Heat of Solution of Hydrogen Chloride A Laboratory Experiment Gregory S. Harms and Thomas A. Lehman Bethel College, North Newton, KS 67117
Many published experiments reflect the importance of thermochemistry in teaching (1-15). They range from exercises suitable for secondary school (I)to experiments suitable only for well-equipped physical chemistry laboratories (2).We have measured the heat of solution of HCI in water using simple techniques. The change can be represented as: H C l ( g ) + excess HzO 3 HCKaq)
The enthalpy change is large for HCI. Experimental Dry HC1 gas is conveniently delivered fmm a lecture bottle. If the experiment is beine done by . onlv . a few students. a more ewu~micalmethod is-to assemble glass distillation apparatus, consisting of a round-bottomed flask with beater, reflux column, water-cooled condensation jacket, and drying tube. Concentrated HC1 solution is placed in the ro&dIbottomed flask and heated. The condensation jacket cools the hot gas. This method has the advantage of using only concentrated acid as the source of the drygas. Two other published methods should work satisfactorily (16,17). We have not tried them. ADewar flask contains a known amount of water, about 150 mL. A thermometer measuring to 0.2 "C and a simple glass stirring rod are in the flask. The initial water tem~ e r a t u r eis noted. The HC1 eas moves throueh ., a deliverv tube from the lecture bottle ror drying tube, to a point in the Dewar flask well below the water surface while the solution is stirred a t a rate that introduces negligible Joule heating. Gas enters the solution thmugb a narrow deliverv tube the first part of which is metal. 1t: thermal conduetiiity and high surface/volume ratio assure that any heat loss due to expansion cooling is replaced by this part of the line. When the increase in temperature is about 10-12' the gas flow is stopped, either by shutting the valve on the leetire bottle or withdrawing the plastic end of the gas line. Stirring continues briefly until the temperature no longer rises; the high temperature is noted. The heat capacity of the calorimeter was found in a separate experiment to be about 6% of the heat released by the resistance heater. Heat loss to the air in the calorimiter is negligible because of the low thermal conductivity of
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air and the brief duration of the experiment, on the order i nDewar of one minute. The concentration of the ~ ~ l 'the flask is determined by titration with standard base. The amount of dissolved HCI is found from the water volume and acid concentration, which is between 0.5 and 1molar. The heat capacity of the dilute acid solution is assumed to be the same as that of pure water, though it is slightly less. Results and Discussion Using dry HCI from a lecture bottle, we find theenthalpy of solution to be 67 kJtmol, about 6 kJ/mol below the estahlished value. Earlier experiments in which HCI was produced by heating a concentrated solution as described above gave values of higher accuracy but poorer repmducibility The experiment provides a simple measure of the enthalpy of solution. Though large enough to be observed easily, this quantity is ignored in most instructional laboratories. Acknowledgment We thank P. G. Wahlbeck for a helpful comment, Literature Cited 1, de Vos, W.; Verdonk, A H. J. Chem. Edue. 1988,63,972-914. [Heat ofreadion end heatof ne"traliration.1 . of formation 2. DeVore, T C.; Oailaher, T. N. J Chem Educ 1 9 8 6 , 6 3 , 7 ~ 1 3 0Meat in a high-temperature i i f f f d dU.1 3. Mahoney D. W.; Sweeney, J. A,; Davenport. D. A: Rsmette, R. W J. Chem Edue. 1981,58,73&731. [Neutralization by continuaus vaviation.1 4. Wentworth, R. A. D. J Ckem. Educ. 19SB.65.1022-1023. (Neutralieation, solution da,~ i ~ ~ t i ~ ~ . ] 5. Smith, R. L.; Rnnic*, H. R. J. Ckem. Educ. 1980.57.320. Frotanation by tihation
..... .,.,
"SI""."~",,-" 6. Smith.R. L.; Pinniek, H. R. J C h e n Educ. 1880,57,15&155.
Phemodynamicsaf ionization ofhydrogen adfate ion by titration calorimehy.1 1. Levinson, G. S.J Chem Educ. 198!2,59,337338. IYapizstion ofwater1 8. Hamlet, F! J. Chem. E d l r 1987.64, 1060. IYapavization ofnitmgen.1 9. Schwentz, R W J. Chem. Educ. 1981,64,551. [Sublimation af iodine.] 10. Burgstah1er.A.W.;Bricker, C. E. J Chom.Educ 1981,68,332333. [Sublimationof dry ice.1 11. AguilTe-Odc, k;Martinez,S. J. Chem. Educ. 19%. 63,1004. [Fusion fmm freezhingpoint depression.1 12. Chen, E.C. M.;Sjoberg, S.L. J. Chem E d w . 1980,57,458. [Reaetionenthal~l 13. R a i m , D.A.: Fung,B. M.;Christisn, S. D. J ChemEduc 19sB.65.932-933. [Enthalpieaofresrnon d ~ f r n i n ~ ~ . ~ 14. Hendemon, G. J Ckem. Educ 1#87,64,8%90. Dimerization of formic s d d l 15. McSwuley, H. 0.J Chem. Edue. 1982,59,165. IGsivanic ceilthemodynamies.l 16. Thom8s.N. C. J. Chem. Educ 1991.67.431. 17. M a u w , M . R . J Chem Educ 1991,67.:974.
Volume 70 Number 11 November 1993
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