Measuring Enthalpy Changes and Gas Laws

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Elfreda Lucas
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1 Mesuring Enthlpy Chnges nd Gs Lws PURPOSE To oserve chnges in enthlpy in chemicl processes. To determine the reltionship etween the pressure nd volume of gs. GOALS 1 To identify exothermic nd endothermic processes. 2 To relte enthlpy chnges nd entropy chnges to chnges in free energy. 3 To lern to use the MicroL syringe ssemly nd interfce. INTRODUCTION Chemicl processes occur spontneously when they lower the free energy of the system. The free energy t constnt temperture nd pressure is the Gis free energy, G, which is defined s follows: G = H T S, (1) where G is the chnge in free energy 1 of the system, H is the chnge in enthlpy 2 of the system nd S is the chnge in entropy 3 of the system. G, the chnge in free energy, must e less thn zero for spontneous process. This follows from the definition of spontneous process; it lowers the energy of the system. Free energy, given the symol G in honor of the Americn chemist, J. Willrd Gis, is the energy ville from spontneous process to do work. For nonspontneous process, it is the mount of energy tht must e supplied to cuse the process to occur. H, the enthlpy chnge, is the mount of het energy sored y process tht occurs t constnt pressure ( H is the het given off). H is often proportionl to T, the temperture chnge cused y the process. The proportionlity constnt etween H nd T is clled the specific het (s). The specific het is property of the sustnce eing heted or cooled. For exmple, the specific het of wter is 4.18 J/(g C) mening tht 4.18 joules of het energy re required to rise the temperture of 1 grm of wter y 1.0 C. Not ll processes tht involve chnge in enthlpy re ccompnied y chnge in temperture. Phse chnges, e.g. melting, oiling, nd sulimtion ll sor het. However, the sustnce undergoing the phse chnge mintins constnt temperture. All the energy supplied to the sustnce is consumed in the phse chnge. 1 free energy c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 1

2 Processes tht give off het energy ( H < 0) re exothermic. 4 Comustion is oviously n exothermic chemicl rection. Condenstion of stem to liquid wter is lso exothermic, lthough wter mintins constnt temperture during the process. Processes tht sor het energy ( H > 0) re endothermic. 5 Swet, liquid wter tht evportes from your skin, cools you y soring het from your ody. S, the chnge in entropy, is relted to the numer of wys the energy of the system cn e distriuted. Entropy is commonly defined s disorder or rndomness. Entropy is high in gses, ecuse the molecules re free to move out in ll directions, fill the spce ville, nd dopt ny orienttion reltive to ech other. Entropy is low in solids, ecuse the molecules cnnot move much reltive to ech other, nd their orienttion is fixed. With this informtion, one cn see tht chemicl process will certinly e spontneous if it is exothermic ( H < 0) nd its entropy increses ( S > 0). If the opposite holds ( H > 0 nd S < 0), the process cnnot e spontneous. Wht if oth hve the sme sign? If the process is exothermic, the process will e spontneous elow the temperture tht stisfies the condition H = T S. It turns out tht chnges in entropy re smll reltive to chnges in enthlpy in most processes. It is usully not possile to tell whether entropy hs incresed or decresed in spontneous exothermic processes. The exceptions occur when gses re produced or consumed. Wter will condense spontneously if the temperture is elow 100 C; the process is sufficiently exothermic to offset the loss in entropy tht ccompnies the phse chnge. If the process is endothermic, the process will e spontneous ove the temperture tht stisfies the condition H = T S. As mentioned ove, entropy chnges re smll reltive to enthlpy chnges when gses re not involved. Therefore, spontneous endothermic processes re reltively uncommon. However, they must involve n increse in entropy. When working with G, H, T, S, nd in fct ll chemicl terms involving (chnge in), the quntity is clculted s finl initil. Thus, if eker contining wter ws originlly t 22.5 C nd chemicl process cused its temperture to fll to 17.3 C, the chnge in temperture would e T = T finl T initil = 17.3 C 22.5 C = 5.2 C. (2) It is importnt to rememer finl initil in order to get signs right. This cn sometimes e confusing. It is importnt to distinguish etween system nd surroundings, 6 the two prts of the thermodynmic universe, when deling with rection energetics. The first line of this document stted tht spontneous processes lower the free energy of the system. The system is the prt of the universe eing studied. The first lw of thermodynmics sttes tht energy is neither creted nor destroyed. The energy relesed y system must e converted to some other form or trnsferred to some other plce. Often, the energy is trnsferred to the surroundings, the prt of the universe (thermodynmics) c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 2

