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ContentsUsing a dewar calorimeter
Determination of enthalpy of neutralisation
USING A BOMB CALORIMETER
Calorimetry is a means whereby energy changes accompanying chemical reactions can be measured quantitatively. This is usually done by precise measurement of the temperature change that takes place during the reaction.
In bomb calorimetry substances are burned in an atmosphere of oxygen and this requires a vessel that can withstand high pressures. This is known as the bomb and is the central vessel of the calorimeter. The top of the bomb has an "O" ring to ensure that the screw cap makes a pressure tight seal. It is essential that this is in good condition. The cap has a pair of electrical connectors that are used in remotely igniting the reactant that resides in the crucible. The cap also has a high pressure valve that is used to charge the sealed bomb with oxygen, and to release the pressure when the reaction has finished. The remainder of the apparatus is to provide remote control of the ignition, measure the temperature and to provide the adiabatic enclosure in which the temperature is monitored. The bomb will sit inside this chamber in a can of water whose temperature is monitored as it is heated by the energy released in the reaction. At the bottom of the chamber locating feet ensure that the can is effectively surrounded by an insulating air space. Like most contemporary bomb calorimeters, this apparatus employs an adiabatic water jacket to eliminate heat loses - heaters will continuously adjust its temperature to match that of the water in the can. This gives improved precision. The lid of the enclosure is equipped with a stirrer to circulate the water in the can so that its temperature accurately follows that of the bomb. The can water temperature is monitored using a high precision yellow thermometer - great care must be taken to protect this as it is very expensive and easily broken. The white thermometer is used to monitor the jacket temperature. The chromed tubes house thermistors, one for the can the other for the jacket. These are the sensors of the heater bridge circuit that keeps the jacket temperature equal to that of the water in the can.
As with all calorimeters, the first step is to determine the effective heat capacity. For bomb calorimeters this is done by burning a measured quantity of a substance of known enthalpy of combustion. This is usually pure benzoic acid which can be purchased in tablet form. Or made from approximately one gram of pure benzoic acid using a pellet press.
Weigh the pellet accurately on an analytical balance and record the result in a lab book. In this bomb the reaction is started by an electrically heated fuse wire which ignites a cotton strand. Measure the distance between the terminals, and cut a piece of fuse wire about 5 cm longer than this distance and a single strand of cotton about 10 cm long. Weigh both to the nearest 0.1 mg on an analytical balance, recording the weights in the lab book. Then fit the wire and cotton thread like this. The terminals must be clean and the fuse wire must make good electrical contact with them. Form a mat from the cotton thread on the bottom of the crucible, and using tweezers, carefully place the pellet on top of the mat. Remember to weigh the empty weighing bottle to determine the exact mass of the pellet. Add one cubic centimetre of distilled water to the bomb. This is needed to absorb any acid gases produced in the combustion process so that they can be estimated by analysis at the end of the experiment and appropriate corrections made if desired. Now assemble the bomb. Screw the retaining collar down until it is hand tight. From now on always keep the calorimeter upright. Place the calorimeter on the metal surface of the console, connect the circuit test terminal to the appropriate terminal on the bomb and check the continuity of the ignition circuit by switching on the mains switch and depressing the test switch. The red light indicates a good circuit. Then switch the mains off. Working with compressed oxygen gas is potentially dangerous and the next stage must be performed in the presence of a demonstrator. Connect the oxygen cylinder to the bomb. Then charge the bomb with 25 atmospheres pressure of oxygen. Turn off the cylinder, and disconnect it from the bomb. Fit the protective cap over the valve. Next place the bomb on the locating pins in the base of the calorimeter can. For this calorimeter 1900 cubic centimetres of water are required to fill the can. Adjust the temperature of the water to a value about 2 degrees above the minimum value marked on the calorimeter's precision thermometer. Carefully add the recommended amount of tap water to the can. Check that the bomb is not leaking, a leak will produce a stream of bubbles. The occasional bubble is perfectly acceptable. With care, place the can on the locating feet in the bottom of the water jacket well. Make the electrical connections and check that the thermometer, stirrer and thermistors can be lowered into place free from obstructions. Then, taking great care, lower the lid assembly into place.
