I The Synthesis and Reactions of a Cobalt - J o h n J. Alexander

and J o h n G. Dorsey University of Cincinnati Cincinnati, Ohio 45221

I A project for freshman laboratory

T h e design of suitable experiments for t h e general chem- istry l ahor .&y presents a challenging prnblem. Appn~priate activities a t the freshman level ought t o introduce s tudents to needed lat~oratory techniques and hasic chemical principles. At the same time the experimenrs should capture students' interest. An important factor in maintaining this interest i s t h e opportunity for successful completion of t h e laboratory work. Students t end to become rather discouraged if their efforts seem t o produce few results.

Added to these reauirements in manv institutions are other pressures resulting from the large sizes nf freshman laboratory sections and the artenrlant loeistical vroblems. The work must f i t conveniently into three-hour modules. Chemicals must not be inordinately expensive. Required equipment mus t b e readily available. T h e principles of t h e experiment mus t be grasped by t h e teaching assistantships who staff t h e labora- tories and have the most direct contact with students. Finally, t h e results should be amenable t o "objective" grading pref- erably involving unknowns andlor product yields to he handed in. Results of this kind encourage useful discussions among s tudents b u t prevent excessive cooperation in writing labo- ratory reports.

With so manv criteria to be satisfied i t is little wonder t h a t large numbers of freshman experiments d o n o t also offer nossibilities for interestine discussions with briehter students, ?or extending t h e work inGolved and for conveying some of the excitement of a developing research problem ( 1 ) .

Description of t h e Experiment and Student Results

At the University of Cincinnati we have had considerable success with a project-type experiment lasting far three laboratory periods during the first quarter of the freshman laboratory.

During the first week students prepare a sample of [Co(NH&- COd(NOd.0.6 H.0 (21 (uide infral. ~ ~ . . , ,. ~ ~ ' . ~ . . .

I'reparation of the reIraamin? rarbonato mmplex from 1Og of Co(NO,<v.ti HzO rrsulted in an awmge yirld of 3.5 I: I4Vh4 from 510 freshmen in the Aurmnn of 19X. The literature repunsa yeld of 7W from n rlmilar prrpnrntwe pr,rrcdurr 12,. I t is likely that our student yirlds n d d haw been increased hy wurking on o larger ~cnle (say, 15 X I and by adding portimr of (I\'H4),CO? to the mother liquor from the fint rn,p tuubtnin a sewnd m,p 111 r r y s u k . Althw~gh the pndurt can be pur~fied by rwrystall:rarron 121, it is ruff&n~ly pure for synthrt~c purposrs. When we askrd some students to actrmpt re- crvsrnllizntim, a significant fraction could not Atain a crystalline pnduct on dirsolurmn in hot water, iiltration, nrJing, and addition of alcohol.

I)urr~g the serond lnbwator\ perid rtudcntc wen givrn unknuwnd and naked to allow there lo rrnet with J g portions of ICdNHd,- CO~l~h'Ozl.0.5 H70. The unknown^ all contarn arid which induces hydhysisof the &rhonato ligand. The presence of anions in solution will lead to anation on heating, the net reactions being

Presented, in part, at the 173rd National Meeting of the American Chemical Societv. New Orleans. March. 1967.

Because the second synthesis and spectral measurements will not require a full six hours of working time, we have found it advantageous to dovetail this experiment with others.

%The cis-[Co(NH3)r(H20)CI]C12 isolated from this procedure is sometimescontaminated with small quantitiesof trnn~-[Co(NH3)~- ClzlCl and [Co(NH3)&1]2. This does not affect the identification from the spectrum. However, purification may be easily effected by con- verting the chloride to the sulfate. See Reference (3).

Spectra 01 Co Complexes'

Complex A d n m l Color

cisCo(~H~).(H~0)Co~+ 510.363 redYiolet cisCo(NH3),(NOd2+ 506, 358. 295(sh) redviolet C ~ S C O ( N H ~ ) ~ ~ H ~ O I ~ ~ + 502, 355 red-violet Co(NHd4NO&' 440 yellow transCo(NH3)&lr2+ 490 (sh) yellowqreen

. ~ ~ a s u r e d in aqueous solution.

