2.01 Combustion Enthalpies by Bomb Calorimetry

Transcription

1 Cmbustin Enthalpies by Bmb Calrimetry (5 pints) 1 Outline Enthalpies f cmbustin can be measured by burning a knwn amunt f material in a bmb calrimeter and determining the temperature change. The bmb is pressurised with xygen t ensure cmplete cmbustin, and sealed t prevent escape f the cmbustin prducts. The sample is ignited by passing a current thrugh a fuse wire within the bmb. Allwance fr the heat capacity f the bmb is made by igniting a knwn quantity f a substance f knwn enthalpy f cmbustin as a standard. Heat lss t the surrundings can be calculated by use f a cling crrectin curve, r, as in this experiment, prevented by use f a jacket arund the calrimeter, maintained at the same temperature as the calrimeter itself; the reactin is then adiabatic. Bmb calrimetry was nce widely used in chemistry, since it prvides thermdynamic data in a very direct way; it is still used in the fuel and fd industries. Nevertheless, enthalpies f cmbustin cannt be used uncritically as a measure f the energy that will be realised by the bdy when fd is cnsumed. Fr example, nt all fds will be cnverted by the bdy int the same prducts as thse frmed by cmbustin in a calrimeter. Other, mre subtle, factrs may als be significant. An example is the St. Bernard dgs wh brught casks f brandy t blizzard victims in the Alps. A primary effect f alchl is t enlarge bld capillaries which, in the case f cld travellers, sends a rush f chilled bld frm the skin's surface t the traveller's cre, thus ptentially speeding up hypthermia and death. Dubtless lst travellers were pleased t see the dgs, unaware that the brandy they brught might hasten, rather than delay, death. In this experiment yu will first calibrate the calrimeter using benzic acid as a standard. Yu will then measure the differences in enthalpies f cmbustin f tw ismeric species t allw yu t investigate resnance stabilizatin energy in an armatic mlecule. 2 Backgrund infrmatin This experiment relies n simple thermdynamic principles, s can be cmpleted at any stage in yur 2nd r 3rd year. Relevant lecture curse: Intrductin t thermdynamics (1st year). Web address fr experiment: Other web sites f ptential interest:

2 2 3 Safety Read the safety instructins in this script befre starting the experiment. The principle hazard in the experiment arises frm the use f a high pressure gas cylinder, s if yu in any dubt abut hw t use the cylinder r regulatr, r are uncertain abut any ther peratin, cnsult a demnstratr r a technician. There is safety infrmatin n the chemicals yu will be using, (which pse n special hazards), available thrugh the web site fr the experiment. 4 Thery The quantity required in this experiment is the standard enthalpy f cmbustin H cb - the enthalpy change n cmbustin t give specified cmbustin prducts such as CO 2 (g), H 2 O(l) and N 2 (g), all reactants and prducts being in their standard states (298 K and 101 kpa). The prcess which takes place in the bmb calrimeter is an adiabatic cmbustin (n heat exchanged with the surrundings) at cnstant vlume with a change in internal energy Ucb. T relate this t H cb we use the first law f thermdynamics U = q+ w (1) N heat enters r leaves the calrimeter, s q = 0. At cnstant vlume V = 0, s w = p V = 0. It fllws frm equatin (1) that the ttal change in internal energy is zer. Three substances are burnt: the sample, and cttn and irn wire fuses. Therefre Ucb,sample + Ucb,irn + Ucb,cellulse = 0 (2) It may seem strange that there is n internal energy change during cmbustin in an adiabatic bmb calrimeter, but n heat enters r leaves the calrimeter and n wrk is dne, s it is reasnable t expect the internal energy f the cntents t remain fixed. After cmbustin, the temperature f the prducts, T 2, is higher than the initial temperature, T 1. T relate the adiabatic cmbustin with its temperature change t the isthermal cmbustin at temperature T 1, the energy change n cling the prducts and bmb calrimeter frm T 2 t T 1 at cnstant vlume must be calculated. This is U = C ( T T) = C T (3) v 2 1 v Since internal energy is a state functin, adiabatic cmbustin fllwed by cling is equivalent t isthermal cmbustin at cnstant vlume and cnstant temperature T 1 with internal energy change Ucb ( T1 ). S U = U U C T (4) cb,sample cb,irn cb,cellulse v If Hcb is independent f pressure and temperature (which is a gd assumptin in the cnditins used fr this experiment),

