ediled by
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GEORGE L. GILBERT -n miversity Granville. Ohio 43023
Oxidation and Chemical Environment Submitted by: Checked by:
Leonard C. Grotz University of Wisconsin C e n t e r Waukesha, Wisconsin 53186 Kenneth H. Lothrop Marshfield High School Marshfield, Massachusetts
Materials Two 800-ml beakers, 2 stirring rods, 400 ml of 0.2 M FeCln, 400 ml of 0.3 M NaI, 25 ml of 6 M NaOH, 25 ml of 6 M H2SO.t. 16 g of NH4HF2,and 150 ml of 12 M HCI. Procedure Place 200 ml of orange colored 0.2 M FeC13 in a 800-ml beaker. Add 200 ml of 0.3 M NaI and stir. The color turns immediately to a deep orange-brown. Add 25 ml of 6 M NaOH, stir vigorously, and let stand for 10-15 min. A brown precipitate partially settles out leaving behind an almost colorless solution. Add 25 ml of 6 M H2S04 and again stir vieorouslv. In about one minute the svstem Eoes hack to a d e e ~ ,,rangt~-l,rnw~~ colored solurion. T o a second 2CnI.ml portion M FeCI, ina800-ml heakrr,add I(ig of NHIHFl and of th(~Il.2 stir until dimolved. 'l'hr omnp6. color turns to a pale yellow. Add 3 R I ml of O.:{ M NaI. Nu visihlc reaction (rcurs except for the precipitation of a small amount of excess white salts. Add 150 ml of 12 M HCI and stir. The system changes to a deep orange-brown colored solution. CAUTION: This part of the demonstration should be performed only with adequate ventilation because of the toxic
nature of hydrogen fluoride. Discussion The ahilitv to oxidize or reduce a snecies is denendent not unlg ulnm thr nature of the spwies itself, but also upon the chrmical environment uf the snecies. Tht. chrmical environment of an ion is composed or the ions and molecules with which it is coordinated. T h e effects of coordinated ions and molecules on an oxidation-reduction reaction are readily illustrated by the reaction between the iron(II1) ion and the iodide ion. When solutions of the iron(II1) ion and the iodide ion are mixed, the iodide ion reduces the iron(II1) ion to the iron(I1) ion. In the process, the iodide ion is oxidized to iodine which forms the orange-brown triiodide ion with excess iodide ion. 2FeW + 3 1 2Fe2++ 13When the system is made basic, the reaction is reversed by the formation of the highly insoluble iron(II1) hydroxide. ~
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T h ~ eaddition of HzS04 removes the hydroxide ion and reforms the iron(I1) and triiodide ions.
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ZFe(0H)r + 31- + 6H+ 2FeZ++ 11- + 6H20 The addition of NHlHFz to the solution of iron(II1) ions forms the hexafluoroferrate(II1)ion which cannot be reduced by the 618 1 Journal of Chemical Education
iodide ion. The addition of a large amount of the strong acid, HCl, frees the iron(II1) ion by formation of the weak acid HF. FeFe" + 6H+ Fe3++ 6HF After reforming the hydrated iron(II1) ion, the iron(II1) ion oxidizes the iodide ion to the orange-brown triiodide ion.
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Simultaneous Equilibria Involving Insoluble Salts Submitted by:
Checked by:
J a c o b B. Baumann T h e Pennsylvania S t a t e University Schuylkill Campus Schuylkill Haven, 17972 Paul T. Ruda Cleveland Hill High School Cheektowaga, New York
Preparation
Place 200 ml of 0.1 M AgN03 in a 1-1 Erlenmeyer flask. In separate separatory funnels, place 200 ml of 0.1 M KzCrO4 and 300 ml of 0.1 M NaC1. Demonstration Stir the AgNOa solution with a magnetic stirrer and slowly add the K2Cr04solution from the separatory funnel. When the addition is completed, stop the stirring and allow the brick-red precipitate of Ag2Cr04 to settle. Observe the appearance of the supernatant liquid. Begin stirring again and slowly add the NaCl solution from the other separatory funnel until the Ag2Cr04 is transformed to AgCI. Again, stop the stirring when the addition is complete and allow the precipitate to settle. The color changes which occur help to confirm the chemical changes: the disappearance of the hrick-red precipitate, the formation of AgCl as a white solid, and the appearance of the yellow color of Cr042- in the supernatant liquid. Remarks The demonstration may he followed by a calculation of the Ag+ concentration in equilibrium with undissolved AgzCr04 and in equilibrium with undissolved AgCI. The former value is 2.6 X 10-4 M , based upon KaP for AgzCr04 = 9 X while the latter value is 1.3 X 1 0 - a , based upon K, for AgCl = 1.6 X 10-'0. These values, applied to the equilibria Ag2CrOqs112 Agt
+ CrOd2-
and AgC
+ C1-
1AgCIls1
are consistent with the shift from Ag2CrOato AgCI, as demonstrated. This demonstration is derived from, and involves the same reactions as. the Mohr titration of chloride ion with silver nitrate solution using potassium chromate HY an indicator. Fnrademonstration which involws the transformation of Ag+ to a succession of precipitates and complex ions and finally through reduction to silver metal, see J. R. Schwenck [J. CHEM. EDUC., 36,45 (1959)l.