Turn on the water supply to the jacket, then the mains switch, and the heater switch. Then to check that the apparatus is thermally stabilising, start a stopwatch and, every 30 seconds, record in the lab book the precise temperature of the calorimeter for a period of two minutes. Then depress the firing switch. A successful firing is indicated by the temperature starting to rise after about 15 seconds. Record the temperature every 30 seconds until the calorimeter has stabilised once more. Switch off the heater and mains switches, and remove the calorimeter from the apparatus. Carefully release the pressure in the bomb and dismantle it. Inspect the interior. No sooty deposits should be present. Any remaining pieces of wire should be weighed and the original mass of wire corrected accordingly.
A plot of the results should look something like this. In order to correct for any heat exchange effects, draw best straight lines through the points where the temperature change is stable. Then draw a vertical line so that the two areas, shown shaded here, are equal. The temperature at the bottom and the top of this line are the corrected temperatures. The corrected temperature change is the difference between them.
Obtain the enthalpies of combustion of the wire and cotton fuses from reference data sheets. For an even more precise measurement the solution in the bomb has to be analysed for sulphate and nitrate by standard analytical methods and corrections made for the acids produced.
The bomb and calorimeter can should be cleaned. Repeat the procedure for the material under investigation. If this is a solid make a pellet using a press. If the sample is a liquid, place it inside a gelatin capsule and insert this into the crucible.
Thermochemical measurements may be made by using an adiabatic calorimeter which eliminates heat exchange between the reaction chamber and the surroundings.
In its simplest form, this consists of a vessel with an evacuated jacket made of thin glass. This is called a Dewar calorimeter. Heat exchange with the surroundings can be further minimised by silvering the surfaces of the vessel, but for the purpose of illustration a clear Dewar will be used here. Thin walls and a vacuum can result in an implosion, particularly if there are scratches on the glass surface. The Dewar should be enclosed in a protective jacket to prevent showers of glass if this should happen. This will be omitted in this demonstration for the sake of clarity. Alternatively, the Dewar can be enclosed in a strong jacket made of an insulating material. This has the advantage of reducing further the heat exchange with the surroundings. Another way of providing additional insulation is to immerse the Dewar in a large volume of water. It is of course necessary to insulate the top of the Dewar, and this is done by using cork lid. Temperature probes and a reactant tube pass through this. The apparatus needs to be set up so that the temperature change which occurs when the reaction takes place can be measured. One or more temperature probes rest in the reaction mixture, and their leads pass through the cork to output displays or recorders. The change in temperature can be determined by manually reading a digital thermometer at times noted from a stopwatch, or by using a chart recorder which plots temperature against time. Alternatively both devices may be used, in which case it is unnecessary to calibrate the chart recorder.
Its essential to bring both reactants to the same measured temperature inside the reaction chamber before the reaction takes place. To do this, accurately pipette a known quantity of one reagent into the Dewar flask. Then, once the lid is in place, pipette a known quantity of the other reagent into the reactant addition tube. This reactant is contained by a bung which can be dislodged with a plunger which also serves to seal the tube. Attach the stirrer, but before switching on ensure that the paddle doesn't make contact with the temperature probes or the bung of the reactant tube.
Take readings of temperature until they remain constant for five minutes. When this has been achieved, press the plunger to start the reaction. The change in temperature should be shown immediately on the chart recorder. Temperature readings should be taken for a further five minutes.
In order to determine the enthalpy of reaction, the effective instantaneous temperature change needs to be determined. This can be done straight from the chart trace, or from a graph of temperature against time. The first portion of the plot is extrapolated forward, and the final portion backwards, to a vertical line which equally divides the line representing the temperature rise at the time of mixing. The vertical distance between the two extrapolated lines is the temperature change which would have been observed had all of the heat been added to the system instantaneously.