3 tht intercts with the system eing studied. In such studies, it is often prcticl to isolte rther smll universe of system nd surroundings in which nerly ll the energy is retined. In Prts A nd B of this l, we will ssume tht the thermodynmic universe is the test tue or eker in which the experiment is eing conducted. Of course, some het is trnsferred outside the test tue (you will e le to feel ekers het up or cool down) ut most is retined. The energy relesed or sored y the systems eing studied (chemicls dissolving or recting) will e trnsferred to or from the wter in which the processes occur. Thus, wter is mjor component of the surroundings. Returning to eqution 2 nd sign chnges. In tht exmple, the temperture of the wter (the surroundings) decresed nd het ws relesed. This mens tht chemicl process (the system) sored het from the wter. Since the process sored energy, it ws endothermic. Now consider n exothermic exmple. If chemicl process (the system) releses het to the wter (the surroundings), the temperture of the wter will increse s the wter sors het energy nd the T of the wter will e positive. In Prt A of the l, you will oserve temperture chnges tht occur s compounds dissolve in wter. In Prt B, you will oserve temperture chnge tht occurs during chemicl rection. In Prt C of the l, you will oserve temperture chnges tht occur s energy is dded to wter in its solid nd liquid phses. Prt D of this l will involve investigting the reltionship of the volume nd pressure of gses. Gses cn e chrcterized y numer of vriles, which include the mount, volume, pressure, nd temperture. The idel gs lw summrizes this reltionship. P V = nrt (3) where P = pressure in tmospheres, V = volume in liters, n = moles of gs, R = liter tmospheres/kelvin mole, nd T = temperture in Kelvin. In this l, you will determine the reltionship etween the pressure nd volume of gs using the MicroL interfce nd syringe ssemly. Two different volume nd pressure reltionships will e studied. The temperture of the system will simply e the temperture of the lortory. However, the different volumes will llow students to investigte the reltionship of volume nd pressure. The volume will serve s the independent vrile, which is chnged y the resercher. The pressure will e the dependent vrile, whose response is mesured. The temperture should remin mostly unchnged during the experiment nd the moles should remin unchnged s the system is seled. EQUIPMENT 1 30 ml eker ml eker c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 3

4 1 10 ml grduted cylinder 1 MicroL interfce 1 MicroL thermistor instruction sheet 1 thermistor 2 smll test tues 1 test tue rck 1 ring stnd nd clmp 1 hot plte/stirrer 1 mgnetic stir r 1 60 ml syringe 1 20 ml syringe 1 connecting dpter 1 deionized wter squirt ottle c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 4

5 REAGENTS 2 g CCl 2 (s, nhydrous) 2 g NH 4 NO 3 (s) 1 ml 1 M NOH (q) 1 ml 1 M FeCl 3 (q) 60 g ice 13 ml deionized wter SAFETY The solutions used in this experiment re dilute, ut corrosive. They cn ttck the skin nd cuse permnent dmge to the eyes. If ny of the solutions splsh into your eyes, flush them in the eyewsh. Hold your eyes open or hve someone ssist you. If you spill ny of the solutions on your skin or clothing, flush the re immeditely with wter. Hve your l prtner notify your instructor out the ccident. You will e working with hot plte. Keep flmmle mterils (ppers, hir, clothing) wy from it. It will sty hot for long time fter you hve turned it off. Do not touch it until you hve felt the ir ner it nd re sure it will not urn you. When using the syringes, moderte mount of force will e needed to hold the syringe plunger t certin volumes ginst reduced pressure. Cre should e tken to grip the syringe firmly to prevent pinched fingers. WASTE DISPOSAL The rection mixture from Prt B of the experiment should e put in the wste continer provided. Use wter in squeeze ottle to rinse the solid mteril out of the test tue nd then wsh the tue with sop nd wter. Solutions from Prts A nd C cn e wshed down the sink. PRIOR TO CLASS Plese red the following section of L Sfety nd Prctices: Prepring Grphs 7 Plese complete your WeAssign prel ssignment. Check your WeAssign Account for due dtes. Students who do not complete the WeAssign prel re required to ring nd hnd in the prel worksheet. LAB PROCEDURE Plese print the worksheet for this l. You will need this sheet to record your dt. Prt A: Het of Solution - CCl 2 nd NH 4 NO 3 7../grphs/mnul.html c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 5

