Bomb Calorimetry Determination of the Energy in a Biodiesel

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1 Bomb Calorimetry Determination of the Energy in a Biodiesel So now we ve got our sample of biodiesel from the previous week, what do we do with it. Well, a fuel is something that ideally is able to be burned to provide energy and so the more energy a given amount of fuel can provide, the better? So the goal is to test the fuel and find out how good it is and compare this to the values from your peers, from the pure oil itself and a sample of a petroleum diesel. The heat of will be measured with a bomb calorimeter see below: We are calculating here the heat of combustion for our fuel. The heat of combustion (ΔH c ) is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat. In this case, we will express this in terms of energy/mass of fuel. It may also be calculated as the difference between the heat of formation (ΔH f ) of the products and reactants which we will not be able to do because of the uncertain structures of our materials we do not know the exact chemical structure of the oils in question that formed the biodiesel. Recommended reading: Zumdahl and Zumdahl does not do a great job of covering calorimetry (there is some information in Section p ) though there example of coffee cup calorimetry concerns itself with specific heat capacity rather than heat of combustion. The Bomb Calorimeter Basically, a bomb calorimeter consists of a small cup to contain the sample, oxygen, a stainless steel bomb, water, a stirrer, a thermometer, the Dewar or insulating container (to prevent heat flow from the calorimeter to the surroundings) and ignition circuit connected to the bomb. By using stainless steel for the bomb, the reaction will occur with no volume change observed. (see figure on next page) In more recent calorimeter designs, the whole bomb, pressurized with excess pure oxygen (typically at 25 atm) and containing a weighed mass of a sample and a small fixed amount of water (to saturate the internal atmosphere, thus ensuring that all water produced is liquid, and removing the need to include enthalpy of vaporization in calculations), is submerged under a known volume of water (ca ml) before the charge is electrically ignited. The bomb, with the known mass of the sample and oxygen, form a closed system - no air escapes during the reaction. The weighted reactant put inside the steel container is then ignited. Energy is released by the combustion and heat flow from this crosses the stainless steel wall, thus raising the temperature of the steel bomb, its contents, and the surrounding water jacket. The temperature change in the water is then accurately measured with a thermometer. This reading, along with a bomb factor (which is dependent on the heat capacity of the metal bomb parts), is used to calculate the energy given out by the sample burn. After the temperature rise has been measured, the excess pressure in the bomb is released. Chem(bio) Spring 2012 Week 10-1

2 The Experiment One of the experimental requirements is the calibration of the bomb calorimeter as every calorimeter has small variations in its construction and may also change slightly over time (not within the day of an experiment so we don t need to worry about between run variation as long as we are precise and accurate in following the protocol). To do this calibration, we carry out a run using a known amount of a standard material (benzoic acid) for which the heat of combustion is known accurately and use this value to determine the heat capacity of the calorimeter. We can then use the determined value in our calculations when we test our unknown substance (biodiesel). Reproducibility is absolutely essential here. Our first run calibrates the calorimeter so for this to be a meaningful calibration, the exact protocol must be repeated each time. Any change in the procedure such as neglecting one step can change the thermal properties of the calorimeter and so the initial calibration will not hold. We only have 4 calorimeters so we will combine to two team groups with one calorimeter per double team and myself on the fourth initially. Because there is a large number of steps here in the procedure, we will carry out our calibration in a follow the leader fashion whereupon I will run the calibration on one calorimeter and you will follow along with your own. You can then run your own samples. While it may also be tempting to try and get this lab done as quickly as possible, you need good data and if something like a failed ignition happens, you need to rerun that sample resulting in the lab taking longer than if you had been careful and systematic! Chem(bio) Spring 2012 Week 10-2

3 Calorimeter Operating Procedure: Note: Bomb is the operative word here. Check calorimeter for frayed/unseated gaskets in the bomb before use (I will show you in class). Make sure there are no leaks and the calorimeter should always be filled and ignited from behind a safety shield. DO NOT plug in the ignition source until ready to ignite and unplug immediately after ignition. The following procedure describes how to complete a single run and for reasons of both safety and reproducibility should be followed exactly with no deviation. This will also be easier to see some of the steps in lab the first time. The bomb unit should be disassembled and both the lid and the ignition assembly suspended in their holders. The stainless steel bucket should be dry and seated exactly on the three feet in the base of the plastic casing. Make sure all parts of the bomb are clean and dry. The volumes of water used should be exactly reproducible - any extraneous water from a previous run will throw off your data. Pipette 1.00 ml in the base of bomb casing (this gives the steam from combustion a surface to condense on which does not easily happen on the stainless steel of the bomb itself this makes our data more reproducible). Remove the metal cup, tare this on a balance and add the sample recording the exact mass of the material added (1 pellet of benzoic acid (about 1 g) and g of your biodiesel). Do not replace the cup in the assembly yet. Replace the cup assembly in the bomb ignition unit and then bend your stainless steel ignition wire such that it touches the material in the cup (biodiesel or benzoic acid) but not the cup itself (if it touches the cup, it grounds the ignition spark and we do not see complete combustion and hence redo!) Tweezers may help in this. Carefully lift the whole ignition assembly into the bomb casing taking care not to tip it or disturb the wire and push down slowly as far as possible in the casing. You should feel some small resistance and it may pop back up a little (this is good as it shows we have a perfect seal). You want to be careful to not tip it as any spill of material or movement of the contact wire will ruin the run. Screw the bomb cover carefully on the unit and insert the carrying tongs into the holes on the casing. Whenever you lift this, you should always keep a hand underneath especially after pressurizing it as the tongs are not infallible! However, when inserting into the water, you cannot get your hands wet as this removes water from the system and destroys the reproducibility of the run. Chem(bio) Spring 2012 Week 10-3