The other quantity required is the heat capacity of the calorimeter. It can be determined in two ways. In the direct method, the reaction mixture should be cooled down, using a water cooler, to its original temperature. Next a heater is needed with a precise, known resistance. Insert the heater and heat the mixture until the temperature is just below the maximum obtained during the chemical reaction. Take readings of temperature and time from 5 minutes before the heating starts until 5 minutes after it finishes. Alternatively, an indirect method can be used, determining the heat capacity of the calorimeter by measuring the temperature change for a reaction which has a known enthalpy change.
The measurement of energy changes can provide much information about chemical and physical changes. A Dewar calorimeter can be used to determine the enthalpy of neutralisation under adiabatic conditions. The neutralisation of hydrochloric acid, for which the enthalpy of neutralisation is known, is used to calibrate the calorimeter - and will demonstrate how to calculate an enthalpy change and make corrections for experimental factors.
To perform the calibration of the calorimeter, pipette 200 cubic centimetre of 0.25 molar sodium hydroxide into the clean and dry calorimeter. The sodium hydroxide solution used must have first been standardised against standard hydrochloric acid. It is important to wear protective gloves when handling the solution at this and subsequent stages since it will burn the skin. The indicator added to the sodium hydroxide in the calorimeter is purely for effect, it is not necessary at this stage. Next put the cork lid on the calorimeter... and remove the plunger from the reactant addition tube. Ensure that the stirrer can rotate freely and that one temperature probe is connected to a digital thermometer, and the other to a chart recorder. Then very carefully, pipette 5 cubic centimetres of 5 molar hydrochloric acid into the reactant addition tube. Carefully replace the plunger. Then connect the stirrer to the motor. Check again that the paddle is clear of the probes, and start the stirrer. Monitor the temperature until the apparatus has reached a constant temperature. Note the temperature. Next set the zero position of the pen, but do not put it at the edge of the chart as a gradual drift in the base line may cause the pen to run off the chart when it returns to zero. Then start the chart running. Record the temperature and time for a further five minutes.
When the temperature is constant strike the plunger to release the acid into the sodium hydroxide solution. Continue to take readings of temperature and time for a further five minutes. Notice that the top of the plunger seals the tube keeping the calorimeter an effectively closed system. When the temperature has stabilised, switch off and note the final reading displayed on the digital thermometer. Remove the lid of the calorimeter. Rinse out a 25 cubic centimetres pipette with the reaction mixture. Then pipette 25 cc's into a conical flask. Titrate this with 0.1 molar hydrochloric acid to determine the exact amount of 5 molar hydrochloric acid used in the neutralisation experiment.
The neutralisation process also involves the enthalpy of dilution of the hydrochloric acid. To determine the correction for this dilution, charge the calorimeter with exactly 200 cubic centimetres of distilled water, that is a volume equal to that of the sodium hydroxide added in the neutralisation experiment. In this case, to ensure the volumes are identical, five drops of indicator are also added. Then reassemble the apparatus. Finally, recharge the addition tube with another 5 cc's of 5 molar hydrochloric acid. Then repeat the calorimetry experiment as for the calibration of the calorimeter. This time the indicator changes colour as there is of course a large excess of acid. A small but reasonably constant amount of acid remains on the walls of the addition tube. So after the run the exact amount of acid entering the calorimeter must be determined. Take a 25 cubic centimetre aliquot from the calorimeter, add to this 25 cc's of 0.25 molar sodium hydroxide and, having washed down the sides, titrate with 0.1 molar hydrochloric acid. Repeat this process with further aliquots until concordant titres are obtained. Then neutralisation and dilution experiments should be repeated.
Once the calorimeter has been calibrated, the temperature changes for the neutralisation and dilution of another acid, say dichloroethanoic acid, can be determined. The experimental procedures are the same as before, but this time the reaction tube should be charged with 2 cubic centimetres of acid. The same care is needed in handling dichloroethanoic acid as with 5 molar hydrochloric acid. Each experiment needs to be repeated once.
The plot of temperature against time will usually show an effectively instant change in temperature, which can be measured directly from the plot. It is, however, possible that the graph obtained is of the form shown here. This arises if there has been a significant heating or cooling during the pre- and post-reaction periods. If this is the case, extrapolate the constant temperature change lines. Then the change in temperature is given by the vertical line which divides the shaded portions into equal areas.