edited by JAMES 0 . SCHRECK University of Nonhern Colorado
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Greeley, CO 80639
Solution-Phase Thermodynamics A "Spontaneity"Activity Thomas H. Bindel Pomona Senior High School, 8101 West Pomona Drive, A ~ a d aCO , 80005 In our first-year chemistry program, emphasis is placed on thermodynamics. One goal of the thermodynamics unit is to determine the spontaneity of reactions, and i t was desirable to have a laboratory experiment that would verify the spontaneity concept. Unfortunately, such experiments did not appear to be reported in the literature. In developing a suitable laboratory experiment, i t was found that thermodvnamic data eiven i n hieh school and colleee chemist6 textbooks oniy allows fortesting reactions carried out i n the "dry way;" that is, with only gases and solids ( I ) . It is undesirable to have such restrictions, especially when most chemistry dealt with is solution-phase chemistry or "wet" chemistry. Fortunately, thermodynamic data is available for aqueous ions (2). follow in^ is a spontaneity experiment, baied on this data, that works extremely weli.
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Student Preparation Our approach1 to spontaneity is exclusively based on entropy (system, surroundings, and universe) and the second law of thermodynamics (3-6). Prior to the laboratory experiment, students calculate (using data i n Tables 1and 2) whether reactions represented in eqs 1-5 are expected to be spontaneous or not.' The spontaneity i s determined from the change in the entropy of the universe The change in Gibb's free energy (AGO) is included for those who desire it.
This author believes, for first-year chemistry students, the spontaneity and the "extent of reaction" concepts are most easily understood, if they are explained in terms of entropy changes (6). 'The calculations could be performed after the experiment. However, if they are done before, then they can be used to demonstrate the predictive powers of thermodynamics.
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Journal of Chemical Education
Sample Calculations for eq I. AH" = AEI~o(F~'+) + AHSO1, a ante prec plate tormeo 15 seconas alter rnwng a.16 sgn I cant q.ant.t er naa forrnea n in n 40 Seconos 'When the concentration of NaCl was 2 M and that of AIz(SO& was 213 M, still no reanion was observed.
+ AS",,,
= 299 J/K +-34
J/K
= 265 J/K
or AG" = AH" -TAP,, = -89.1 kJ - 298.15 K (0.034 kJ/K) = -79 kJ
AG" = - TASoUni, = -298.15K (0.265 kJK) = 79.0 kJ
Materials 0.5 M aaueous Nat (NaC1.0.029 dmL) 0 5 .M nqurous SO," I K ~ S O , 0.018~2 , g ml,, urA. SO,,?. 18H20,0.111pml. 0.5 M aqueous Ca'. \Cat NO., > .4H,O, O l l X p mL. 100 mg iron powder (a full spitula tip) 1-3 pieces of copper shot (3.2 mm in dial 1M aaueous Hi (1mL 12 M HC1 diluted ta 12 mL) sadwrn metal pea-sned quantity luur 13 * 100-mm test tubes. and a test tube rack Experiment Reaction 1.Aqueous Ht (1mL) is added to 100 mg iron powder in a test tube. (Caution: Aqueous hydrochloric acid is corrosive.) Reaction 2. Aqueous H+(1 mL) is added to 1-3 pieces of copper shot in a test tube. Reaction 3. Aqueous Ca2+(1 mL) is mixed with 1mL of aqueous SO4'- in a test tube. Reaction 4. Aqueous Na+ (1mL) is mixed with 1mL of aaueous SOL2-in a test tube. Observations are made for a period of five minutes. Results are presented i n Table 3. After the completion of laboratory observations, the instructor may demonstrate the spontaneity of eq 5 (7).
.
Caution:Sodium is extremely reactive in water, and spattering from the reaction vessel may occur.
3Thistranslates to a thermodynamic equilibrium constant of one for eq 4. This number also represents the reciprocal of the Ksp for sodium sulfate.Therefore,the thermodynamic Ksp of sodium sulfate is calculated to be one. In order for a Drecidtate of sodium sulfate to form.me Ion concentratfonsw have to exceeo 1 M, so that me Ion proom [ha']' [S0,27 exceeos a vaue ol one Tnls expalns wny no prectp tale ot soown sulfateformed
Discussion As expected from the calculations, reactions expressed i n eqs 1,3, and 5 are spontaneous. The reaction expressed i n eq 2 was not observed. Of course, one cannot say definitively whether a given reaction is nonspontaneous, since a long enough period of time may not have occurred i n which to observe a reaction. Therefore. the result observed for the reaction represented by eq 2 is consistent with the calculation of a nonspontaneous reaction. The calculations involving eq 4 are not easy to predict spontaneity, because the value for the entropy of the universe i s near zero. This is a desirable problem because i t introduces students to the notion that "real world" problems are not always clear cut. I t also provides for exploration of concepts of "extent of reaction" and equilibrium. Nons~ontaneousreactions ~ r o c e e d to such a small extent that itappears, to the observer, as if nothing is happening. The extent of reaction is mirrored i n the thermodynamic equilibrium constant, K,,. The constant3is related to ASomi, by eq 6. eq
-e 4 n i v ~ ~ ~ (6)
The thermodvnamic calculations in Table 2 are for solu~~-~ tions of aqueous species a t a concentration represented by unit activity. Generallv. ionic s ~ e c i e sa t a concentration of 1 M have a n activity ikss t h a i one (8).The effect of this would be to decrease the AS?S",,i,. This is not a problem for the reaction represented by eq 3, because spontaneity was observed. On the other hand. this mav be a contributine factor for the absence of any ot,served reaction from eq 4. Ofcoursc, the ~osiibilitsof a kmetic-related on)blrm cnnnot be dismissid. ~ a s t l < i is t interesting to speculate about the thermodynamic change if a hydrated salt were to orecipitate in reactions repr&entedby eqs 3 and 4. surprisingly, calculations for the reactions represented by eqs 7 and 8 do not appreciably differ from those from eqs 3 and 4 as shown in Table 2. ~
~
~~~
.,
Ca2'(aq) + ~ 0 , ~ 7 a + q )2 H20 (1)+ CaSO, .2H20(s) 2 Nai(aq) + ~ 0 ~ ~ ?+a10 q H20(l) ) + Na,SOcIOH20(s)
(7)
(8)
Literature Cited 1. Tykodi, R. J. J. Chem. Educ. 1986,63,107.
2. HondhookofChemislryandPhyshre,67th ed: Weast, R C.. Ed.;The Chemirsi Rubber Co.: Cleveland, OH, 19861987. 3. Lowe. J. P J. Cham. Educ. 1988, 65, 403. 4. Barrow G.M. J. Ckcm Educ 1988,65,122. 5. Zumdahl. S. S. Chemidry; D.C. Heath Co.: Lexingtion M A . 1986:p 683. 6. Bmansn. T J Chem Educ. 1980.67.48. 7. Hutton.A.T. J Chsm Educ. 1981.58.506. 8. Dordur,A. A. J Chem Educ. 1991,68,139.
Volume 72 Number 1 January 1995
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