Chem 30A Test 5
Nov. 14, 2005
Show work as always!
1.
A student drew the three following Lewis structures for the nitric acid molecule:
H
O
N
O
2.
O
H
O
N
O
H
O
O
N
O
O
a.
Put in the correct number of lone pair electrons on the appropriate atoms.
b.
Which Lewis structure is wrong and why is it wrong?
c.
What geometric difference, if any, is predicted between the two valid Lewis structures?
For each of the two valid Lewis structures indicate the expected bond angles.
d.
Optional: Of the two remaining structures which is most favored and why? Hint: In the
favored Lewis structure more atoms have their normal “valence” (combining power) .
The conjugate base of nitric acid is the nitrate ion: NO3- (read NO3 minus). In nature it is found
that all the NO bonds in NO3- are identical.
a.
Draw all the correct “resonance” Lewis structures for the NO 3- ion.
b.
What is the bond order for any NO bond in the nitrate ion?
c.
What is the electron domain geometry around N?
d.
Indicate the molecular geometry and bond angles of the NO3- ion.
e.
Is the nitrate ion polar? Why or why not?
3.
Consider the molecule F2CO (both fluorines are attached to the carbon as is the oxygen.)
a. Draw a valid Lewis structure for the molecule (including lone pair electrons).
b.
c.
4.
5.
Draw the approximate dipole vector and estimate the magnitude. Is the molecule polar?
Indicate central atom geometry and bond angles for F2CO.
Consider the molecule cyanogen fluoride: FCN
a.
Draw a valid Lewis structure for the molecule (including lone pair electrons).
b.
Draw the approximate dipole vector and estimate the magnitude. Is the molecule polar?
c.
Indicate central atom geometry and bond angles for FCN.
Give ground state electron configurations for the following elements and ions:
a.
Mn
b.
Mn2+
c.
S
d.
S2-
e.
Ar
f.
F-
g.
Cl
h.
Br
6.
Periodic Table, quantum numbers, & electron configurations - Label and use the following
cartoon of the periodic table and your own words to:
a.
Describe the connection between principle quantum number n and the periodic table.
b.
Describe the connection between the angular momentum quantum number l, and the
periodic table.
c.
What is the most important pattern regarding electron configurations in the periodic
table?
d.
Explain the widths of the s, p, d, and f blocks.
7.
8.
Sketch the following orbitals:
a.
1s orbital
b.
2px orbital
c.
3pz orbital
A chemist wanted to prepare hydrazine, N2H4, (a type of rocket fuel) by the reaction:
2NH3 + OCl-  N2H4 + Cl- + H2O. To do this she mixed 3.0 mol NH3 with 2.0 mol OCl-.
a.
What is the limiting reactant?
b.
How many moles of hydrazine will be obtained?
c.
What is the excess reactant and how many moles remain?
9.
Take home problem due Wednesday!! The attitude control rockets on the space shuttle use the hypergolic rocket
fuel monomethyl hydrazine (CN2H6 or CH3NHNH2) with dinitrogen tetroxide (an oxidizer). Hypergolic fuels
spontaneously ignite upon contact. In this case the reaction products are nitrogen, carbon dioxide, and water.
a.
Write a balanced chemical equation for the reaction.
b.
Given 1.00 kg each reactant, which is the limiting reactant? And what is the mass (theoretical yield) of
each product?