(X = NOS NOz, Br)

[Co(NH3)&03](N03) + 2HaOf + 3 C 1 - A

cis-[Co(NH3)a(HzO)Cl]Clz + CO? + NOs- + 2HzO (2) red-violet

If the reaction mixture is immediately cooled in an ice bath without the heating step, a diaquo product can be isolated.

[Co(NH3)&03](NO3) + 2H30+ + 3X- - ~is-[Co(NH3)~(HzO)z]X3 + CO1 + NOa- + Hz0 (3)

(X = NO3, CI, Br)

Average student yields for the products are as follows: cis-[Co- (NH~)~(H~O)CI]CIZ 64%; [Co(NH3)~(N02)2](N0z) 59%; trans- ICo(NH3)dBrzJBr 61% (Lit. 80% (3)).

Our students did not prepare the dinitrato complex (Lit. 33% yield (3)) because its spectrum was indistinguishable from that of [CO(NH~)~(H~O)CIIC~Z in the region accessible on Spectronic 20 in- struments. If more sophisticated spectrophotameters were available, the compound could be included. The diaquo visible spectrum is also indistinguishable from that for the dinitrato compound. However, infrared measurements could distinguish these.

A few students who were given 48% HBr as an unknown obtained a brown product containing COO instead of the anticipated green trans-[Co(NHa)bBrz]Br. We are uncertain as to the reason for this behavior unless it could have resulted from heating to too high a temoerature.

~ i r i n e the remainder of the lahoratorv time (after drvine the ~, ~ ~ .~ . ,. ~~~~

complex in air fur at least 15 minl students were asked to make up solutions of the products they prepared and tu mrasurs the G l r l e spectrum using Spectrunic 20's. The pmduc~5 wuld thrn he identified by comparison of the measured spectrum with posted spectra of the complexes.' A solution of proper concentration can be obtained by weighing precisely around 0.5 g of product and using a 250-ml volu- metric flask. ICdNH~IaBnlBr is not very soluble and onlv about 0.2 e will dissolv; in 250ml &water. "

Spectral paramrtcrs of products mcaiured on a Cary 11 arc re- pndutnl in the table. 'The apema reveal that c ~ . ~ - I O ~ ( ~ H ~ , l ~ ~ N 0 3 1 ~ ~ ~ end c i r - IC~) (NH~l~l H20~Clli* are dirtinguishnhle mly in the region below 370 nm. Thus, unless either an instrument having a range in this region or an infrared spectrophotometer is available, only one of these complexes is suitable for student preparat i~n.~

In our laboratories some 86% of freshman students correctly identified their complexes from the visible speetnun. After completing identification, students were asked to write a balanced chemical equation descrihing the reaction of [Co(NH3)&03]N03 with a salt containing the appropriate anion in acid solution. They were given the information that carbonate ion reacts with acid to afford carbon dioxide and water. This was a particularly challenging aspect of the experiment for many students who were accustomed to balancing

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chemical equations as a formal exercise rather than using them to deserihe actual reactions.

From the balanced equation students were asked to calculate the yield. Although mast were able to do this correctly, it came as acon- siderable surprise to many that a yield of considerably less than 1W was obtained and that this was to be expected in actual chemical preparations.

Finally, students located the positions of peak maxima and calcu- lated molar absorptivity. The values of r varied considerably and were usually not particularly close to literature values owing presumably to the fact that no recrystallization was carried out and to inadequa- cies in technique in using volumetric flasks.

Procedure Preparation of [Co(NH&C03] N03.0.5H20

About 10 g of Co(N03).6H20 is weighed by difference into a 50-ml beaker and dissolved in 10 ml distilled water. This solution is poured into a 250-ml Erlenmeyer flask containing 15 g (NHdzC03 dissolved in 20 rnl of distilled water to which has been added 25 ml conc. NHs. After all the C O ( N O ~ ) ~ . ~ H ~ O solution is rinsed into the ~r lenmeyer the flask is swirled gently to mix the reactants.

The solution is then cooled to 5" in an ice bath while the student obtains 10 ml30% H2OZ3 (CAUTION: Keep H202 off skin and eyes; it is a powerful oxidizing agent and can cause painful burns.) The hydrogen peroxide is added dropwise to the Erlenmeyer flask with swirling. The addition should he carried out sufficiently slowly that the solution temperature does not rise above 10". (Alternatively, air could he bubbled through thesolution for two hours (2). Oxygen has also heen cmphyed m similar rcnctimr 11)).

