Sci 101
Final Exam
Jan 14, 2010
KEY
1. (1 point each) Fill in the blanks.
a) Periodic Table is arranged according to the atomic number of element.
b) Valence electrons are electrons in the outer most shell of elements
c) There are only 3/5 electrons in the valence shell of Group 5 (13) elements.
d) Mass number is equal to the total number of protons and neurtons in the nucleus.
e) Intermolecular attractions between molecules by the distortions of electrons around
molecules are known as dispersion forces.
f) Water has high specific heat because of strong hydrogen bonding.
g) At high temperatures and low pressures most nonmetals and their compounds generally
exist in gas phase.
h) Ice floats on water because it has a lower density than liquid water.
i) When the electronegativity difference between atoms are less than 2, polar
covalent bonds form.
j) Total number of (p) electrons in Si (Silicone) is 8.
k) CO2 contain polar covalent bonds but it does not have dipole moment.
2. (4 points each) From the list of ions/compounds given, choose and write in the space designated
the one that relates best to the statement below. USE EACH SPECIES ONLY ONCE!!
BH3
BaI2
N2
BeH2
Na+
SiH4
a) Species that has a Noble gas electronic configuration
Pb
HF
SO2
Na+
b) Species that has polar covalent bonds but has no dipole moment BH3 / BeH2
c) Species that has a triple bond N2
d) Species that violates the Lewis octet rule BH3 / BeH2
e) Species that has the most ionic bond BaI2
f) Species that has a tetrahedral structure SiH4
g) Species that can form hydrogen-bonding HF
h) Species that has metallic bond Pb
i) Species that has a bend structure SO2
3. (6 points) Explain why the atomic radii of elements decrease as one moves from left to right
within a given period?
With in a given period, the outer shell remains the same, however, as one moves right, the
number of protons in the nucleus increases. This increases the positive electrostatic attraction of
nucleus on the electrons and thus the electrons cloud shrinks and atomic size become smaller.
4. (6 points) By using the data below, calculate the H-H bond energy.
H2
+
Cl2
2HCl
H-Cl bond energy is 431 kJ/mol
Cl-Cl bond energy is 243 kJ/mol
(Missing data: The heat of this reaction is -183 kJ/mole)
Heat of reaction = Bond energy of reactants - Bond energy of products
-183 = H2 + 243 - 2(431)
H2 = 436kJ/mole
5. (6 points) Explain why alkali metals have lowest ionization potentials in each period.
Alkali metals are the elements in the 1st group of periodic table where a new principle
quantum shell begins by each element as one moves down the group. When an electron is
lost the cation formed has the stable noble gas configuration of the previous group. Also the
single electron in the outer most shell is shielded (screened) from the nucleus and does not
feel the attraction of the positive nuclear charge.
6. (6 points) Briefly explain why it is very important for chemist to determine the three dimensional
structure of compounds.
Three dimensional structure of molecules determine the chemical properties (such as
reactivity, acidity, Basicity, Flammability, oxidizing-reducing ability as well as various pyhsical
properties such as melting - boiling points, density, specific heat, magnetism, state, electrical
conductivity etc.
7. (6 points) Which of the following (A or A+ have higher ionization energy? Explain
Case 1
A
e-
Case 2
A+
e-
+
+
A+
A2+
In the second case. It is the same atoms thus the number of protons are the same. In the
second case however, one has to do much more work not only to overcome the attractions by
the nucleus but the positive charge that already exist on the ion A+ .
8. (6 points) Briefly explain how water can dissolve ionic compounds like NaCl. (You can also
show this by drawing simple picture)
Water is a very polar compound. Oxygen end has a partial negative charge, Hydrogen side
has a partial positive charge. The water molecules abstract the Cl- and Na+ ions and Negative end
of water surround the Na+ and positive end surrounds the Cl-. The interactions stabilize the ions
and NaCl spontaneous dissolve in water.
9. Lewis dot structure
a) (5 points) Write the Lewis dot structure of Cl2O.
b) (5 points) Draw its three dimensional structure
c) (4 points) Write is proper name and state if it is polar or not.
Tetrahedral (bend), the molecule will be polar with definite dipole moment
Bonus!!! (5 points)
Water has its highest density at 4o C. Explain the environmental impact of this unique property.
Water at the bottom of deep lakes remains at 4oC since it is the most dense form of water.
Late fall, when the weather cools and the surface of the lake reaches 4oC before it freezes, the
oxygen rich surface water will sink to the bottom causing the fall be a vertical mixing. After the lake
is frozen, fish as the bottom can survive all winter long. When spring comes, first ice will melt and
then the surface water will begin to warm up to 4oC. At this stage again the oxygen rich water will
sink to the bottom causing the spring vertical mix. This vertical mixing will provide oxygen rich
water for all the aquatic life at the bottom of deep lakes and seas.