6 1 Open the MicroL progrm. Clirte the thermistor s descried in the MicroL instructions provided in the l. 2 After the clirtion is complete, set the MicroL collection increment to 2 seconds using the instructions provided. 3 Use spoon-type sptul to dispense nhydrous solid CCl 2 into dry 30 ml eker. One spoonful, not heping, is plenty. 4 Otin 10 ml of deionized wter in grduted cylinder. 5 Mesure the initil temperture of the wter in the grduted cylinder y plcing the thermistor in the wter nd pressing strt on the MicroL dt collection progrm. Record the initil temperture in Dt Tle A. 6 Pour the wter into the eker contining the CCl 2. 7 Plce the thermistor in the eker. Wtch the temperture chnge on the monitor. Record the mximum or minimum temperture reched in Dt Tle A. Stop the MicroL dt collection progrm. 8 When finished, dispose of your solution in the sink, rinse the eker with deionized wter nd dry. 9 Repet steps 3 through 8 with solid NH 4 NO 3 in plce of the solid CCl Clculte the T for ech rection nd record them in Dt Tle A. Dt Tle A: Het of Solution Question 1: For dissolution of CCl 2, plese nswer - c. c Ws het given off or sored? Could you feel it? Ws the process exothermic or endothermic? Did the entropy increse, decrese, remin the sme or cn you not tell from your results? Question 2: For dissolution of NH 4 NO 3, plese nswer - c. c Ws het given off or sored? Could you feel it? Ws the process exothermic or endothermic? Did the entropy increse, decrese, remin the sme or cn you not tell from your results? Question 3: Which chemicl would you use in cold pck, CCl 2 or NH 4 NO 3? Prt B: Het of Rection - FeCl 3 (q) + NOH(q) 1 Tke two test tues to the side shelf nd otin out 1 ml of 1 M FeCl 3 in one nd out 1 ml of 1 M NOH in the other. c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 6

7 2 Strt the MicroL dt collection progrm nd use the thermistor to mesure the temperture of the FeCl 3 solution. Also, note its color. Enter this informtion in Dt Tle B. 3 Add the 1 M NOH to the FeCl 3 test tue. 4 Plce the thermistor in the test tue contining the mixture. Wtch the temperture chnge on the monitor. Record the mximum or minimum temperture reched in Dt Tle B. Stop the MicroL dt collection progrm. 5 Record the ppernce of the rection mixture in Dt Tle B. 6 Clculte the T for the rection nd record it in Dt Tle B. 7 When finished, dispose of your solution in the wste ottle provided. Use wter in squeeze ottle to rinse the solid mteril out of the test tue nd then wsh the tue with sop nd wter. Dt Tle B: Het of Rection Question 4: For the rection of FeCl 3 nd NOH, plese nswer - d. c d Wht evidence indictes tht rection occurred? Did the rection give off or sor het? Could you feel it? Did the entropy increse, decrese, remin the sme or cn you not tell from your results? Ws the rection spontneous? Justify your nswer. Prt C: Enthlpy nd Phse Chnges 1 Chnge the MicroL collection increment to 30 seconds. 2 Plce mgnetic stir r in 100 ml eker. Fill the eker to the 60 ml mrk with crushed ice nd dd tp wter to the 60 ml mrk. 3 Clmp your thermistor in the ring stnd with hot plte/stirrer on the se. 4 Plce the eker on the hot plte/stirrer nd lower the thermistor into the ice slurry. The tip of the thermistor should e out third of the wy down in the slurry. 5 Strt the stirrer; set the dil to out 6 for gentle stirring. 6 Initite the MicroL dt collection progrm. 7 Turn the hot plte to 100 nd continue llowing MicroL to record the temperture. 8 After two minutes, turn the hotplte to Note the temperture when ll the ice hs melted in Dt Tle C nd turn the heter to 400. Continue llowing MicroL to record the temperture. c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 7

