of combustion for an unknown sample in the second part of this experiment.

C cal is defined as:

C cal = H st " m st + e wire

# T (3)

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• A standard sample, benzoic acid, with a known heat of combustion will be used to calibrate your

calculations – this will determine the heat capacity of the calorimeter.• Once this is known, the heat of combustion will be determined for an unknown sample, sucrose.• Compare your results to literature values for the heat of combustion of sucrose.• Prepare long lab report.

D. Prelab Assignment

The following prelab question must be answered in your lab notebook before you can participate in the

lab.• Why is w = 0 in equation 1?• In equation 5, we find that the change in internal energy ( ! U ) is equal to the heat of combustion,

yet in the preceding discussion of First Law of Thermodynamics (eq. 1) we said that the ! U = 0.

How did the description of ! U change between equation 1 and equation 5?

• What is the literature value of the heat of combustion of sucrose?

E. Apparatus

The apparatus used to measure your heats of combustion is the bomb calorimeter described above. An

oxygen tank is supplied to allow pressurization of the bomb cell. The temperature will be read off

periodically and the temperatures are recorded in your lab notebook. A pellet press is provided to

prepare a solid pellet of your samples before combustion in the calorimeter. The TA will demonstratethe correct usage of the pellet press. An analytical balance is used to measure the mass of the pellets to

be analyzed, and a ruler is used to measure the amount of fuse wire before and after combustion.

Safety!

You will be working with a bomb cell that is pressurized with oxygen. Oxygen is extremely flammable

and should not be used near open flames.

F. Procedure

Preparing the sample and charging the oxygen bomb

• The benzoic acid comes in pre-made pellets, simply choose one and be sure to record its mass. Thesucrose will need to be prepared using the pellet press. See a TA for hints on operating the press.

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• Attach the fuse to the bomb: Set the bomb head on the support stand and fasten a 10 cm length offuse wire between the two electrodes. Insert the ends of the wires into the eyelet at the end of eachelectrode stem and push the cap downward to pinch the wire into place. Place the fuel capsule withits weighed sample in the electrode loop and bend the wire downward toward the surface of thecharge. It is not necessary to submerge the wire in a powdered sample. In fact, better combustionwill usually be obtained if the loop of the fuse is set slightly above the surface. When using pelletedsamples, bend the wire so that the loop bears against the top of the pellet firmly enough to keep itfrom sliding against the side of the capsule. It is also good practice to tilt the capsule slightly to oneside so that the flame emerging from it will not impinge directly on the tip of the straight electrode.

• Close the bomb: Care must be taken not to disturb the sample when moving the bomb head from thesupport stand to the bomb cylinder. Check the sealing ring to make sure it is in good condition andmoisten it with a bit of water so that it will slide freely into the cylinder. For easy insertion, push thehead straight down without twisting and leave the gas release valve open during this operation. Setthe screw cap on the cylinder and turn it down firmly by hand to a solid stop.

• Fill the bomb: The oxygen filling connection should already be attached to the oxygen tank. The pressure connection to the bomb is made with a slip connector on the oxygen hose which slides overthe gas inlet fitting on the bomb head. Slide the connector onto the inlet valve body and push itdown as far as it will go. Close the outlet valve on the bomb head; then open the oxygen tank valveno more than one-quarter turn. Open the filling connection control valve slowly and watch thegauge as the bomb pressure rises to the desired filling pressure (usually 30 atm., never more than 40atm.), then close the control valve. Release the residual pressure in the filling hose by pushingdownward on the lever attached to the relief valve. The gauge on the oxygen cylinder should nowreturn to zero.

Operating the Calorimeter

• Fill the calorimeter bucket with 2000(+/- .5) mL of water every time you do a run.

• Set the bucket in the calorimeter

• Set the bomb in the calorimeter bucket: Attach the lifting handle to the two holes in the side of thescrew cap and lower the bomb into the water with its feet spanning the circular upraised guides inthe bottom of the bucket. Be very careful with the bomb so as not to disturb the sample. Removethe handle and shake any drops of water back into the bucket; then push the two ignition lead wiresinto the terminal sockets on the bomb head (the other end of the ignition wires should be attached tothe ignition unit, one wire attached to the 10 cm lead and the other to the middle (ground), be carefulnot to remove any water from the bucket with your fingers.