3 3 H ( T) = H. cb 1 cb Therefre U = H U C T (5) cb,sample cb,fe cb,cellulse v By definitin, changes in enthalpy H and internal energy U are related by the equatin H = U + ( pv) (6) Assuming the gaseus prducts bey the ideal gas law and that the (pv) terms f slids and liquids are negligible ( pv) = n RT (7) gas where n gas is the change in the number f mles f gas during cmbustin. Therefre H = U + n RT = U C T + n RT (8) cb,sample cb gas 1 cb,fe cb,cellulse v gas 1 The heat capacity C v may be fund by using the cmbustin f benzic acid as a standard, since U cb fr benzic acid has been accurately measured by internatinal standards labratries. Prvided that the values f U cb fr the cttn and irn fuses are knwn, C v may then be calculated frm equatin 5. C v is the heat capacity at cnstant vlume f bth the cmbustin prducts and the calrimeter assembly. The calrimeter has a much greater heat capacity than the reactin prducts, s it is reasnable t assume that C v is cnstant fr different samples, and t use the value btained frm the cmbustin f the benzic acid standard. Prcedure The bmb is placed in a water-filled can, which is itself inside a water jacket frm which it is separated by an air gap. The apparatus has stirrers and thermmeters as shwn in the diagram by the apparatus. A cntrl unit supplies pwer t the stirrer mtrs and the ignitin circuit. The temperatures f the liquids in the can and the uter jacket are mnitred by a pair f thermistrs and the jacket cntents may be heated by passing a substantial current directly thrugh the liquid, raising its temperature by hmic heating. The rise in temperature due t cmbustin is measured using an illuminated reader n a precisin thermmeter. There is a small danger in the peratin f bmb calrimeters bth frm explsin and frm electrical shck. It is essential that yu take the fllwing precautins: Keep the bmb clean; when yu dismantle it, place it n a clean flded twel r stand. Be careful nt t scratch r drp the bmb. Switch ff all pwer when adding the bmb r remving it frm the calrimeter. D nt use mre than 1.0 g f slid. D nt use an xygen pressure greater than 2 MPa (20 atm).

4 D nt fire the bmb if bubbles f gas shw it is leaking (less than 1 bubble per three secnds is insignificant.) Stand clear f the tp f the calrimeter when it is fired, and fr 20 secnds afterwards. Be careful when mving the calrimeter lid, since the thermmeters which stick thrugh it are very vulnerable. Bmb calrimeters require care t perate, s d nt hesitate t ask fr assistance frm a demnstratr. 1. Weigh ut rughly 1g prtins f benzic acid, 9,10-dihydranthracene and bibenzyl (C 6 H 5 - CH 2 -CH 2 -C 6 H 5 ), using a tp-lading balance. 2. Frm the slids int pellets with the press, place each in a labelled weighing bttle and weigh the pellets as precisely as pssible. 3. Cut a piece f fuse wire rughly 5cm lnger than the distance between the electrdes in the bmb. Cut a piece f cttn abut 25cm lng and weigh bth items precisely. 4. Sand lightly the bmb terminals t ensure a gd electrical cntact with the fuse wire, then thread the wire thrugh the hles in the terminals and wrap arund t make a gd cntact. D nt leave the wire ends prtruding, r they may earth against the walls f the bmb. Place the crucible in psitin. Knt the middle f the cttn, which acts as a secndary fuse, arund the centre f the wire, dangle the tw ends int the crucible and frm them int cils there. Place the pellet f benzic acid nt the cttn. 5. Weigh the empty weighing bttle t find the mass f the pellet. 6. Intrduce 1 cm 3 demineralised water int the bmb (NOT the crucible!) 7. There is a thin O-ring lcated in a grve in the lwer surface f the bmb lid. Check that this is prperly psitined, then assemble the bmb, tightening the tp firmly using hand pressure nly. 8. T check fr cntinuity between the terminals, stand the assembled bmb n the tp cver f the calrimeter unit. Pull ut the banana plug n the end f the white wire n the calrimeter lid and insert it in the bmb firing scket. Press the test switch. The lamp incrprated in the test switch will light if the fuse wire is cnnected prperly. If the light des nt cme n, disassemble the bmb and cnnect a new fuse wire. 9. Cnnect the xygen line t the bmb. Open the main xygen cylinder valve, then fill the bmb t 2 MPa (20 atm) by turning the bar handle n the cylinder. Clse bth cylinder valves, then discnnect the sealed bmb by unscrewing the cnnectr. Sme xygen will escape frm the pressurised line, but this shuld quickly stp. 10. Place the bmb in the can n the scales and adjust the weight t 6000 g by filling the can with tap water. The temperature f water in the can shuld be clse t that f the water in the calrimeter jacket (measure with a thermmeter); if necessary, yu can use warm water frm the dark rm. Check that the high accuracy thermmeter supplied cvers the apprpriate temperature range - the temperature f the water in the can will rise by up t 2K during the experiment. If the thermmeter des nt cver the right range, ask at the service rm fr ne that des. (Several thermmeters cvering different ranges are available.) TAKE GREAT 4