8 10 As the wter hets, record the temperture t which you oserve ules, stem, nd true oiling in Dt Tle C. 11 Once oiling egins, red the temperture every 30 seconds for three minutes. Then stop the MicroL dt collection progrm. 12 Mke sketch of the MicroL grph on your worksheet, noting where the phse trnsitions occur. Mke sure to give your grph title. Lel the xes with the quntities eing plotted nd the units in which they re mesured. Dt Tle C: Temperture nd time during the heting of wter Question 5: Were there times when the temperture styed constnt for severl redings? Wht ws hppening during these times? Question 6: Wht hppened to the entropy of the system for ech of the following processes? Did it increse gretly, increse slightly, decrese gretly, decrese slightly, sty the sme or cn you not tell from your results? c As the ice melted? As the wter ws heted? As the wter oiled? 13 When you hve completed your mesurements, wsh nd dry ll your equipment nd return it netly to the set-up re where you found it. Mke sure the hot plte is turned off. Prt D: Gs Lws 1 Open the MicroL progrm nd select New Experiment. 2 Enter your experiment nme, Volume vs. Pressure, nd select OK. 3 Click ADD SENSOR, nd use the pulldown menu to select PRESSURE. The progrm will lso request the input. Choose the only input ville with red squre for the pressure. Enter torr for the lel. Then select NEXT nd mke sure the toggle utton is selected for Fctor clirted torr, followed y FINISH. 4 Click ADD SENSOR nd use the pulldown menu to select KEYBOARD. The lel will e VOLUME. Enter the prompt s volume? nd the units s ml, followed y FINISH. 5 To view dt s it is collected, click on the keyord sensor nd drg the icon to the x-xis of the grph, nd column A of the spredsheet. Click on the pressure sensor nd drg the icon to the y-xis of the grph, column B of the spredsheet, nd the lower right disply re. 6 Compress the 60 ml syringe to the 0 ml mrk nd ttch the hose to the MicroL pressure sensor. NOTE: Do not compress the syringe fter ttching it to the MicroL unit s this will rek the pressure sensor inside the unit. c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 8

9 7 Press strt on the MicroL console. 8 Hold the syringe to the 0 ml mrk, which is ctully 4 ml of gs, s the connector hose is 4 ml. Request tht your l prtner enter 4 in the keyord nd tke reding. 9 Tke dditionl redings t the 10, 20, 30, nd 40 ml syringe mrks. (Plese note: this is two-person opertion. One person will need to hold the syringe open ginst vcuum, while the other enters volumes using the keyord nd tkes pressure redings. Don t forget to dd 4 ml to your volumes!) Enter your dt in Tle D. You will e sked to plot the dt in different wys in your WeAssign In-L nd postl ssignments. 10 Remove the 60 ml syringe nd replce with 20 ml syringe, compressed to the 0 ml mrk. 11 Strt new experiment on the MicroL console. 12 Hold the 20 ml syringe to the 0 ml mrk. Request tht your l prtner enter 4 in the keyord nd tke reding. 13 Tke dditionl redings t the 5, 10, 15 nd 20 ml syringe mrks. Enter your dt in Tle D. Dt Tle D: Volume vs. Pressure 14 Before leving, go to computer in the lortory nd enter your results in the In-L ssignment. If ll results re scored s correct, log out. If not ll results re correct, try to find the error or consult with your l instructor. When ll results re correct, note them nd log out of WeAssign. The In-L ssignment must e completed y the end of the l period. If dditionl time is required, plese consult with your l instructor. Question 7: Prepre grph of the 60-mL syringe dt in Excel R y plotting the volume on the x-xis nd the pressure on the y-xis. Be sure to use proper leling nd include title. For ssistnce in the formt of grph, plese see Prepring Grphs 8 under L Sfety nd Prctices in the l mnul. For ssistnce using Excel R, plese consult with your TA. Finlly, uplod your grphs s n Excel R file with mximum size of 1 MB. Question 7: When the volume increses, wht is the effect on the pressure? Question 7c: When the volume decreses, wht is the effect on the pressure? Question 7d: Wht type of reltionship is this etween vriles? Reference for Prt D MicroLAB Dt Collection Technology. Gses: Boyle s nd Chrles Lws, pge.html 9 (ccessed Septemer 28, 2010). 8../grphs/mnul.html 9 pge.html c Advnced Instructionl Systems, Inc. nd North Crolin Stte University 9

Measuring Enthalpy Changes and Gas Laws

Measuring Enthalpy Changes and Gas Laws PURPOSE A B To observe changes in enthalpy in chemical processes. To determine the relationship between the pressure and volume of a gas. GOALS To identify exothermic