Download Fall 2008 (3rd exam) Answer Key

Name:
10 Dec. 2008
Chemical Principles I
Third Midterm
Constants:
c = 3.00 x 108 m/s ; h = 6.63 x 10-34 J-s
PART A1 Circle the letter for the correct answer. (1/2 pt each)
Answers have been posted on the web for several weeks.
PART B Write answers on exam. Show calculations.
1) (8 pts) A life-vest is designed to contain the hydrogen gas produced when 100g of
CaH2(s) reacts with 100g of water to form Ca(OH)2(aq) and hydrogen. Calculate the
pressure inside the 10.L vest at 20˚C after the vest inflates. (R = 0.080 L∙atm/(mol∙K)
Balance:
CaH2(s) + 2H2O  Ca(OH)2(aq) + 2H2 (g)
MW CaH2 = 40. + 2(1.0) = 42. g/mol, H2O = 16. + 2(1.0) = 18. g/mol
100 g CaH2 / 42. g/mol = 2.38 mol CaH2 /
2.38 mol CaH2 ∙ 2 mol H2 / 1mol CaH2 = 5.26 mol H2
100 g H2O / 18. g/mol = 5.56 mol CaH2 /
5.56 mol H2O 2 mol H2 / 2 mol H2O= 5.56 mol H2
CaH2(s) is Limiting Reagent (4 pts)
PV = nRT
P=nRT/V
T = 20o +273 = 293K
P = 5.26 mol ∙ 0.080 L∙atm/(mol∙K) ∙ 293K / 10. L
P = 12. atm
2) (4pts) It takes helium 10 s. to diffuse through a porous membrane. Calculate how
much time it would take methane, CH4 to diffuse.
Graham’s Law: Self Test 4.15B
1
3) (6 pt) Display on the two graphs the effect of raising the Temperature, (T higher
and lower) of a gas and changing the molecular mass, (M) of a gas. Label the axes
of the graphs
Class notes and figures 4.27 and 4.26
PART C Write answers on exam. Show calculations.
1) What is an orbital? (5 pts)
An orbital is a mathematical function the describes the wave-like behavior of one
electron.
2) What is the difference in energy for a hydrogen atom with its electron in a 2s
orbital and in a 2p orbital? (2 pts)
None (for the H atome)
3) The physical property that demonstrates the wave-like behavior of light is called
___diffraction___. (2 pts)
4) Calculate the energy of a photon that is released when the electron in hydrogen
moves from n = 6 to n = 2. (RB = 2.18 x 10-18 J) (2 pts)
2
5) Fill in the numerical value of quantum numbers n and l corresponding to the
following orbital designations: (2 pts)
Orbital
n
l
4f
4
3
2s
2
0
3p
3
1
5d
5
2
6) Fill in the maximum number of electrons that occupy each of the subshells in the
following table: (2 pts)
Max number e-
3d
4f
2s
5d
10
14
2
10
7) Write the electron configurations for the following atoms or ions, using the
appropriate noble-gas core abbreviations: (2 pts)
Electron configuration
As
[Ar] 4s23d103p3
Cu
[Ar] 4s13d10
Ni2+
[Ar] 3d8
Se2-
[Kr]
8) Fill in the letter for the quantum number(s) that govern the following properties of
hydrogen-like orbitals: (2 pts)
3
Quantum number (or numbers)
shape
l
energy
n
spin
s
spatial orientation
m
9) Arrange the following atoms or ions in order of increasing size: (2 pts)
Lowest, middle, highest
Ca, Mg, Ba
Ga, Br, Ge
Se2-, Te2-, Se
Ni2+, Cu2+, Zn2+
Cr6+, Ti4+, Sc3+
10) For each of the following pairs, circle the element with the higher metallic
character. (2 pts)
Li or Be
Li or Na
Sn or Pb
Al or B
11) Predict which of the following oxides are ionic or molecular by putting the letter I
under the ionic and M under the molecular. (2 pts)
SO2, MgO, Li2O, P2O5, N2O, XeO3
M
I
I
M
M
M
12) Write a balanced equation for the reaction that occurs in the following cases: (2
pts each)
a) Hot magnesium metal reacts with water:
Mg + H2O  MgO + H2
b) Strontium oxide is added to water:
SrO + H2O  Sr(OH)2
c) Powdered zinc metal reacts with sulfur:
Zn + S  ZnS
d) Iron(II) oxide reacts with phosphoric acid:
3FeO + 2H3PO4  Fe3(PO4)2 + 3H2O
4
e) Sulfur trioxide reacts with water:
SO3 + H2O  H2SO4
f) Carbon dioxide reacts with aqueous sodium hydroxide.
CO2 + NaOH (aq)  NaHCO3 (aq)
13) Write an equation (defining your symbols) that accounts for the large increase in
lattice energy in the series of isoelectronic substances KF < CaO < ScN (2 pts)
E = k Q-Q+
----d
14) Circle the most electronegative atom in each of the following sets: (2 pts)
(a) P, S, As, Se;
(b) B, ν, C, Si;
(c) Sn, Ga, Ge, As; (d) Na, Mg, K, Ca
15) Sketch the 1s, 2s, and 3s orbitals on the following graphs (2pts) Fig 1.34
1
1
1



