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Inorganic Chemistry 411/511
Midterm Exam # 2
60 minutes. Show all work, partial credit will be given. Supplemental information is provided on
the last page.
1.
(a) [Co(Br)2(NO2)2(NH3)2]1- is an anionic complex with octahedral coordination around
the Co(III) center. Draw two different isomers and give their point group symmetries
(ignore ligand shapes for symmetry determination). [12 pts]
Any two of the following are possible:
(b) Do symmetry rules require that either of the isomers you sketched in part (a) be nonpolar? Explain briefly. [6 pts]
Non-polar molecules have either cubic point groups, inversion centers, or belong
to D groups. All others have a net dipole (large or small). Thus, symmetry
requires only the D2h structure to be non-polar.
2.
The orthophosphate anion (PO43-) exists only in highly basic aqueous solutions.
(a) Write a balanced chemical reaction for the formation of P2O74- (aq). [7 pts]
3−
4−
−
2 PO4 (aq) + H2O (l) → P2O7 (aq) + 2OH (aq)
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(b) Identify each reactant in your answer to part (a) as being an oxidant, a reductant, an
acid or a base in the reaction. Some of these terms may not be appropriate. [5 pts]
PO4
3−
is a base, H2O is an acid; neither is an oxidant or reductant
(c) Draw the dimeric species produced in part (a), showing clearly the geometry and
ligands around phosphorus. [8 pts]
Note the approximate Td coordination around P, the apical O ligands have a
partial negative charge (on each P two are often drawn with single bonds and a
negative charge, and one with a double bond, all are equivalent due to resonance).
There is a P-O-P bridge.
(d) Which is a stronger acid, phosphoric acid or boric acid? Explain. [9 pts]
Phosphoric acid, H3PO4 has one oxo ligand and pKa ≈3 according to Pauling’s
rules. Boric acid, H3BO3 has no oxo ligands and pKa ≈ 8. H3PO4 is thus a much
stronger acid.
3.
(a) Explain why the oxidation
potential for Cu(s) is pH independent
below pH=4. [6 pts]
Cu(s) → Cu2+ (aq) + 2eThe reaction neither consumes nor
forms H+ or OH-
(b) Is the reaction associated with the vertical line near pH=4 an acid/base reaction, a
redox reaction, both, or neither? [5 pts]
It’s an acid/base reaction. (all vertical lines are acid/base reactions)
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(c) Calculate the value of the negative slope of the line indicating the Cu2O / Cu(OH)2
equilibrium using the Nernst equation: E = E0 – (0.059 V/n) log Q
[10 pts]
2 OH- (aq) + Cu2O (s) + H2O (l) → 2 Cu(OH)2 (s) + 2eMagnitude of the slope will be (0.059V/2) log (OH-)-2 = 0.059 V / pH unit
4.
Will peroxide spontaneously disproportionate in aqueous
acid or aqueous base? Explain, using the Frost diagram
provided. [8 pts]
Since H2O2 and HO2- are both above the tie lines
of O2 and water or hydroxide, the peroxy species will
disproportionate in either acidic or basic solution.
5.
Sketch the NH2F molecule and draw or explain the location of all symmetry elements
besides identity. [8 pts]
The only symmetry elements are E (identity) and a mirror
plane (σ) that contains the N-F bond axis and bisects the
HNH bond angle. There is no C2 axis!
Circle the one best answer for each. [4 pts each]
6.
What do Pauling’s rules predict for the pKa of the anionic species HAsO42- ?
(a) 0
(b) - 2
(c) 2
(d) 5
(e) 8
(f) 10
(g) 13
7.
Which of the following is the strongest acid in aqueous solution?
(a) Cr2+
(b) Fe2+
(c) Ni2+
(d) Na+
(e) Ca2+
(f) all of these have similar acidities in water due to solvent leveling
8.
Which of the following has an inversion center?
(a) N2O
(b) C2H2
(c) NO
(d) BF3
9.
(e) SO42- (f) fac-PF3Cl3
SiF2Cl2 shows only a single 19F NMR peak at room temperature. What do you expect to
occur at low temperature?
(a) Only a single peak because the two F ligands are equivalent by symmetry.
(b) Two equal intensity singlet peaks emerge as the fluxional behavior slows.
(c) The single peak broadens and a triplet peak emerge.
(d) The single peak broadens and two triplet peaks emerge with equal intensities.
(e) Two singles peaks appear from cis and trans isomers in the tetrahedral structure.
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(f) One doublet peak arise due to F-F coupling in the spectrum.