Chemistry 120 Laboratory Solution Calorimetry: A Hess' Law Investigation

Transcription

1 Chemistry 120 Labratry Slutin Calrimetry: A Hess' Law Investigatin Objectives: T measure the heat capacity f a calrimeter T measure the heat f reactin fr tw chemical reactins T use Hess' Law, in cnjunctin with the abve measurements, t calculate the heat f reactin fr the cmbustin f magnesium Backgrund: The heat f reactin is defined as the heat gained r lst by a chemical system as a reactin prceeds frm reactants t prducts. It is ften given as part f a thermchemical equatin, such as that shwn fr the cmbustin f hydrgen gas: (a) H 2 (g) + 1/2 O 2 (g)! H 2 O(l) ΔH = -286 kj The negative sign indicates that the chemicals lse thermal energy t the surrundings in the reactin. Thus when ne mle f hydrgen cmbines with ne-half mle f xygen t make ne mle f water, 286 kiljules f energy are released t the surrundings. This is an example f an exthermic reactin. The symbl "H" stands fr the enthalpy, the thermal energy at cnstant applied pressure. The symbl "Δ" refers t change. S "ΔH" refers t the enthalpy change fr the cmbustin f tw mles f hydrgen gas. Cnditins f cnstant pressure are achieved by carrying ut the reactin in a calrimeter that is pen t the cnstant applied pressure f the atmsphere. The heats f reactin are reprted fr 25 C, althugh they vary nly minimally with temperature. In this experiment yu will measure the ΔH values fr the fllwing reactins: (b) (c) Mg(s) + 2 HCl(aq)! MgCl 2 (aq) + H 2 (g) MgO(s) + 2 HCl(aq)! H 2 O(l) + MgCl 2 (aq) Using the thermchemical equatins fr these tw reactins alng with that previusly given fr the hydrgen/xygen reactin, yu will then calculate ΔH fr the frmatin f magnesium xide (which is the same as the heat f cmbustin f magnesium) Mg(s) + 1/2 O 2 (g)! MgO(s) ΔH f by applying Hess' Law as described belw and in yur textbk. A slutin calrimeter is a device fr measuring heats f reactins that ccur in slutin. Fr aqueus slutins, a Styrfam cffee cup wrks well. One carries ut the reactin in the cup and measures the temperature change (Δt) ver the curse f the reactin. Assuming adiabatic cnditins that is, all f the heat f reactin is cnfined t the insulated cup and its cntents with n heat leak t the utside f the cup ne can calculate the heat f reactin in the calrimeter as fllws: 1. Ttal heat frm the reactin is equal t the prduct f the heat capacity f the calrimeter and the temperature change f the calrimeter and its cntents: Q = Ccalrimete r Δt 2. The heat capacity f the calrimeter is btained frm the heat capacity f the {cup, lid, thermistr, and magnet} assembly plus the heat capacity f the slutin. The apprximate heat capacity f the cup assembly can be estimated t be abut 30 jules/ C; and that f the 100 ml f HCl and Mg t be 1

2 Thus the ttal heat capacity f the system is abut Ccntents 3.86J / g/ C 100mL 1.00g / ml = 386J / Ccalrimete r 386 J / C + 30J / C = 420J / See the next page fr a methd by which yu will determine the actual heat capacity. 3. The temperature change is btained by determining the difference between the extraplated preadditin and pst additin baselines at the pint where a vertical line cnnecting them passes thrugh the midpint f the temperature rise. 4. The balanced equatins are used t cnfirm that the limiting reagents fr each reactin are, respectively, Mg and MgO. Frm the actual mles f limiting reagent and the actual number f jules f energy released, yu can determine the heat evlved per mle f limiting reagent. 5. The chemical equatins are as fllws: C C (a) H 2 + 1/2 O 2! H 2 O ΔH a = -286 kj (given, nt measured) (b) Mg + 2 HCl! MgCl 2 + H 2 ΔH b = - measured heat/mle f Mg (c) MgO + 2 HCl! H 2 O + MgCl 2 ΔH c = - measured heat/mle f MgO Nte the (-) sign indicates an exthermic prcess. Heats need t be cnverted frm Jules t kiljules. Adding the chemical equatins (a) + (b) (c) gives: Mg + 2 HCl + H 2 + 1/2 O 2 + H 2 O + MgCl 2! MgCl 2 + H 2 + H 2 O + MgO + 2 HCl which simplifies t: d) Mg + 1/2 O 2! MgO This is the reactin fr the frmatin f ne mle f MgO. T get the heat f reactin fr this prcess, we add the heats f reactin fr the equatins in the same sense that we added the equatins: ΔH d = ΔH a + ΔH b - ΔH c ΔH d is the mlar heat f frmatin f MgO. Thus ΔH f MgO (kj/mle MgO) ΔH d. T determine the heat capacity f the calrimeter assembly, ne can add a f knwn heat capacity at the temperature f biling water (t ht ) t the calrimeter cup cntaining 100 ml f HCl at rm temperature (t cld ). The whle system cmes t sme temperature in between the tw temperatures, t. The assumptin is that all f the heat is transferred frm the ht ingt t the calrimeter, and all is cntained within the cup, s nne escapes t the surrundings: Q + Q calrimeter = 0 (eq. 1) c mass ( t t ) + C ( t C calrimeter = c mass ht ( t calrimeter t ht )/( t t t cld cld ) = 0 ) (eq. 2) 2