0
0
0
5
x
1s = e -| x |
0
0
5
0
5
x
x
2s = (5 – x) e -| x/2 |
3s = (x2-6x +5) e -| x/3 |
5
16) A 1s orbital placed 2 units to the left on the x axis has the form, e -| x +2 | and
another placed 2 units to the right has the form, e -| x - 2|. Sketch the  MO on the
graph on the left and the * MO on the right, and write the mathematical function
for each (2 pts). Fig 3.25 and 26


1
1
x
x
-5
5
-1
 = ___________________________
5
-5
-1
* = _________________________
BONUS (3 pts) HeH+ is a molecular ion found in space. Sketch its  MO with its nuclei
at +/- 2 and write its mathematical function, knowing that the 1s orbital for different
atoms is e -| Zx | ,where Z = atomic number.
6
17) Draw any “good” Lewis structure for each and predict the electron-domain name,
molecular structure, and set of hybrid orbitals used by the central atom: (15 pts)
All Lewis structures can be found in textbook.
Lewis structure
+
electron-domain
tetrahedral
H3O
SCN
molecular
hybrid
structure
orbital
Trigonal
sp3
pyramidal
linear
linear
sp2
linear
linear
sp2
tetrahedral
Trigonal
sp3
-
CS2
ClO3
pyramidal
-
octahedral
Square planar
d2sp3
Trigonal planar
Trigonal planar
sp2
tetrahedral
Trigonal
sp3
-
ICl4
BF3
CF4
Pyramidal
(or tetrahedral)
7
Trigonal planar
bent
sp2
tetrahedral
bent
sp3
tetrahedral
Trigonal
sp3
-
NO2
SF2
Pyramidal
-
AlCl4
(or tetrahedral)
Trigonal
linear
dsp3
Trigonal
sp3
bipyramidal
KrF2
tetrahedral
pyramidal
-
HSO3
Trigonal planar
Trigonal planar
sp2
tetrahedral
Trigonal
sp3
SO3
pyramidal
2-
SO3
Trigonal
I3
-
linear
dsp3
bipyramidal
8
18) Using each of the Molecular Orbital diagrams: 1) fill in the number of electrons of
+
the two species CO, and CO , 2) calculate the bond order and 3) write the word
to describe the magnetic property. (4 pts)
a) CO
z
x*y*
px py pz
px py pz
x y
E
z
s
s
s
s
C
Bond Order: __3_______
CO
O
Magnetism: ___diamagnetic_____
9
+
b) CO
z
x*y*
px py pz
px py pz
x y
E
z
s
s
s
s
N
NO+
O
Bond Order: ___2.5____
Magnetism: _____paramagnetic___________
10
11