Hess Law - Enthalpy of Formation of Solid NH 4 Cl

Transcription

1 Hess Law - Enthalpy f Frmatin f Slid NH 4 l NAME: OURSE: PERIOD: Prelab 1. Write and balance net inic equatins fr Reactin 2 and Reactin 3. Reactin 2: Reactin 3: 2. Shw that the alebraic sum f the balanced net inic equatins frm Reactin 2 and Reactin 3 and Reactins 4-7 equals the equatin fr the frmatin f slid NH 4 l. (Use Hess' Law) 3. Why is it imprtant in Part B f the prcedure that bth slutins are at the same temperature befre they are mixed? 4. Hw many mles f NH 4 l are used Part A f the prcedure? Assumin n vlume chane n disslvin, what is the mlarity f the NH 4 l in Part A? 5. Which reactant is the limitin reaent in Part B f the prcedure? Supprt yur answer with calculatins. 1

2 Hess Law - Enthalpy f Frmatin f Slid NH 4 l In this experiment the heat f slutin f NH 4 l (s) and the heat f neutralizatin f NH 3 (aq) and Hl (aq) are measured and cmbined with the knwn heats f several reactins t determine the heat f frmatin f NH 4 l (s) usin Hess' Law. Heat is invlved in all chemical reactins. hemical reactins are either exthermic r endthermic. The heat f reactin fr any chemical reactin is determined as fllws: H = Σ H f (prducts) - Σ H f (reactants) Fr exthermic reactins, H is neative, whereas H is psitive fr endthermic reactins. A neative H indicates the prducts are at a lwer enery state than the reactants. A psitive H indicates the prducts are at a hiher enery state than the reactants. In the fllwin reactins, H is expressed in kiljules f heat. Example: H 2 () + 1/2 O 2 () H 2 O () H 2 () +I 2 () 2 HI () H = -242 kj (exthermic) H = kj (endthermic) Many substances can be frmed usin mre than ne technique. Fr example, carbn dixide can be frmed by burnin carbn in xyen. 1) (s) + O 2 () O 2() H = kj Anther methd f prducin carbn dixide invlves a three step prcess. 2) H 2 O () + (s) O () + H 2 () H = kj 3) H 2 () + 1/2 O 2 () H 2 O () H = kj 4) O () + 1/2 O 2 () O 2 () H = kj The sum f equatins 2, 3, and 4 is: 5) H 2 O () + (s) + H 2() + 1/2 O 2() + O () + 1/2 O 2() O () + H 2() + H 2 O () + O 2 () T determine the ttal amunt f heat enery released r absrbed usin the three-step prcess, add the heats f reactin fr the three reactins. H= ( kj) + ( kj) + ( kj) = kj Equatin 5 reduces alebraically t equatin 6 6) (s) + O 2 () O 2 () H = kj which is identical t H fr the reactin in equatin 1. 2

