Chem 241-2012 Exam #2 Sample question Answers
1. Using the table below, decide if the element aluminum (Al) should be found mainly in the
core of the earth, or mainly in the mantle and crust. Explain your answer.
ANS: Al will be mostly in the crust because Al3+ ion cannot be reduced by Fe.
Fe  Fe2+ + 2eAl3+ + 3e-  Al
3 Fe + 3 Al3+  3 Al + 2 Fe2+
Eox = +0.44 V
Ered = -1.66 V
Ecell = -1.22 V Disfavored
2. Consider the diagram below for the structure of the mineral Na2SiO3.
a. What type of solid does this represent? (example: molecular solid, ionic solid, extended solid,
etc.).
ANS: It is an extended covalently bonded silicate anion, ionically bonded to Na+ cations. So, it is
covalent, extended and ionic.
b. What atoms (name the element or elements) in the formula are not explicitly shown in the
structural representation? (That is, by either a line or a corner.)
ANS: Si is not represented. Each tetrahedron has a Si atom in its center. Na + ions are also not
shown. They are found between silicate chains.
3. Which of the following elements do you expect to be extracted from their ores by electrolysis?
Circle all that apply.
Ag
Cl
K
Fe
Ar
ANS: We didn’t talk about Cl, but it is very difficult to oxidize to Cl2, so it is done electrolytically.
K+ is very hard to reduce to K, so it too is done electrolytically.
4. a. Draw a reasonable structure for the B2H7- anion.
b. Which of the following best describes the reactivity of this species?
Bronsted Acid
Bronsted Base
Lewis Acid
Lewis Base
5-6-7. Consider the following portion of the p-block of the periodic table.
This question concerns the structures of the elements themselves.
For each question, give all applicable answers.
ANS: The answers to these questions all revolve around the ability of the elements or oxides to form pi
bonds. Entropy favors small molecules. If that does not require pi bonding, then small molecules will be
formed. If pi bonds are necessary to avoid extended structures, then only molecules that can form pi
bonds will form small molecules. Also remember, entropy rules at high temperature; enthalpy rules at
low temperatures.
9. Consider the thermodynamic data given below. In what temperature range will the
decomposition of silver sulfide to elemental silver and sulfur be favored? Be sure to say
whether it is favored at high or low temperature, and specifically what temperature. Show your
work.
Ag2O(s)  2 Ag(s) + ½ O2(g)
Hfo(kJ)
So(J/K)
Ag2O(s)
Ag(s)
O2(g)
-31.1
0
0
121.0
42.6
205.1
ANS:
ΔHrxn = (2 x 0 + 1 x 0) – (-31.1 kJ) = + 31.1 kJ
ΔSrxn = [(2 x 42.6 + 1/2 x 205.1) – (121.3)] = + 66.5 J/K
The reaction is favored by entropy; disfavored by enthalpy; so it is favored at high temperatures
and disfavored at low temperatures. The cutoff temperature is where ΔGrxn = 0;
𝑇=
∆𝐻𝑟𝑥𝑛
∆𝑆𝑟𝑥𝑛
31100 𝐽
= 66.5𝐽/𝐾 = 486𝐾
10. Explain the bonding in the compounds with empirical formulas AlCl3 and BF3. Determine
if they are electron rich or electron poor, and how they alleviate the resulting instability.
11. What is the cell voltage for a cell running on the reaction:
Cd2+(aq) + Sn2+(aq)  Cd(s) + Sn4+(aq)
ANS: See Q1. Answer is -0.25 V
12. Here’s an easier one. Which two pairs of reactants will form a more powerful battery (in
terms of voltage), provided that the appropriate metals and/or cations are used as needed:
Al and Ni
or
Al and Ag
ANS: Al and Ag. For the exam I would need to give you the reduction potentials. The pair of
reactants with the greater difference between their reduction potentials makes the battery
with the greater voltage.
13. What is the difference between a battery and a regular redox reaction happening in a
beaker?
14. What is the most important technical difference between a rechargeable battery and one
you can’t recharge?
15. The formula for lattice energy, Uo, is:
Compare each term for the two salts: MgO and NaI (I- is much larger than is O2-, Both take the
same structure)
a) Circle for which each is larger:
Z+Z-
MgO or
NaI
or
same
M
MgO
or
NaI
or
same
(1 – 1/n)
MgO
or
NaI
or
same
b) Which overall do you expect to have a stronger lattice energy?
MgO or
c) Which term do you think is most important in any difference between them? Z+Z-
16. What is the purpose of the Madelung constant, M?
NaI