4 Carefully carry the bomb unit to the oxygen cylinder and place on the bench. Connect the bomb to the cylinder and myself or the TA will slowly open the oxygen valve. Flush the bomb unit for about 60 seconds then shut off the cylinder, close the release valve on the bomb finger-tight and then reopen the oxygen valve and pressurize the bomb unit to atmospheres. DO NOT PRESSURIZE ABOVE 25 ATM! Make sure the oxygen valve is closed and the bomb maintains pressure (it may drop 1 or 2 atm this is normal). Now flip the pressure release on the cylinder regulator and lift off the connector from the bomb. Carefully carry this to your bench and place in the stainless steel bucket on the raised circular platform. Connect the bomb ignition wires (polarity doesn t matter) Fill the 2 L graduated cylinder to the 2.0 L mark with DI water then pour this into the stainless steel bucket submerging the bomb unit. Don t splash any water outside the bucket! Place the lid on the calorimeter unit, assemble the high tech stirrer mechanism connector (rubber band) and turn on the stirrer motor making sure it stirs. Record the temperature once it has stabilized (this is a precision thermometer so make sure you are reading it right (should read to 0.01 degrees C) Connect the ignition electrodes on exterior of bomb to ignition unit (centre and 10 cm connection) Plug in the ignition unit and ignite the sample by pressing the ignition button a red light will come on for a few seconds on the unit then go out this shows the circuit is complete, powered and goes off as the circuit breaks when the ignition wire is consumed! UNPLUG THE IGNITION UNIT! Record the temperature immediately and at 1 minute intervals until the highest value is reached. It may take a few moments for the temperature to start to rise and up to 15 minutes for max temperature to be achieved. Once final (max) temperature is achieved, remove the lid from the calorimeter, lift the bomb unit out using the tongs and carefully place on bench it is still pressurized! Slowly open the vent on the top and release the excess pressure Open up the bomb unit, make sure all sample has been consumed and there is no ash left, dry out the interior and exterior of the bomb and the bucket You are now ready to carry out another run. Return to the first step, start again. Chem(bio) Spring 2012 Week 10-4

5 Results The first step is carry out the calibration of the calorimeter using the following equation: C bomb = ΔH c /ΔT (1) Where C bomb is the specific heat capacity of the calorimeter, ΔH c is the heat of combustion for our material and ΔT is the observed change in temperature. We can calculate ΔH c for our benzoic acid system using the known heat of combustion for this material (6318 cal/g from bottle) and the known mass of the benzoic acid pellet ΔH = heat of combustion per gram x Mass of material (in grams) (2) Use the data from your first calibration run to determine a value for C bomb using equation (1) which you will then use in all future calculations. For each run on your biodiesel sample, calculate a value of ΔH then calculate the specific heat of combustion per gram for each run. Are the values comparable? If so, average these to obtain your final answer. I will also try to run the oil we all used as a starting material so this heat of combustion for this should be compared with the biodiesel which is better and why? A literature paper (J. Chem. Ed., 2006, 83, p ) reports their biodiesel value as kj/g and the value of petroleum diesel as kj/g. How comparable is your data to these values (noting these are in different units so you will have to convert). The value for your biodiesel should be recorded on the table in the class wiki asap and you should also compare your sample with that of other groups. How much variation is there in the class data? Grading of Week 9/10: Full report covering background, synthesis, calorimetry of the biodiesel. You ll note up to this point, I ve not given a lot of background on the biodiesel, I am leaving for you to do but I would like to see some information on both the reaction (hint show this) that creates the biodiesel from the oil and why this is potentially an important reaction in the world outside the lab. Also consider the questions I asked on the calorimetry above. Chem(bio) Spring 2012 Week 10-5