3 Hess's Law f heat summatin states that when a reactin can be expressed as the alebraic sum f tw r mre ther reactins, the heat f the reactin is the alebraic sum f the heats f these reactins. The standard heat f frmatin f a cmpund is defined t be the heat released r absrbed when ne mle f the cmpund is frmed frm its elements each in their standard states. The standard heat f frmatin f NH 4 l (s) is iven by the equatin: Reactin 1 1/2 N 2 () + 2 H 2 () + 1/2 l 2 () NH 4 l (s) The cefficient f the cmpund bein frmed is set equal t ne since ne mle is bein frmed which results in fractinal cefficients fr sme f the reactants. The heat f slutin f NH 4 l (s) (reactin 2) and the heat f neutralizatin f NH 3 (aq) and Hl (aq) (reactin 3) are measured experimentally. Reactin 2 NH 4 l (s) NH 4 +1 (aq) + l -1 (aq) Reactin 3 NH 3 (aq) + Hl (aq) NH 4 l (aq) They are cmbined with the knwn heats fr the fllwin reactins t determine the heat f frmatin f NH 4 l (s). Reactin 4 1/2 N 2 () + 3/2 H 2 () NH 3 () H = kj Reactin 5 NH 3 () NH 3 (aq) H = kj Reactin 6 1/2 H 2 () + 1/2 l 2 () Hl () H = kj Reactin 7 Hl () H +1 (aq) + l -1 (aq) H = kj Prcedure: Preparin the alculatr and BL and llectin Data 1. Attach the BL temperature prbe t the BL in hannel Attach the BL t the calculatr usin the unit t unit link cable. Attach the vltae adapter t the BL. 3. Turn n the calculatr and BL. 4. Press [PRGM] n the calculatr. 5. Use the dwn arrw t select the prram HEAT. Press [ENTER]. 6. The PRGM HEAT will appear. Press [ENTER]. (On the TI-83, Heat V1.2 appears. Press [ENTER]. Turn n the BL appears. Press [ENTER]. Nw checkin the alculatr-bl Link. Please wait. appears. If the BL is n, Status OK appears. Press [ENTER].) 7. Fr the secnds between pints, type in 3. Press [ENTER]. 8. Press enter t start cllectin data appears. D Nt press [ENTER] at this time. Part A) Heat f Slutin f NH 4 l (s) 1. Mass an empty small weih bat. Add abut 4.00 f slid NH 4 l t the weih bat. Remass the weih bat and NH 4 l. Recrd the values in the data table. alculate the mass f NH 4 l used. 3

4 2. Measure 50.0 ml f distilled water usin a raduated cylinder. Add this t the calrimeter and place the cver n it. Place the temperature prbe thruh the lid f the calrimeter and int the water. 3. Press [ENTER] n the calculatr. Wait fr abut 3-4 pints t appear n the calculatr screen. These temperatures shuld be relatively cnstant. 4. Lift the cver and add the NH 4 l t the water in the cup. Place the cver back n the cup, with the temperature prbe thruh the hle and in the water. Stir the slutin ently with the temperature prbe while the slutin prcess takes place. It is necessary t stir the cntents f the cup while slutin is ccurrin t be sure that the temperature measured is the averae temperature f the cntents f the cup. The temperature will drp rapidly and then level ff. Yu are lkin fr the stable plateau temperature. The temperature will then bein t rise as the slutin ains heat frm the surrundins. Gd stirrin is essential! 5. The BL will shw DONE at the end f the data cllectin. The raph is displayed n the calculatr. 6. Press [ZOOM]. Select: ZmStat. Press [ENTER]. 7. Press [TRAE]. Use the arrw keys t find the initial cnstant temperature (T 1 ) rather than the first temperature value and the lwest cnstant temperature which will be the final temperature (T 2 ). Recrd these values t the nearest 0.01 in the data table. alculate the chane in temperature. Use the abslute value f the chane in temperature. 8. T see the data, press [STAT]. EDIT shuld be hihlihted. Press [ENTER]. The Time shuld be in L3 and the Temperature in L4. 9. Dispse f the slutin accrdin t yur instructr s directins. Thruhly rinse and dry the calrimeter and the temperature prbe fr Part B. Press [LEAR] n the calculatr until yu et t the hme screen. Part B) Heat f Neutralizatin f Hl (aq) and NH 3 (aq) 1. Repeat steps 3-8 fr preparin the calculatr and BL. Keep the time per sample at Usin a raduated cylinder, measure 50.0 ml f 1.60 M Hl. Usin a different raduated cylinder, measure and pur 50.0 ml f 1.50 M NH 3 int yur clean dry calrimeter. Place the temperature prbe thruh the lid f the calrimeter and int the slutin. 3. Press [ENTER] n the calculatr. Wait fr abut 3-4 pints t appear n the calculatr screen. These temperatures shuld be relatively cnstant. 4. Lift the cver and pur the Hl slutin int the calrimeter cntainin the NH 3 4

