Download Exam #3

CHEM 31
Introductory Chemistry
EXAM #3
November 14, 2001
Name:
Keye, Francis Scott
SSN:
Lab T.A.:
INSTRUCTIONS: Read through the entire exam before you begin. Answer all of the
questions. For questions involving calculations, show all of your work -- HOW you
arrived at a particular answer is MORE important than the answer itself! Circle your
final answer to numerical questions.
The entire exam is worth a total of 150 points. Attached are a periodic table and a
formula sheet jam-packed with useful stuff. Good Luck!
Page
2
Possible
Points
30
3
4
5
6
7
30
9
45
16
20
TOTAL:
150
Points
Earned
30
30
9
45
16
20
150
1. For the Hydrogen atom:
a. (10 pts) Calculate the energy change (in Joules) associated with an n=6
to n=1 electronic transition.
∆E =
=
=
=
=
=
RH(1/(ni)2 - 1/(nf)2)
(2.1798741 x 10-18 J)(1/(6)2 – 1/(1)2)
(2.1798741 x 10-18 J)(1/36 – 1)
(2.1798741 x 10-18 J)(-0.9722222)
-2.1193220 x 10-18 J
-2.119322 x 10-18 J
b. (5 pts) Is a photon emitted or absorbed as a result of this transition?
EMITTED
ABSORBED
circle one
c. (10 pts) Calculate the wavelength (in nm) of the photon having the same
energy as the energy change associated with this transition.
E = hc/λ
→ λ = hc/E
λ = (6.62606876 x 10-34 J-s)(2.9979 x 108 m/s)
2.1193220 x 10-18 J
= 9.37294623 x 10-8 m
= 9.37294623 x 10-8 m x 109 nm
m
= 9.3729 x 101 nm or 93.729 nm
d. (5 pts) Is the photon in the visible portion of the electromagnetic
spectrum?
YES
NO
circle one
2
2. (5 pts each) Write the ground state electron configurations for the following
atoms and ions (use noble gas abbreviations for core electrons where
appropriate):
a. Cl:
[Ne]3s23p5
b. Ti:
[Ar]4s23d2
c. Ti2+:
[Ar]3d2
d. Ag:
[Kr]5s14d10
3. (10 pts) Give the values for the quantum numbers n, l, and ml for each orbital in
the 4d subshell.
n = 4
l = 2
ml = -2, -1, 0, 1, 2
3
4. (3 pts each) Circle the number next to the appropriate response for each of the
following:
a. The azimuthal quantum number (l) governs:
1. The energy of an orbital
2. The shape of an orbital
3. The color of an orbital
4. The spin of an electron in an orbital
5. The spatial orientation of an orbital
b. Which of the following orbital diagrams shows the lowest energy electron
configuration for Si?
1. [Ne] ↓↑
3s
2. [Ne] ↓↑
3s
3. [Ne] ↑_
3s
4. [Ne] ↑↓↑
3s
5. [Ne] ↑_
3s
↓↑ __ __
3p
↑_ ↑_ __
3p
↑_ ↑_ ↑_
3p
↑_ __ __
3p
↓↑ ↑_ __
3p
c. Which ONE of the following statements is TRUE ?
1. First ionization energies are always exothermic.
2. Atom electron affinities are always endothermic.
3. Atomic size always increases with increasing atomic number.
4. Electronegativity values have no units.
5. The Born-Haber cycle describes the reproductive process for
spineless invertebrates.
4
5. (5 pts each) For the following, circle the species in each row with the desired
property:
a. smallest atomic radius
Rb
K
Li
F
Na
Ca 2+
S2-
Cl-
Ar
K+
c. greatest electronegativity
O
At
Br
I
Cl
d. greatest ionization energy
Na
Na +
Mg
e. greatest lattice energy
NaF
CrN
MgCl2
f. smallest ion separation
NaF
NaCl
NaBr
b. largest radius
Mg2+
Al
LiF
MgF2
NaI
LiF
6. (15 pts) Using the information provided on the formula sheet, calculate the
enthalpy change (kJ) for the combustion of 1 mol of ethanol (C2H5OH):
C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)
∆ Ho = Σ n∆
∆ Hof(products) - Σ n∆
∆ Hof(reactants)
∆ Ho = [2(-393.5 kJ) + 3(-285.83 kJ)] – [(-235.1 kJ) + 3(0)]
= [-787.0 kJ – 857.49 kJ] + 235.1 kJ
= -1409.39 kJ
= -1409.4 kJ
5
7. (16 pts) The Born-Haber Cycle is really just an application of Hess’s Law that
enables us to calculate the lattice energy (∆Hlatt) for an ionic compound. Given
the reactions below (and their associated enthalpy changes), calculate ∆Hlatt
(kJ/mol) for MgBr2:
MgBr2 (s) → Mg2+ (g) + 2Br- (g)
∆Hlatt = ?
Here are the reactions you’ll need:
Rxn 1:
Rxn 2:
Rxn 3:
Rxn 4:
Rxn 5:
Rxn 6:
∆Hof = -524 kJ/mol
∆Hof = 147.1 kJ/mol
∆Hof = 111.8 kJ/mol
IE1 = 738 kJ/mol
IE2 = 1.45 x 103 kJ/mol
EA = -325 kJ/mol
Mg (s) + Br2 (l) → MgBr2 (s)
Mg (s) → Mg (g)
½Br2 (l) → Br (g)
Mg (g) → Mg+ (g) + e Mg+ (g) → Mg2+ (g) + e Br (g) + e - → Br- (g)
NOTE: In order to receive full credit, you need to clearly show how you arrived at
your answer!
-(Rxn 1):
Rxn 2:
2(Rxn 3):
Rxn 4:
Rxn 5:
2(Rxn 6):
MgBr2 (s) → Mg (s) + Br2 (l)
Mg (s) → Mg (g)
Br2 (l) → 2Br (g)
Mg (g) → Mg+ (g) + eMg+ (g) → Mg2+ (g) + e2Br (g) + 2e- → 2Br- (g)
MgBr2 (s)
→ Mg2+ (g)
+
2Br- (g)
-(-524 kJ/mol)
147.1 kJ/mol
2(111.8 kJ/mol)
738 kJ/mol
1.45 x 103 kJ/mol
2(-325 kJ/mol)
2432.7 kJ/mol
∆ Hlatt = 2433. kJ/mol
6
8. (5 pts each) Draw the most probable Lewis structure for each of the following
molecules. Indicate the formal charge on the central atom and the electrondomain geometry for each.
a. SO2
6 + 12 = 18 e..
..
..
:O = S = O:
AX3 – Trigonal Planar
fc = 6 –(4 + 2) = 0
b. SF6
6 + 42 = 48 e..
..
:F: :F:
..
\ /
..
:F – S – F:
..
/
\
..
:F: :F:
..
..
AX6 – Octahedral
fc = 6 –(6) = 0
c. NH3
5 + 3 = 8 e..
H – N
/
\
H
H
AX4 – Tetrahedral
fc = 5 –(3 + 2) = 0
d. XeF2
8 + 14 = 22 e..
..
..
:F – Xe – F:
..
..
..
..
AX5 – Trigonal Bipyramidal
fc = 8 –(2+6) = 0
7