5 CARE WITH THE THERMOMETERS - they are very expensive and cntain a large amunt f mercury, which presents a cnsiderable clean-up prblem if they are brken. 11. If a stream f bubbles rises frm the bmb, remve and depressurise it, adjust r replace the sealing ring in the lid, and reassemble. Seek advice frm the technician befre cntinuing. A bubble exiting every few secnds can be ignred. 12. Psitin the can n the three lugs n the base f the uter jacket assembly. The calrimeter lid swings ninety degrees frm the up psitin, and is mved by first lifting it slightly, then rtating it int psitin. Mve this with great care - it is quite heavy, and the thermmeters which prtrude frm the lid are very vulnerable. Lwer the lid gently, ensuring the thermmeters pass freely thrugh the apprpriate hles in the calrimeter. Hld dwn the electrde cntact as the lid is being lwered, making sure the cntact engages in its hle n the bmb lid. If the electrde cntact des nt stay dwn when the lid is fully in psitin, it is nt crrectly engaged in the bmb firing scket. 13. Switch n the calrimeter and turn n the water supply at the sink t prvide a flw f cling water t the uter jacket. (The calrimeter maintains a balance in the uter jacket between heat lst t the cling water and heat supplied by the heater.) 14. Recrd the temperature in the inner can as precisely as pssible every 30 secnds fr tw minutes using the high-precisin thermmeter. 15. Nte the time and depress the firing buttn fr 3 secnds nly. There is n nise when the bmb is fired. (The wrd bmb' refers t the ally cmbustin vessel itself, nt the nise it makes; it is a sealed cntainer in which a high pressure reactin can take place safely.) If cmbustin has taken place, the temperature will start t rise within a minute. Recrd it every 30 secnds until it stabilises (abut 7 minutes). 16. If the bmb des nt fire (n temperature change after 3 minutes), r when the temperature has stabilised, switch ff all electrical circuits and dismantle the bmb. D this using the capshaped internally-threaded metal fitting which screws dwn ver the xygen inlet n the bmb. Screw this dwn until gas begins t escape frm the bmb. Wait until the bmb reaches atmspheric pressure, then und the bmb. 17. Clean and dry the bmb, then repeat the prcedure fr the tw ther pellets. 5 Calculatin Fr each run, plt temperature against time. Determine the temperature rise upn cmbustin. Frm the benzic acid data, calculate the heat capacity f the calrimeter. Use this t determine the standard enthalpy f cmbustin f 9,10-dihydranthracene and bibenzyl. D nt frget in yur calculatin that, when cnverting internal energy changes t enthalpy changes, the (pv) term must take int accunt the amunt f material actually burnt, nt simply the stichimetry f the cmbustin prcess. Discuss surces f errr and their magnitude, and explain the rigin f the differences yu bserve in the cmbustin enthalpies.

6 Frm yur calculated enthalpies f cmbustin, determine which f 9,10-dihydranthracene and bibenzyl has the greater armatic stabilisatin energy. The simplest way f ding this is t determine what bnds are brken and which are frmed in ging frm 9,10- dihydrxyanthracene t bibenzyl, and then d a Hess s cycle in which (a) 9,10- dihydranthracene is burned t give carbn dixide and water, and (b) 9,10-dihydranthracene is first cnverted int bibenzyl, which is then burned. Yu can use the fllwing average bnd enthalpies (all per mle f bnds): C-C: 364 kj, H-H: 436 kj, C-H: 461 kj, O=O: 498 kj, O-H: 462 kj Cmment n yur result. If yu have dne the mlecular mdelling practical yu may wish t use the Spartan sftware t cnfirm (r refute!) yur findings. Answer the fllwing questins: 1. Why is 1 cm 3 water added t the bmb befre firing? 2. Why is it that, althugh the temperature reached during the cmbustin is very high, reflecting the large amunt f energy released during the reactin, we need t knw nly the initial and final temperatures in rder t calculate that energy? 6 Additinal Infrmatin U cb,benzic acid = kj g -1 U cb,fe = kj g -1 U cb,cellulse = - 18 kj g -1,