Name (Last, First): _____________________________
ID Number: _____________________
High School Name: _____________________________________________________________
Washington University Chemistry Tournament
April 2, 2016
Team Round
Please write your school name and ID number at the top of every page.
50 minutes is allowed for this examination. This exam is 7 questions long, and has 18 numbered
pages in total. Please check to make sure that your exam is complete, and report to the exam
proctors if any pages are missing.
Necessary equations and constants, as well as a periodic table, can be found at the end of the
exam. Do not write on the scoring sheet on the last page of the exam.
Scratch paper will not be permitted during this exam. If you run out of room on the front of the
page, you can continue to work on the back of the page, provided that all answers for parts are
clearly labelled. Only write work on the backs of pages that correspond to the question on the
front of the page.
Four blank copies of the exam will be given to each team, and teams may work on any number
of these copies. It is not recommended, however, that the entire team do every question together
as time would become an issue. At the end, the team must submit a single copy of each page of
the exam to the proctor for grading. Please make absolutely sure that you are submitting every
page on the team exam, otherwise the team will lose all points for the page(s) that were not
submitted. Submitting multiple copies of a page will also lead to a loss of points. The team
number and high school must be written on each page.
Make sure to circle or box your final answer if appropriate. Correct answers with appropriate
work will receive full credit. If work contains reasoning or justification that is partially correct,
partial credit may be awarded. Explanations should fully answer the question and provide
supporting evidence and logic. These should also be given in complete sentences. Correct
answers without reasonable supporting work will not receive credit.
If you cannot answer a question or the entire question, it is advised that you do not spend too
much time on that question and proceed onto other parts of the exam. No electronics of any kind
are allowed during the exam, with the exception of a non-programmable scientific calculator.
Cell phones must be turned off, and watches must be removed. Time remaining will be
announced by the proctor in the room. The scoring policy can be found on the WUCT website.
Any appeals must be made in writing in the appeals room, Lab Sciences 400.
Cheating will not be tolerated on this exam. The cheating policy that has been listed on the
WUCT website will be followed. Violators of this policy will be referred to the directors of the
competition.
1
School Name: ____________________________ ID Number: _______________________
1. By some unfortunate turn of events, you have found yourself locked inside of a Menards
while picking up some compact fluorescent light bulbs for your home on a late night –
the store has closed while you were shopping, and the manager has turned off the power
to the automatic doors. Luckily, you are able to remember some electrochemistry from
your high school chemistry course and think you can create a battery that would be able
to create enough power to open the automatic door, bringing you to freedom! The
temperature is currently 25 °C.
Walking around the empty store, you procure some copper wiring, a retro tin kitchen
backsplash square, a lead fishing weight, and a 3" x 3" Zinc T-Plate (whatever that is
used for). You also find two alligator clip leads to connect your battery, 1 kg of kosher
salt, a pack of 24 plastic water bottles, a 12-pack of coca cola, drink glasses, a pack of
paper towels, and sulfuric acid (!). The table of standard reduction potentials is shown
below. You only have enough materials to make a single battery.
Half Reaction
+ 2e− → Cu(s)
+ 2e− → Pb(s)
+ 2e− → Sn(s)
+ 2e− → Zn(s)
Cu2+
(aq)
Pb2+
(aq)
Sn2+
(aq)
Zn2+
(aq)
E° (in Volts)
+0.34
-0.13
-0.14
-0.76
a. You plan to make the most powerful standard battery possible using only these
materials. What is the maximum voltage that can be produced? Assume all
conditions are standard.
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School Name: ____________________________ ID Number: _______________________
b. Propose a plan for this battery. Draw a picture of your battery, making sure to
clearly label each of the following parts, and indicate what available materials
you would use to make each of those.
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Anode electrode
Anode solution
Cathode electrode
Cathode solution
Salt bridge
Container for anode and cathode
Connection from electrodes to door
Direction of conventional current flow
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School Name: ____________________________ ID Number: _______________________
2. Examine the table below. Trouton’s rule indicates that most standard entropies of
vaporization of liquids are close to 85 J/mol∙K. Why are liquids such as water, methanol,
ethanol, and ammonia exceptions to Trouton’s rule? Briefly justify your answer on the
lines provided below.