• Set the cover on the jacket with the thermometer facing toward the front. Turn the stirrer by hand to be sure that it turns freely; then slip the drive belt onto the pulleys and start the motor.

• Let the stirrer run for 5 minutes to reach equilibrium before starting a measured run. At the end ofthis period record the time or start a timer and read the temperature to one-tenth of the smallest scaledivision. Always tap the thermometer lightly with a pencil or rod to vibrate the liquid before takinga reading.

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• Read and record temperatures at one-minute intervals for 5 minutes.

• At the start of the 6 th minute: Stand back from the calorimeter and fire the bomb (be sure ignitionunit is plugged in!) by pressing the ignition button and holding it down until the indicator light goesout. Normally the light will only glow for about ! second but release the button within 5 secondsregardless of the light. Continue to stand clear for 30 seconds after firing.

• The bucket temperature will start to rise within 20 seconds after firing. This rise will be rapid duringthe first few minutes; then it will become slower as the temperature approaches a stable maximum.

• Take temperature readings at 45, 60, 75, 90, and 105 seconds after firing and interpolate betweenthese readings to identify the 60% point after the total rise has been measured. These readings can

be taken without a magnifier since estimates to the nearest .02 oC are sufficient.

• After the rapid rise period (about 4 or 5 minutes after ignition) adjust the reading lens and recordtemperature to one-tenth of the smallest scale division at one minute intervals until the difference

between successive readings has been constant for five minutes.

Cleaning Up

• After the last temperature reading, stop the motor, remove the belt and lift the cover from thecalorimeter. Wipe the thermometer bulb and stirrer with a clean cloth and set the cover on thesupport stand. Lift the bomb out of the bucket; remove the ignition leads and wipe the bomb with aclean towel.

• Open the knurled knob on the bomb head to release the gas pressure before attempting to remove thecap. This release should proceed slowly over a period of not less than one minute to avoid losses.After all pressure has been released, unscrew the cap; lift the head out of the cylinder and place it onthe support stand. Examine the interior of the bomb for soot or other evidence of incompletecombustion. If such evidence is found, the test will have to be discarded.

• Remove all unburned pieces of fuse wire from the bomb electrodes; straighten them and measuretheir combined length in centimeters. Subtract this length from the initial length of 10 cm to obtainthe net amount of wire burned.

• Remember to repeat the experiment with sucrose after your first run with benzoic acid.

H. Data Analysis

Plot your recorded temperatures as a function of time. The plot should look similar to Figure 2 above.

Use a non-linear regression in Excel to fit the data through the region from the ignition point until the

temperature stops rising. From this fit you can interpolate to determine the time when the temperature

has changed by 60%, which will allow you to determine t 1 and t 2. The rates of change before ignition

and after plateau, r 1 and r 2, can be determined from a linear fit to the data points in these two regions.

For your calculations, the known value of the heat of combustion of benzoic acid is 6318 calories/gram,

and the heat generated from the fuse wire is 2.3 cal/cm.

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I. Discussion Questions• If your measured heat of combustion for sucrose is not identical to the literature value, what are

some of the possible reasons for this discrepancy (sources of error)? In fact, we have omitted

some of the details in performing a “careful” calorimetric experiment that would improve your

accuracy. What are some of the procedures that could have been included in this lab to improve

your measured value accuracy?• Is the rate of temperature change after ignition in the bomb calorimeter the same or different for

the combustion of benzoic acid versus sucrose and why?• In your data analysis usage of equation 4, explain why you need to know the rate of temperature

change before ignition and after the temperature plateau (in other words, what is the purpose of

the “- r 1t 1 - r 2t 2” part)?• Compare the heat of combustion of benzoic acid (provided below) to that measured in this

experiment for sucrose. What molecular information does this provide? If you wanted to

develop a machine that ran on solid fuel, would it be more advantageous to use benzoic acid or

sucrose as your fuel source? Justify your answer from a thermodynamic perspective.• In the first sentence of this lab manual chapter we mention enthalpies of reactions, and we

proceed through the chapter as if enthalpies are equivalent to the ! U that we measure for ourunknown sample. In fact, ! U is not quite equal to ! H. What is the relationship between these

quantities (give an equation)? Then describe, in words, the difference between a change in

internal energy and a change in enthalpy.

expt11_2006

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