Gary F. Riley and William H. Eberhardt
I
Thormnrlynarnics and Solubilities of SdtJ itive Ions
We would like to briefly describe a n experiment of rather simole desien which has been used successfullv in our nonmajbrs general chemistry laboratory t o illustrtke the application of t h e principle t h a t a decrease in free energy is a criterion for t h e spontaneity of a chemical reaction. T h e experiment t o be described also serves t o exoose t h e student t o a few common chemical species a n d t o dbserve some of their physical properties. The Experiment Pre-Lab Calculations The students are presented with stable listinga special set of values of free energies of formation for most of the ions and salts to be studied in the ea~riment.Thevalues are piven for a state of0.1 M rather than for the &"a1 standard state. 'Ihus;one finds small differencesbetween the values listed in Table 1and those found in published tables. To display this difference,the symbol AG* is to be used for the state of 0.1 M. .~~~~ For each possible renetion, the studentashdd
calcuiav the IG' whirh orcum when the two ionsat 11.1M react togiveandid pnduct For example, for Caz+
+ CO?
e CaCOds)
AG* = AG*r,caco3c.)- AG*t.c.z+ - AG*r,co3z= (-270) - (-133) - (-128) = -9 kcallmole Hence, the studenrs ran conclude that when thpy m u Ca" and CO?ions at 0.1 A1 rorh, thc ions should mart u, give solid CaCO, The students tabulate the results of their calculations in Table 2. In same cases, e.g., BaSOds), no data are available and no decision is possible.
Procedure Equipment needed: test tubes, graduated cylinders. Reagents needed: 0.2 M solutions of the following ions: cations (all as nitrates): Ba2+,Caz+,Mg2+,CuZ+,Pb2+,Zn2+. Anions (all as sodium salts): C03Z-, CI-, OH-, S042-. Totest the conclusions of your calculations, mix small equal volumes of the solutions of the various cations and anions (about 5 ml of eaeh solution should be sufficient for a test). If no reaction occurs eaeh ion will he at 0.1 M. Some reactions are quite slow so wait about 5 min before concluding that there is no reaction. Indicate in Table 2 whether the reaction occurs to form a precipitate. Note also what characteristics the precipitate has, e.g., whether it is gelatinous,finely divided, coarse, colored, or anything else that seems noteworthy. For those species where no data appear in Table 1, deduce limits far the value based on your experimental results. That is, if a precipitate forms, you know AG* must be negative and since you know the values for the ions, you can calculate an upper limit for the AG* for the solid.
Discussion T h e experiment works well and students enjoy this uniting of theoretical calculations with experimental results. T h e experiments can be expanded readily to include more ions and their salts b u t this either results in many more calculations (of which t h e students mow wearv) or (usuallv) introduces several more blanks in Table 1d u e t o insufficient data. One interesting variation is t o include a ligating species such a s ammonia with t h e anions and t o list t h e free energies for varisuch species a s Cu(NH&Z+ and Z ~ ( N H Z ) ~A~further +. ation would be t o then have t h e students make predictions about exchange reactions such as
+
-
+ +
Cu(0H)z 4NH8 Cu(NHa)2+ 20HCUCOB 4NH3 C U ( N H ~ ) *+~ COJZ+ Table 1. AG,', Free Energies of Formation in Solutions of 0.1 M or Solids (In kcallmole) 61Ions
-33
135 133 Mg2+-110 Cu2+ 14
-310 w2 -179 -142 -42 -75 -88
Ba2+ -
Ca"-
+
-
7 Zn2+ - 37 Pb2+
SOr -179
COs2-
OH-
-128
-39
...
-272 -270 -246 -124 -150 -175
-205
-429 w2 -281 -442 w5 -194 -612 w7
...
-199 -85 -101 -133
M a : If a number appears wima w l a w2 following it, he solid haoone wtwo moles olHPptarmlawei@?lolsaU.YwwlllhawfotaLBinmaaxumheAq~dwater.whid is -56.7 kcallmale. Th~lmsfmsolidsare identloal m standardvsluea.mevalueafaapeciesin solution have been carreled to the 0 1 Mstate wing he relstionohip A W = AGO + 2.303 RT lw(0.1)= A 4 ' - 1.37 kcallmole.
-
Table 2. Format for Reporting Student Resubs indicate in this chart the results of your predictions and experiments. For each Square write:
1) AG' far the proposed reaction, if it has been calculated. 2) Yes or No for whether a precipitate should appear. 3) The nature of the ppt: its color, gelatinous, fine, coarse, etc. Ions
CI-
SOF
Cop
OH-
