ADVANCED LABORATORY I
FALL, 2000
CALORIMETRY, MICRO-BOMB

 

Reference:  S,G,&N pg. 145 – 158.

  

Part I:   In Part I, you will determine the specific heat of your calorimeter (bomb, water, and container) by igniting a known amount of benzoic acid in a bomb pressurized with oxygen (ca. 30 atm).  The specific heat of combustion for benzoic acid is 6318 cal/gm. 

Addendum to procedure in S,G&N:   For each run, cut a length of fuse wire about 2 inches long.  Make a coil in the center of the fuse wire by wrapping it around a paperclip.  Obtain the mass of the fuse wire on an analytical balance.  

Take about 0.2 g (you don't need to know this exactly yet, you can just use an amount of material consistent with this weight) of benzoic acid and press it into a pellet.  Melt the hot iron fuse wire into the pellet by passing current through the wire.  Trim the pellet so that the final mass of the pellet is ca. 0.1 g.  Obtain the mass of the fuse wire and pellet on an analytical balance.  Since you know the mass of the wire, you know the mass of the sample alone.  Hang the sample on the inside of the bomb.  Seal and pressurize the bomb according to instructions provided by your TA.  The oxygen pressure in the bomb should not exceed 40 atmospheres.

Add exactly 500 mL (using a volumetric flask) of water to the dewar flask.  This should be enough so that the stirrer tip and bomb are immersed in the water.  CAUTION:  THE DEWAR FLASKS ARE EXTREMELY FRAGILE.  DO NOT DROP THE BOMB INTO THE DEWAR!  

Carefully suspend the bomb into the dewar flask.  Begin stirring and take temperature/time points (a stopwatch is provided).  Obtain data points to construct a plot of temperature vs. time as follows:  (1) Take about 6 points, at 30 second intervals, before igniting the sample.  This will give you a pre-ignition line from which you can obtain an accurate slope.  (2) Press and hold the ignitor for 10 seconds. Release the ignitor and take another temperature/time reading.  NOTE:    IF A TEMPERATURE RISE IS NOT IMMEDIATELY OBSERVED, IT IS LIKELY THAT YOUR BOMB WAS NOT PRESSURIZED (IT WAS CLOGGED) OR YOUR SAMPLE FELL OF THE LOOP.  If this is observed, open the bomb and repeat the trial.   (3) Record the temperature at 10 second intervals until the temperature levels off.  (4) Record about 5 more data points at 30 second intervals.  

Remove the bomb from the dewar and open it.  Remove any unburned (not the black pieces!) pieces of iron from the hooks or inside of the bomb.  Weigh these on the analytical balance.  The specific heat of combustion of the iron wire is 1600 cal/gm. Weigh the sample with and without the wire and also weigh the remnants of the wire after combustion to determine the amount of wire burned.   Knowing the mass of the sample and iron that burned, along with the temperature change observed, you can calculate the heat capacity of the bomb, water, and the rest of the system.   

Repeat the above procedure two times so that you have three good trials. 

 

Part II:  Determine DHcomb of the unknown compound.  Use the amended procedure described for Part I, accounting for the mass of the iron burned.   The heat of combustion of the unknown is determined using the heat capacity of the system (just determined in step 1) and the measured change in temperature.  From your measured value of the enthalpy of combustion, along with the molecular formula of the unknown, suggest what the compound might be.   Perform three trials for your unknown.