Download THE UNIVERSITY OF LETHBRIDGE DEPARTMENT OF CHEMISTRY

THE UNIVERSITY OF LETHBRIDGE
DEPARTMENT OF CHEMISTRY
CHEMISTRY 2000
PRACTICE TERM TEST #1 ANSWER GUIDE
Winter 2005
Explanations in red text are not necessarily required for full marks on the question.
Instructor: Dr. R.T. Boeré
Time: 50 minutes
Value: 50 points
No. of Pages = 4.
1.
[10]
a) What is the coordination number of the Ti atom in [Ti(NH3)4(OH)2]Cl?
Six. There are four N and two O atoms directly bound to Ti
b) Write out the electron configuration of neutral Ti
1s22s22p63s23p64s23d2
c) What is the dn configuration of the Ti atom in [Ti(NH3)4(OH)2]Cl? Justify your answer.
Ti is in 3+ oxidation state, i.e. [Ar]3d1, so this is a d1 configuration
d) Write an acceptable name for the complex salt [Ti(NH3)4(OH)2]Cl.
Dihydroxotetramminetitanium(III) chloride List cation before anion. List ligand names before metal
Use “o” name for anionic ligands (hydroxo), ammine for coordinated NH3, roman number for oxid. st.
e) Predict the number of unpaired electrons in the complex ion in [Ti(NH3)4(OH)2]Cl. Justify your answer
with a d-orbital splitting diagram
There should be one unpaired electron. The splitting diagram looks like:
↑
The geometry is octahedral (the only one we have considered so far.) The sole u.p.e. must therefore fill in
lower orbital with “up” spin. All other electrons are core electrons, which are always paired.
2.
[4]
What inter-molecular forces must be overcome to:
a) Melt ice
Hydrogen bonds are the dominant IMF in the structure of ice; these must be partially broken to allow
liquid water to form. Water is though to have a structure near the m.p. of “flickering clusters” of the IceI structure. Additional IMF that contribute to the structure are DP-DP and dispersion forces.
b) Melt solid I2
I2 is a non-polar molecule. Thus only Induced-DP – Induced-DP forces operate. With the very large,
easilly polarizable iodine molecule, these dispersion forces are sufficiently strong to cause it to be a
solid.
c) Remove the water of hydration from MnCl2.4H2O
Ion-DP forces dominate, because the water in this formula is most likely coordinated directly to the Mn,
and is not merely water of crystallization as the “bottle label” formula would indicate. Dispersion forces
are of much less importance.
The true formula for this compound is likely to be either [Mn(OH)4]Cl2 (ionic) or [Mn(OH)4Cl2]
(molecular)
d) Convert liquid NH3 to gaseous NH3.
Hydrogen bonding dominates. General DP-DP and dispersion forces also contribute.
1
3.
Cuprite is an oxide of copper with the following cubic structure:
[6]
Oxide ions are at the cube corners and in the cube center. The
central oxide ion is tetrahedrally surrounded by four copper ions.
a) What is the formula of cuprite? Justify your answer.
8 corner O2- x 1/8th = 1
1 central O2– = 1
4 internal Cu ions.
Therefore the formula is Cu2O,
b) What is the oxidation state of the copper in this structure? Explain.
Oxygen is invariant in its anionic state, except in peroxide and superoxide, neither of which fit the
geometry of this lattice. From the formula, 2 Cu ions to 1 O2– anion, it is Cu(I), not Cu(II)
c) Would you expect cuprite to be paramagnetic or diamagnetic? Justify your answer.
The electron configuration of neutral Cu is [Ar]4s13d10
Cu+ would lose the s electron to give: [Ar]3d10, which has all subshells filled. Hence it would be
expected to be diamagnetic.
4.
The graph at the right presents the normal boiling points
of 2nd, 3rd, 4th, and 5th period element hydrides of the
Group 14 and 16 elements.
Boiling Points of Group 14 and Group
16 element hydrides
[10]
Explain the position of each of the eight points in the
graph in terms of the kinetic molecular theory of liquids
and the kinds of intermolecular forces that operate
between the molecules in pure liquids of the given
composition. Discuss all the intermolecular forces that
apply in each system.
The group 14 compounds follow the series:
bp. CH4 < SiH4 < GeH4 < SnH4
i.e. the bp. goes up with mass, as predicted by the kinetic
molecular theory. The only forces that operate among these
symmetrical (tetrahedral) molecules are induced-DP –
induced-DP forces
Temperature - deg C
150
100
H2O
50
Period
H2Te
0
2
3
-50
H2S
-100
5
SnH4
GeH4
SiH4
-150
-200
4
H2Se
CH4
Grp14
-250
Grp16
This trend is also seen for the bp. H2S < H2Se < H2Te, but all have higher bp. than their Group 14
analogues SiH4, GeH4 and SnH4. This is due to DP-DP forces operating in the polar (angular shape)
Group 16 compounds.
H2O has a bp ca. 200 degrees higher than expected for a molecule of its size. This shows the dominant
influence of H-bonding in this compound.
2
5.
The density of copper metal is 8.95 g cm–3. If the unit cell edge length is 361.5 pm, is the copper unit
cell simple cubic, body-centred cubic, or face-centred cubic?
[8]
We calculate the density for the three types of possible unit cells:
The density can always be calculated if you know have a postulate for a possible unit cell contents, as
well as the geometry of the unit cell.
d=
m
# atoms per unit cell × 63.546 g mol
=
= # atoms per unit cell × 2.234
V 6.022 ⋅ 1023 atoms mol × (3.615 ⋅ 10−8 cm)3
CELL TYPE
# atoms/unit cell
calc’d density
SIMPLE CUBIC
1
2.234 g/cm3
BCC
2
4.467 g/cm3
g
cm3atom
FCC
4
8.935 g/cm3
The density for FCC matches well with experiment. Therefore copper metal is FCC.
6.
Determine for each of the following molecule the normal or “average” geometry, and demonstrate for
each whether or not it is a polar or non-polar molecule. For polar molecules, show the net molecular
dipole moment using a Vector (please point the vector towards the negatively charged end of the
molecule.) Central atoms are underlined. Show enough work to justify your choice of geometry.
[12]
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