1. J. H m a n and Assa Lifshik
The Hebrew University 01Jerusalem Israel
I
The Pre~arationand Kinetics of h u a h n of Bromopentaaquothromium(III)
1
A new multipurpose experiment
O n e of the n ~ h j r c t srnrrly trr:~trdproperly in the stwl~mt1:tborntory is drccriptivc rhemistry. \I:my ~ ~ n i v r wirs i t inrluclr in their grnrr:d or inorpmic Inhorntory roursrs n wrim of boring rookbook rsprrimrnts nimcd :it showing t h r stuilrnt ?me of the proprrtirs of t h r r l r m r n t ~ . Othrr nnivrrsitics, srnrrhing for more time fnr nclvnnml rsperimrnt?;, h n w droppnl the formrr from their curriculum without tilling in the wp. We hnvc rrccntly inrlodrd snmr new multipnrpow fourth trimnter undrr~rndunte esprriments in I inorg:u~irli~bnrntorycourse, with :I thrrcfoltl aim. The stuclrnt follo\\.s some of the drwriptivc rhrmistry of nn rlrmrnt whilr rnrrying out nn ndvnnrnl rsprrimrnt, tlrm:u~cling:in nrrurntr rrcult, nntl nt the snmc time lrwns somr importnnt trchniqurs. T h r esprrimrnt prrsrntnl hcrr rovrrs pnrt of tlw drsrripti\.e and prcpnrntivr rhrmistry of rhromium, n typirnl trnnsition m r t : ~ lrlemrnt. I:our r h m m i ~ ~ compounds m in thrir thrrr most stablc osidntinn st:~tm,romplrs fornlntion, nnd disx~rintio~t nre rncuuntrrnl by t h r studrnt., while c n r r y i ~ ~out g t h r w dilTrrrnt osid:~tior~-rnluetio~~ rrnction* : ~ n d kinrtic nw:~.mrcmrnts. At t h r snme time vnlunblr rsprrirncc is g:1inn1 in the tcchniqurs of using n donr.; rrclurtor, sl)ertrol)l~otomctrr,nnd \vorkin.g in nn inert ntniosphrrr. T h r rsprrimrnt is bnsed in pnrt on the kinrtic work of Guthric : ~ n dKingt on t h r nquntion of bmmopentanquochromium(l1 I) ion in ncicl solution. (1) ICr(llrO).Brll+ + 11~0 ICr(llrOh]'+ + Br-
-
The renction is nciddepcndeat nnd hns n rate lnw'
which is pseudo first-nrrlcr a t conntnnt hydrogen ion conrrntrntion. At [ H + ] = 1.00 .11 nnd n tmiprr:~ture r is k = 1.40 X of (iO°C the pseudo f i ~ t a r d e constnnt 1 w 4SW-I. T h r stuclrnt is given n solution of CrO, (colored redI (;I.TIIIIIE.
F. A,.
.\TI> Iilsc.
1.:
I,.. Inorg.
Chma., 3. !llli
brown) in n suitnblc concentrntion of prrrhlorir ncid, which he rrducrs to hrsnnquochmmium(Il1) ion (colored dirty blur), using hydrogen perositlr (renction (2)). 2C& 3HIO. + 6H 2Cra+ 30, 61120 (2)
+
+
-
+
+
This solution is then p a s 4 through :I Jonrs r d u r t o r strnight intn :I s t i r r ~ lmixture of prrchlorir arid and liquid bromine under nitrogen. The hrsn:~qunrhromium(lI1) is rnlucnl on thc rrdurtor to hrsanquorhromium(I1) (rolornl light sky blur), renrtion (3,\vhirh in turn is oxiclizrd to bronmprntnnquochromium(1ll) (colored preen), renction (4), by the bromine.= (3) 2Cra+ Zn(amalmm) 2Cr'+ + Zn'* 2[Cr(IlKIM'+ + Dm- 2[Cr(llt0)J3rl~+ + 21L0 (4) (.;) [Cr(HrO)anrlS+ K O [Cr(A,OhlB+ Br-
+
+
-
+
T h e aquntion (reaction ( 5 ) ) of the bromopentanquorhmmium(1II) is then follo\vnl spcctmphotomrtric:~lly :it fi:O mpa nnd the psrudo lirst-ortlrr rntr conctnnt a t CWC is m l c u l n t ~ l . T h r esprrimrnt r:m h r completed by a pnir of ztuclents within s four hour Inborntory period. The Experiment Jlolrrinla and Apparatus. 0.2 Jf rolulion of CrOl in 1.6 JI IIC'IO,, O.i.11 aolulion nf Ildh in L J1 IICI04,prcpurificd n i t r n ~ e n , bromine, n Jmw redwtur rnnnrrtccl 10 mnirnl fln4 with r i d e arm (Fig. I), n s n t e r lmth therna~stnl.edat ,iO0C, and n sir* truphotrmcter. I'repnrnltnn qf I l r o n ~ o p m l n ~ u o c h r o n ~ i ~ ~ m lPlnw l l I ) . 10 ml of i h r Cd). r w l d r m i n n ItWn~llwnkcr mrl d d I l l ml of t h e HO: solt~tions i l h mrc. I( thc re-ul1111~~olutioni.i ~reen,nn intprmcdiste pen~xorhromium mmlxwd', has been formecl, which m n tie demmpwerl by hen tin^ the ~ k l i o nfor n few minutes till it n=wmesthe dirty I~htc mlor of h e x n q u o r h m mitun(lll). llentinp will nlso dmcvmpse most. uf the rr-s of ]I& and the r e t r i l l hc tnkm carp of Ity !.he Jonn rrdacbr. 1'wi 1 0 ml of 1 .ll 11('104 thnmph the d u c t o r nnd diwnrd the el~wnt. I'lnm 50 ml 1 JI IICIO~ i n the c o n i d R w k , a mwnrtic s t i r r i n ~rod, and about 5 ml hn,mine(tnke cnre!),and quickly
Volume 47, Number 3, March 1970 / 231
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