JOHN J. ALEXANDER University of Cincinnati Cincinnati. OH 45221
Brz + I-
Stoichlometry of Redox Reactions
+ Hz0
-
103-
+ HC + Br-
(5)
At the same time HCN is converted to BrCN.
-
Gordon A. P a r k e r University of Toledo Toledo, OH 43606
Br2 + HCN BrCN + H+ + Br(6) Excess bromine is now destroyed by adding formic acid
Generally mole ratios of reacting species are small numbers. Occasionally, however, by careful selection of a reaction sequence one can alter these ratios to allow a small amount of reactant to produce a large amount of final product. Such a procedure is known as an amplification reaction sequence'. From the opposite point of iiew, determination o f a small amount of product allows quantitative determination of an even smaller amount of reactant. Thus, minute amounts of a chemical substance can be quantitatively determined from measurement of t h e oroduct of a series of amnlification reactions. T h e f 0 1 l o w i ~questions ~ illustrate these points and at the same time orovide oractice in the balancine of oxidation-reduction re&tions. +be problem would be &table for the analytical course or perhaps for a n honors freshmen course. It requires cognitive skill a t the applications level. Question T h e followine.. amolification seauence of reactions allows one . to determine minute amounts ot'bismuth. Bi is first precipitated ns a comolex molecule. The orecioirate is then dissolved and, through a-series of chemical changes, iodine is eventually titrated with standard sodium thiosulfate solution. 1) How many moles of thiosulfate will eventually be required for each mole of bismuth initially present? 2) How many mg of Bi (at. wt. 209) were initially present if 32.46 ml of a 0.100 M solution of sodium thiosulfate was required to titrate the iodine formed from the reaction sequence? Bismuth is first precipitated by addition of hexathiocyanatochromate(II1).
-
(1) Bi3++ [Cr(SCN)6I3- B~[CI(SCN)G]I After filtering and washing, the precipitate is transformed by adding bicarbonate ion until the p H is 8.2.
-
Bi[Cr(SCN)s]L+ HC03- + H20 (BiO)zC031 [Cr(SCN)6I3- Ht
+
+
(2)
Following filtration, iodine is added to the filtrate and the
-
cyanogen iodide is also produced. S0.2-
-
Br- +Con + H+
(7)
A large excess of iodide ion is now added.
T h e iodine formed is titrated with standard sodium thiosulfate solution. Iz + 5.2032- -1-
+ S4062-
(10)
Acceptable Solution Coefficientsof the balanced equations are
----
(1) 1,1- 1
(2) 2,1,2 1,2,5 (3) 1,24,24 6,6,42,48,1 (4) 1, 1 , l - 1 , l (5) 3,1,3 1,6,6 (6) 1,l 1, 1 , l (7) 1,1- 2,1,2 (8) 1,5,6 3,3 (9) 1,2,l 1,1 , l (10) 1,2 2 , l Part 1. Moles of thiosulfate equivalent to bismuth: From reactions (I), (2), (3) 1mole Bi[Cr(SCN)s] 2 mole [Cr(SCN)& 1mole Bi X 1 mole Bi 2 mole Bi[Cr(SCN)c] 42 mole I1mole lCr(SCNM-. 1mole Bi = 1mole Bi[Cr(SCN)s]= 1mole [Cr(SCN)6J3= 42 male IFrom reactions (3), (4)
'
1 mole I6 mole ICN = 6 mole I1 mole Bi 1mole ICN Six moles of I- are consumed in reaction (4) for each mole Bi. Fortytwo minus six or thirty-six mole of I-, from reaction (31,is available for reaction (5). From reactions (5), (S), (10) 1mole 1033 mole 12 2 mole 5 ~ 0 3 ~ 36 mole I- X 1mole I1mole 1031 mole Iz 36 mole I- = 36 mole 103- = 108 mole Iz = 216 mole Sz0z21mole Bi X
bexatbiocyanatochromate(1II) ion oxidized to sulfate ion; [Cr(SCN)d3- + Iz+ HzO
BIZ + HCOOH
+ ICN + I- + H+ + Cr3+ (3)
T h e reaction mixture is acidified to p H 2.5. ICN+I1-+Hf-Iz+HCN (4) Iodine present in the reaction mixture is now removed by extraction into chloroform. Bromine water is added to the separated aqueous phase and oxidizes iodine present to iodate ion. 1 Belcher, R., Liamangrath, S., and Townahend, A,, Talonto, 24, 590 (1977).