Liquid
acetone
ammonia
argon
Benzene
Ethanol
Mercury
Methane
Methanol
Water
ΔSvapo (J/mol∙K)
88.3
97.6
74
87.2
124
94.2
73
105
109
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School Name: ____________________________ ID Number: _______________________
3. Dimethyl Sulfoxide (DMSO) is an extremely useful reagent used in research laboratories
when scientists perform certain reactions. DMSO is used to inhibit the formation of
secondary structures in DNA primers and templates that may interfere with successful
amplification of a DNA sequence.
a. The chemical formula of DMSO is (CH3)2SO. The connectivity of the atoms in
DMSO has been replicated below four times. Use the connectivity to complete all
major Lewis structures for DMSO, indicating all lone pairs and non-zero formal
charges. It is possible that not all the diagrams will be used. One of the resonance
forms has charge separation between sulfur and oxygen. Circle this form.
b. Let’s suppose that we examined trimethyl sulfoxide, a molecule similar to
DMSO. In trimethyl sulfoxide, there were three methyl (-CH3) groups attached to
the central sulfur atom. Would this molecule or DMSO have a more polar S-O
bond? Explain your reasoning in on the lines provided below. (Electronegativity
of Oxygen is 3.5; Electronegativity of Sulfur is 2.5).
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School Name: ____________________________ ID Number: _______________________
c. Dimethyl ketone has a similar structure to DMSO except that the central sulfur
atom is replaced by a carbon atom. Given this information, determine which of
the two molecules will have a greater bond angle. Explain your reasoning on the
lines provided below.
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d. Ka is a measure of acidity of a proton. More acidic protons would have greater
Ka’s, whereas less acidic protons would have lower Ka’s. However, chemists
usually express acidity in terms of pKa, which is equal to the negative log (Ka).
The pKa of the methyl groups in DMSO is 35, suggesting that the hydrogens are
weakly acidic. Given the following bases and their Kb’s, how many reagents
could deprotonate the hydrogen atom? Justify your answer on the lines provided
below.
Base
Methylamine
Ethylamine
Caffeine
Pyridine
Potassium Hydroxide
LDA
Kb
4.4 x10-4
5.6 x10-4
4.1 x10-4
1.7 x10-9
1013
1022
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School Name: ____________________________ ID Number: _______________________
4. You decide to investigate the properties of the carbonic acid – bicarbonate buffer system
in vitro.
The most important buffer in maintaining pH in the blood is the carbonic acid – hydrogen
carbonate buffer system as illustrated below:
+
𝐻(𝑎𝑞)
+ 𝐻𝐶𝑂3− (𝑎𝑞) ⇄ 𝐻2 𝐶𝑂3(𝑎𝑞) ⇄ 𝐻2 𝑂(𝑙) + 𝐶𝑂2(𝑔)
An equation for the pH of blood in terms of the concentration of [CO2] and [HCO3-] can
be derived. We, the question writing committee of WUCT, have performed this
dangerous and exhausting derivation. The result follows:
𝑝𝐻 = 𝑝 (
[𝐶𝑂2 ]
𝐾𝑎
) − log
[𝐻𝐶𝑂3− ]
𝐾2
a. Hyperventilation can result from strenuous exercise, anxiety, high fever, or high
sodium diet. How does this affect the pH of blood? Justify your answer on the
lines provided below using the equation derived in part (a).
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b. A common technique we use with hyperventilating patients is to have them
breathe in a paper bag. Why does this work? Explain your answer in 3 complete
sentences or less on the lines provided below.
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School Name: ____________________________ ID Number: _______________________
c. Consider the following reaction for the dissociation of carbonic acid in water:
+
𝐻2 𝐶𝑂3(𝑎𝑞) + 𝐻2 𝑂(𝑙) ⇄ 𝐻3 𝑂(𝑎𝑞)
+ 𝐻𝐶𝑂3− (𝑎𝑞) . Is the reaction exothermic or
endothermic? Use the following information in your explanation on the lines
provided below:
Ka (25oC) = 4.5 x 10-7
Ka (180oC) = 1.6 x 10-10
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School Name: ____________________________ ID Number: _______________________
5. In organic chemistry, there are two main types of substitution reactions that a compound
can undergo. In a substitution reaction, a covalent bond is broken to allow for another
group to make a bond there – using these methods (and many others), organic chemists
can start to build up molecules that can be useful for and in a multitude of things, such as
pharmaceuticals, dyes, and fragrances.
Shown below are simple examples of the two types of substitution, called bimolecular
nucleophilic substitution (SN2) and unimolecular nucleophilic substitution (SN1),
respectively (R or R’ just indicates an alkyl group – there is no need to worry about what
this alkyl group could be; dashes and wedges indicate covalent bonds going into and out
of the plane of the paper, respectively). A compound listed above the reaction arrow acts
as a reactant for that reaction.
