Name_______________________________________
Chem 1311
Second Exam
6 October 2000
In taking this examination, you are expected to adhere to the GT academic honor code. At a
minimum this requires that you utilize only the materials supplied to you, and that you do not
give help to, or accept help from, others. Non-programmable calculators only are allowed for
this exam.
1. a) Molecular oxygen has a triplet ground state (two unpaired electrons). However, it can form two
different singlet state species (zero unpaired electrons). Use the energy level diagrams shown below to
indicate how two different species can be formed in which the total net spin for the molecule is zero.
c*
_*
c*
_*
p
p
p
p
_
c
_
c
c*
c*
s
s
c
s
s
c
b) Singlet oxygen emits photons upon conversion to the triplet form as demonstrated in class. What
color was the emitted light for the experiment done in class?
RED
2. Provide a thermochemical cycle utilizing the heat of formation of lithium peroxide, Li2O2, which given
the necessary numeric values would allow the calculation of the lattice energy of the salt. Write the
algebraic expression relating the lattice energy to the other thermodynamic quantities in the cycle.
Remember that the lattice energy is defined as the energy required to dissociate a mole of solid salt to
its gaseous constituent ions, and that the electron affinity of an atom or moleculed is defined as the
energy associated with the reaction anion S neutral + e.
2 Li(s) + O2(g)
2 Sub
2 Li+
2 IE
FHfo
Li2O2(s)
U = - ∆Hfb + 2 Sub + 2 IE -EA1 - EA2
- EA1
O2- EA2
∆Hfb = 2 Sub + 2 IE - EA1 - EA2 -U
-U
2 Li+(g) + O22-(g)
3. Silver bromide has the same structure as sodium chloride (shown on the cover sheet) and a unit cell
dimension of 5.77Å. Calculate the density of silver bromide.
4 formula units cell-1 x 187.772 g mol-1
6.022 x 10 23 formula units mol-1
d =
= 6.49 g cm -3
-8
3
(5.77 x 10 cm)
4. A large class of oxide minerals have the cubic perovskite structure shown to
the right. These compounds contain two different metal ions. Use the unit cell
to the right to determine the empirical formula, which is of the form AwBxOy. In
the drawing the shaded sphere is A, the large open circles are oxide ions and
the small open circles are the B atoms.
A1B1O3 A is fully contained in cell, B is on corners (8x1/8), O is on edge (12 x
1/4)
Name______________________________________
5. Circle the letter that corresponds to the correct response to each of the following.
Which of the following salts would be expected to be most insoluble
a) BaSO4
« Al2(SO4)3
b) Na2SO4
d) all are sulfates and should have the same solubility
Which of the following drawings best represents the antibonding π molecular orbital of the nitrite ion?
The shaded portions of the orbitals indicate that the phase (sign) is different from the unshaded portions.
The relative sizes of the orbitals reflect the relative contributions of the individual atomic orbitals to the
molecular orbital.
O
a)
N
O
b)
N
«
O
N
O
O
O
d)
O
N
O
Which of the following cations should have the least exothermic (least negative) enthalpy of hydration?
a) Mg2+
b) Fe3+
c)Li+
« Cs+
Lithium selenide (Li2Se) has a structure that can be considered to be based upon cubic closest packing
of Se2- ions with lithium occupying holes in the cubic closest packed array of anions. Which of the
following occupancies is most probable given the formula of the salt and ion radius ratio. (T stands for
tetrahedral holes, O for octahedral holes)
a) 50% O
b) 100% O
« 100% T
d) 50% T
Using the sheet that shows the molecular orbitals of nitrite ion (NO2–), indicate the nature of the lowest
energy unoccupied molecular orbital. The lowest energy orbital is #12 (upper right corner); highest
energy is #1 (lowest left corner). The view is toward nitrogen along an axis bisecting the O-N-O bond.
a) σ antibonding
« π antibonding
c) non-bonding
d) π bonding
Which of the following substances is most likely to be a high-melting solid?
a) SF2
b) ClF3
c) SiF4
« CaF2
Which of the following molecular orbitals for O=C=C=C=O is fully nonbonding?
a)
b)
«
d)
The primary reason that molecular bromine (bp 58.8b C) boils nearly 40b lower than ICl (bp 97.4b C) is
because:
a) The molecular weight of ICl is greater than that of Br2
« ICl is polar, whereas Br2 is nonpolar
c) London dispersion forces are absent for Br2 since it is a nonpolar molecule
d) ICl is an ionic compound whereas Br2 is molecular
Band structure diagrams are shown below for three substances. Which is most likely to show metallic
conductivity? The shaded area represents the fraction of the band that is filled.
a)
p
s
p
«
b)
p
d) none of these
s
s
6. Which of the manganese oxides, MnO, Mn2O3, or MnO2, is most likely to have the highest lattice
energy? Briefly (35 words or less, whole sentences only) explain the basis for your selection.
MnO2 has the most highly charged and therefore smallest cation. It will have the largest lattice energy
because U is proportional to the product of the ion charges and inversely proportional to the internuclear
separation.