From reactions (3), (4) 6 mole ICN 1 mole HCN = mole HCN 1 mole Bi X 1mole Bi 1mole ICN Six moles of HCN are formed in reaction (4) for each mole of Bi. From reactions (6),(9), (10) 1mole BrCN l mole Iz 2 mole S20s26 mole HCN X 1 mole HCN 1mole BrCN 1 mole I, 6 mole HCN = 6 mole BrCN = 6 mole Iz = 12 mole S ~ 0 3 ~ Volume 57, Number 10, October 1980 1 721
Twelve additional moles of SsO? a1.e needed to react with the 12 eenerated from HCN. reaction (4). C~nrlucwnOne m& Ri r e q k s 21fi + 12or 228 n>oleolS,0~2' to titratr the iudine liberated lnnn the rearrim zequmre. 1mole Bi = 228 mole S2OS2-
Part 2. Mg of Bi:
mg Bi = 32.46 ml X
0.100 mmole 5 ~ 0 3 ~ - 1mmole Bi ml 228 mmole S2OZ2209 mg Bi -X mmole Bi
CO Stretching in Metal Carbonyls Wai-Kee L i The Chinese University of Hong Kong Shatin, N. T., Hong Kong This question is suitable for a senior undergraduate or beginning graduate course in physical or inorganic chemistry which includes elementary group theory in the syllabus. T h e question tests students' abilities to apply the group theory techniques a s well as to make spectral assignments and structural determination by qualitative arguments. Fnrthermore, the students' understanding of bonding in metal complexes is also examined. Question 1) When two of the carbonyl groups of F e ( c 0 ) ~ (trigonal bipyramidal structure, D3h symmetry) are substituted by two L ligands, there are three possible structures for the product LzFe(C0)a: both L's are in axial positions (D~J,), both L's in equatorial positions (Czu),and one L in axial and one L in equatorial positions (C,). For each of the three possibilities, determine the symmetry and spectral activities of the CO stretching motions. 2) T h e following CO stretching bands are found in the IR spectra of (&P)zFe(C0)3and (MeNC)zFe(CO)s (these two compounds are expected to have the same symmetry), (&P)2Fe(C0)3 1887 cm-', (MeNC)zFe(CO)a: 2009 cm-' (0.5),1927cm-' (101, with the relative intensities heing given in brackets In view of these data, which of the three structures rnenrioned in part 1is most likely? Make assignments for the observed bands. Comment on why there are two observed bands in (MeNChFe(C0)3 and only one in (&P)2Fe(C0)3. 3) Given the following C-N stretch frequencies,
GO ~tret~hing motions fw the three possible Wctures of LnFe(C0h.
tensity since the non-linearity of the Me-N-C group slightly perturbs the D3h symmetry of the MeNC-Fe(CO)&NMe grouping. Cotton and Parish' also suggest another reason. 3) Resonance structures for RNC are R-fi = c: R-&=C: (1) (11) . . When the carbon atum is honded to a metal such that negative charge is drained hum C, there is a tendency for the rontrihution of rll to incrrasr, t h u i raising the NC hond order. On the other hand. hark donation from metaidr orbitals tends to lower the NC bond order. ~~~, in irppcnltim rc, rhc atbrementicmtd (inductive)rftrct.'l'he dau given suegrit thar the indurtiveeff~t dominatesin tMe,CNClFelCO~,nnd Couun and I'ari\h c~ back dunation prevmL~in (hle~.SiNC~t'c~C'O,~. a full explanation of why this is the case.'
-
~~
~
'Cotton, F. A. and Parish, R. V., J. Chem. Soc., 1440 (1960).
Free MeGN-C: 2145 cm-llA)
Note that frequency A (free) is about 40 cm-' lower than B (coordinated), while frequency C (free) is about 60 cm-' higher than D (coordinated). Comment.
Erratum
I n the August 1979 installment of "Exam Question ExAcceptable Solution change" the matrix in the solution to the question on Hy1) DZ,,: A,,(R), E Y I R ~ )C: 2 U : z ~ 1 ( ~BI(IRIR): ~ m ) , C.: ~ A Y I R ~ ) , bridization and Bond Angle should read A"(IR/R).IR denotes infrared active vibrations; R denotes Raman active vibrations. These motions are illustrated in the figure. 2) Since there is only one intense CO stretch hand in each of the spectra, theDsh structure is heavily favored.In (MesNC)nFe(CO)s, the totally symmetric A1' mode may have gained a little (IR) in-
!;)
722 1 Journal of Chemical Education