5 slutin. Stir the slutin ently with the temperature prbe while the reactin takes place. The temperature will rise rapidly and then level ff. Yu are lkin fr the stable plateau temperature. Gd stirrin is essential! 5. The BL will shw DONE at the end f the data cllectin. The raph is displayed n the calculatr. 6. Press [ZOOM]. Select: ZmStat. Press [ENTER]. 7. Press [TRAE]. Use the arrw keys t find the initial cnstant temperature rather than the first temperature value and the hihest cnstant temperature. Recrd these values t the nearest 0.01 in the data table. alculate the chane in temperature. 8. T see the data, press [STAT]. EDIT shuld be hihlihted. Press [ENTER]. The Time shuld be in L3 and the Temperature in L4. 9. Dispse f the slutin accrdin t yur instructr s directins. Thruhly rinse and dry the calrimeter and the temperature prbe. Press [LEAR] n the calculatr until yu et t the hme screen. alculatins: The heat absrbed by the water r the slutin may be calculated as fllws: J Heat( J) = MassSlutin, SpecificHeatSlutin,, ( ) ( Temperature ) The mass f the slutin is in rams, the specific heat f water is in J/, the temperature r chane in temperature is in. ***These assumptins are bein made in rder t carry ut the calculatins.*** a) The density f all slutins equals the density f water, 1.00 /cm 3. b) The specific heat f all slutins equals that f water, J/. c) All substances react r disslve cmpletely. d) All heat enery released by a reactin is absrbed by the slutin (nne by the cup). 1. In Part A use the mass f the water in the equatin abve. 2. In Part B use the ttal mass f the Hl and NH 3 slutins in the equatin abve. 3. alculate the heat absrbed by the water r the slutin in J and cnvert it t kj. Include the crrect sin fr the heat chane. 4. alculate the heat released by the reactin in terms f kj/ml NH 4 l. In Part A, the mles f NH 4 l are calculated frm the mass f slid NH 4 l. In Part B, the mles f NH 4 l are calculated frm the vlume and the mlarity f the NH 3 slutin used. The mle rati between NH 4 l and NH 3 is 1:1. Include the crrect sin fr the heat chane. 5

6 5. alculate the experimental value fr the heat f frmatin f NH 4 l. 6. Usin table values fr the H f, calculate a theretical heat f reactin fr reactins 2 and 3. alculate a percent errr usin yur experimental value. 7. Usin a table value fr the H f fr NH 4 l, alculate a percent errr usin yur experimental value. This experiment is adapted frm Experiment 11-2: Thermchemistry-Enthalpy f Frmatin f NH 4l (s), Labratry Experiments fr Fundatins f hemistry, Tn and Ellis, Hlt, Rhinehart, and Winstn, Inc., 1973, p

7 Hess Law - Enthalpy f Frmatin f Slid NH 4 l NAME: OURSE: PARTNER S NAME: PERIOD: 1 Mass f NH 4 l (s) ) and weih bat 2 Mass f weih bat Data Table Part A Part B 3 Mass f NH 4 l (s) 4 Vlume f H 2 O 5 Mass f H 2 O 6 Vlume f NH 3 (aq) ml ml 7 Mass f NH 3 (aq) 8 Vlume f Hl (aq) 9 Mass f Hl (aq) 10 Ttal mass f slutin (calculated) ml 11 Initial temperature, T 1 12 Final temperature, T 2 13 Temperature chane, T= T 2 -T 1 14 Heat absrbed r released by the slutin (with sin) J J 15 Heat absrbed r released by the slutin (with sin) kj kj 16 Heat absrbed r released by the reactin (with sin) kj kj 17 Mles f NH 4 l used r prduced ml ml 18 Heat evlved/ml f NH 4 l Experimental kj/ml kj/ml 19 Heat evlved/ml f NH 4 l Theretical kj/ml kj/ml 20 Percent Errr % % alculatins: 1. Shw yur calculatins fr the experimental H fr reactins 2 and 3. 7

8 2. Usin Hess Law, the experimental H fr reactins 2 and 3, and the iven H fr reactins 4-7, shw the calculatin fr the experimental value fr the H f f NH 4 l (s). Shw hw yu rearraned the reactins in the summatin prcess. 3. Discuss any surces f errr yu encuntered in the experiment. 8