3 T get a reasnable temperature change, yu wuld need at least 30 grams f. Yu need a that will nt react with the HCl. Cpper is a gd chice, since it is inert t actin by acids. Its specific heat capacity c Cu = jules/ C/gram. Pre-Labratry Assignment: Answer the fllwing questins, shwing all steps in the slutins. The crrect answers are given fr each. Q. 1 Suppse 30.0 grams f cpper at 99.0 C are added t a calrimeter cntaining 100 ml f 1.00M HCl at 23.5 C and the temperature rises t a value f 25.2 C. What is the heat capacity f the calrimeter in Jules/ C? (Answer 500 J/ C) Q. 2 What is the experimentally measured heat f reactin between Mg and HCl per mle if 0.25 grams f magnesium added t 100 ml f 1M HCl in the calrimeter frm Q 1 causes a temperature rise f 12.3 C? (Answer -600 kj/mle Mg) Q. 3 T hw many significant figures shuld the heat capacity f the calrimeter and the heat f reactin f Mg with HCl each be reprted? Explain. 3

4 Prcedure: All slutins can be discarded in the drains. Slids shuld be discarded int the trash receptacles. Cpper shuld be returned t the lcatin frm which it was taken. Prper safety precautins shuld be taken and gggles shuld be wrn thrughut the lab perid. 1. Thermistr Calibratin [Nte 1] This prcess is the same that yu used the first week f lab. Turn n the MicrLAB unit, then the PC. Plug the thermistr temperature sensr int the jack. Open the MicrLAB prgram and design a prgram that measures time and temperature every half secnd. When yu add the temperature sensr, yu will be given the ptin t read frm a calibratin file r d a new calibratin f the thermistr temperature prbe. Yu may wish t use yur earlier calibratin file, but nly if yu are using the same PC and thermistr prbe. If yu chse t recalibrate, use at least three pints spanning the range f the experiment, rughly 0 t 40 C. Be sure t add a line fit and save the calibratin t a file with a name that makes sense. Add the time t clumn A and t the x-axis; add the temperature t clumn B and the y-axis. It might be a gd idea t add the temperature t the Digital Display windw as well. Run the prgram nce t make sure all is well, mving the thermistr frm ice t warm water. 2. Heat Capacity f the Calrimeter: Bring 100 ml f DI water t a bil in a 250 ml beaker. Weigh abut 30 grams f Cu t the nearest mg; alternatively, just weigh ne r tw cpper cylinders t get abut 30 grams. (If the cpper is tarnished, first clean the tarnish ff with sme dilute HCl, fllwed by a rinse with DI water, and a thrugh drying.) Add the weighed Cu t a 150 mm test tube and place the test tube in the biling water bath fr at least 10 minutes. The Cu shuld then be at the temperature f the biling DI water. Yu can measure that temperature r lk it up if yu knw the atmspheric pressure. Set up the calrimeter: place nestled Styrfam cups int a supprting beaker. Add magnetic stir bar. Measure 100 ml f 1.00M HCl in a graduated cylinder. Pur the HCl int the calrimeter cup. Psitin the assembly ver the magnetic stirrer securely. Adjust the stirring t a mderately brisk speed fast enugh but n slshing r trnads inside the cup. Insert the thermistr int the lid, using a rubber band lped abut the prbe t adjust its height. Put the lid n the cup and allw a minute r s t let everything thermally equilibrate. START the prgram. Cllect readings fr at least 2 minutes t establish a baseline n the temperature/time plt. [See NOTE 2 at end f handut.] Nw wrk quickly but carefully: remve the lid frm the calrimeter and the ht test tube frm the water bath. Add the cpper t the HCl and replace the lid assembly. Yu shuld see the temperature begin t climb and then level ff t a cnstant temperature plateau. Cllect data fr at least 5 minutes and then STOP the experiment. Turn ff the stirrer. Save yur data t the hard drive and t yur flash drive. EXPORT yur data as well. Determine the initial and temperatures f the calrimeter and cntents. A really gd way t d this is t get best linear fit equatins fr the baseline and fr the temperature line. Cmpute the tw values fr temperature (y) at that cmmn time (x) that represents abut the midpint f the temperature change regin f the plt. The difference between these tw temperature values is the best-fit temperature change, Δt, fr the calrimeter in that run. Place this Δt int equatin 2 t determine the heat capacity f the calrimeter. It wuld be reasnable t assume that yur heat capacity wuld be abut 400 t 600 jules/ C. Repeat at least nce and determine the average heat capacity f the calrimeter. 4