After dl tlw H,O> hsr bwn added, the tlnrk is heated to 5040" for twent\, minutes wth t,co&iunnl swding. r\lwrnatirrlg the Flmm hath may be employed.

The flask is then cooled in ice to complete precipitation of the

water and methanol. We have found it best to isolate the product during the first working

period since a week's standing of the reaction mixture sometimes vields red ICO(NHI)ICO~I(NO~. Many students obtain a second crop of crystalsof the t&aa&e after a week, however.

Dr. Glen Direen of the University of Wisconsin. Madison, has suggested the use of 10% Hz02 to decrease the likelihood of peroxide hurns.

Reaction of [CC(NH~)~CO~](NO~) with Unknown

Students are prvvid~d with sut'rhent unknown t < r react with :l y of thc rarhonatocomplex. Suitable unknowns arc. 10 ml conr. HCI; ISml 1 HBr: 10ml I:] e m r . HSO,: H:O: 10ml 1:1 r u n r . H N O : H z O and I g N ~ N O ~ in separate vials.-~he 3-g sample of the carbonato complex is suspended in 15 ml distilled water and the liquid unknown added a little at o time followed by the solid, if any. The solution is maintained at 55-65' for half an hour using a burner. At the end of this time, the mixture is m l e d in an ice hath and the product collected on a Biichner funnel and washed with cold water and methanol.

Student Discussion and Extension of Work Several features can serve to stimulate discussion and suggestions

for further work with bright students. Far example, some possibilities include the purpose of using ammonium carbonate in the synthesis as opposed to other carbonates (4). the mechanism of CO1 loss on hydrolysis (4) and the mechanism of anation (5 ,6) . Extension of the work could involve elemental analyses (7). synthesis of related products (9, 20, l l ) , or infrared spectra af products (2'413).

We are currently pursuingsome of these problems in the freshman laboratory at the University of Cincinnati and will report our results in future publications in this Journal.

Literature Cited (11 One experiment dealing unthasimiiarsystem which ineorporatesaome of these features

isdoscribed in0lsen.G. R., J. CHEM. EDUC.,44508(19691. (2) The preparation ia a slight modification of that described by Schlessinger. G., lnorp

Synfh.. 6,173 (19601. (3) Sehlesringer, G.."lnorgenic Laboratory Preparations." Chemical Publishing C a , New

York. 1962. (4) Dasgupte,T.S.and Harria,G. M . , J Amer Chrm. Soc.. 91,32W (19691. (5) (a) Chang, F. C.,end Wondlandf W. W., Thermmhimicc Aclo, 3.59 119711. IblSee

also Ref. (31. (8) Lsngf",d. C. H. and Gray, H. 8. l'Ligsnd Svhrtitution Prcrernes.'' W. A. Benjamin,

New York, 1965. (71 (4 H U E ~ ~ , R. G.. Endiwft, J. F. Hoffman, M. 2. and House, D. R.. J. CHEM. EDUC.,

46. 440(19691. (bl Wilson, L. R., J. CHEM. EDUC..46.U7 11969). (81 Werner. A., 2. Anorg. Chm. 43,338 119051. (9) Mellow,J. W.."ACompreh~nsivoTrestiseon InorganicandThmretical Chemistry,"

Vol. 14, Lungmans, Green and Co., London, 1935, p. 688 (10) (a1 Jorgcmen, S. M., 2. Anurg Chem., 14,415 (18971. lbl Palmer, W. G., "Experimental

Inarganie Chemislry,nCambridgo University Press. Cambridge. 1954. p. 546 (11) Jorgens~n, S. M.L Anorg. Chrm.. 16.184 118981. (12) Nakamoto, K.,'lnfrared Spectra of Inorganicand CwrdinationCompounds", Wilcy

Interncience. New Yurk, 1910.pp. 98. 16912 (13) See, for example, Alexander, J. J. and Steffd, M, d.. "Chemistry in the Laborstary,"

Harcourt B m e Jovanovieh. New York, 1976. Experiment 8.

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