5 3. The Mg/HCl Reactin: It is imprtant t maintain the experimental cnditins f this part f the experiment as clse as yu can t thse used in getting the heat capacity f the calrimeter. Clean and dry the calrimeter cup, stir bar, thermistr prbe, and lid. Add the stir bar t the cup. Measure 100 ml f 1.00M HCl and add it t the cup as befre. Insert the thermistr int the lid and place the lid n the Styrfam cup assembly and begin stirring. Allw the system t equilibrate thermally fr several minutes. Meanwhile, weigh t the nearest gram abut 0.25 grams f Mg turnings. Begin data lgging and cllect data fr at least tw minutes t establish an initial baseline. Remve the lid and quickly add all f the Mg t the calrimeter. As the reactin ccurs, the temperature shuld increase, since the reactin is exthermic. Cntinue taking data until a cnstant temperature plateau is reached and maintained fr at least three minutes. Stp the experiment and turn ff the stirrer. Save and Exprt yur data t the hard drive and a flash drive. Rinse the calrimeter and all cntents with DI water and dry everything. Repeat the calrimetry run at least nce. The Δt can be determined in the same way as in Part The MgO /HCl Reactin This experiment is carried ut the same as Part 3 abve except that 1 gram f magnesium xide, MgO, weighed t the nearest gram, is used in place f Mg. MgO is a fine pwder. Take care that the pwder is transferred as cmpletely as pssible t the HCl slutin, leaving as little as pssible in the weighing tray r n the sides f the Styrfam cup. It shuld be cmpletely reacted and disslved at the end f the run. D at least tw runs. The Δt can be determined in the same way as in Part 2. Data analysis: Obtain printuts f ne thermgram f each type. Calculate the heat capacity f the calrimeter in Jules/ C. Use the heat capacity and measured temperature changes frm parts 3 and 4 t determine the number f jules f energy released in each run. Cnvert the amunt f Mg and MgO int mles f each in each run. Calculate the heats f reactin per ne mle f Mg and per ne mle f MgO. Use Hess' Law t calculate the heat f frmatin f MgO as shwn n page 2 f this handut. Reprt: Yur reprt shuld include the page that fllws as a cver sheet as well as the fllwing: sample thermgram frm each sectin f the experiment sample calculatins frm gd runs f Δt, calrimeter heat capacity, heats f reactin, and heat f frmatin f MgO tables f raw data (mass, temperature changes) used in calculatins table f results fr heat capacity, heat f reactin between Mg and HCl and between MgO and HCl average heat f frmatin f MgO and a cmparisn with the accepted value 5

6 Hess s Law Grading Sheet Name: Attach this as the cver sheet t yur lab reprt Pints Pssible Pints Earned Objective: 1 Data: Thermgrams 3 Table f Raw Data 2 Masses 2 Temperatures 1 Sample Calculatins: Δt 1 C cal 2 q Mg 2 ΔH Mg rxn 1 q MgO 2 ΔH MgO rxn 1 ΔH f 2 % Errr 3 Results: Table f Results with ave. ΔH fmgo 2 Ave. C cal 1 Ave. ΔH Mg 1 Ave. ΔH MgO 1 Summary Discussin f Results: 1 Reference t Surce f Accepted Value: 1 Ttal Pints 30 6