SN2:
Alkyl halide
Alcohol
SN1:
Alkyl halide
Carbocation
Alcohol
In both of these reactions, bromine (Br) is getting replaced by a hydroxyl group (OH).
The reactions may look identical but they are not. One reaction rate depends on both the
concentration of the nucleophile (either -OH or H2O – these reactants have the same
purpose for both SN2 and SN1 reactions and can be treated as essentially the same) and
the concentration of the alkyl halide. The other rate depends only on the concentration of
the alkyl halide.
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School Name: ____________________________ ID Number: _______________________
a. Using the fact that the equilibrium reaction in the SN1 reaction is the slow, ratedetermining step, come up with rate equations for both SN2 and SN1 reactions,
making sure to include k, the rate constant. Your answer should look something
like “rate = k[A][B][etc.]”.
b. Classify the rates you have written above as zero order, first order, or second
order.
c. Why would the first equilibrium reaction of the SN1 substitution reaction be the
rate-determining step? (Hint: look at the products of this first reaction compared
to the reactants). Justify your answer on the lines provided below.
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School Name: ____________________________ ID Number: _______________________
d. For the equilibrium discussed in part (c), is ΔS positive or negative? Given that
this equilibrium reaction favors the reactants, is ΔH positive or negative? Justify
your answer on the lines provided below.
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e. For the equilibrium discussed in part (d), is it possible to make this reaction
spontaneous, and if so, are high temperatures or low temperatures better for
spontaneity? Justify your answer on the lines provided below.
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School Name: ____________________________ ID Number: _______________________
6. A single bond is composed of one ϭ (sigma) bond. A double bond consists of one ϭ bond
and one π (pi) bond. A triple bond consists of one ϭ bond and two π bonds. Furthermore,
a ϭ bond is significantly stronger than a π bond.
Based on the above information and your understanding of ϭ and π bonds, answer the
following questions:
a. Rank the single, double, and triple bonds in terms of strength from strongest to
weakest.
b. Rank single, double, and triple bonds in terms of length from longest to shortest.
c. Amides are used in the formation of fabrics and nylons. Consider the structure of
an amide on the left; the R groups are irrelevant to the following question.
..
..
Compare the rotational energy barrier of the C-N bond in the amide to the
rotational energy barrier of the C-N bond in the amine on the right. Resonance
structures and/or other diagrams may be used in your answer.
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School Name: ____________________________ ID Number: _______________________
d. Many molecules with double and/or triple bonds are linear; that is, all atoms lie in
the same plane (examples include acetylene C2H2 and CO2). That being said, are
any of the 3 possible isomers of C3H4 linear molecules? Explain why or why not?
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School Name: ____________________________ ID Number: _______________________
7. Consider the following reaction profiles that start with the same reactant. For the
following parts, please round the energies of Reactant C, Product A, Product B, and the
two reaction profile transition states to the nearest integer.
Ea (Product B)
Ea (Product A)
a. We can classify the two profiles as “thermodynamic control” or “kinetic control.”
Kinetic control is when the product forms more quickly, whereas thermodynamic
control is when the product forms more slowly but is also more stable. Given this
information, which of the reaction profiles corresponds to thermodynamic
control? Which of the reaction profiles corresponds to kinetic control? Explain
your answers on the lines provided below.
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b. What can we add to lower the activation energy of the reaction that forms product
B so that it is similar to that of the reaction that forms product A?
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School Name: ____________________________ ID Number: _______________________
c. Which product would be favored if Reactant C was subjected to high
temperatures for a long time? Explain your answer.
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d. What product would be favored if Reactant C was subjected to lower
temperatures for a short period of time? Explain your answer.
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e. For the following question, round the energies of Reactant C and Product B to the
nearest integer. Assuming that Reactant C is in equilibrium with only Product B,
calculate the ratio of B to C at equilibrium at 25 ℃. Round your answer to the
nearest integer. 1 kcal corresponds to 4.184 kJ.
(End of Exam)
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School Name: ____________________________ ID Number: _______________________
Equations and Constants
Mega (M) = 106 , Kilo (k) = 103 , Deci = 101 , Centi (c) = 10−2, Milli (m) = 10−3
Micro (μ) = 10−6, Nano = 10−9
R = 0.0821
L∙atm
K∙mol
= 8.314
J
mol∙K
c = 3.0 × 108 m⁄s
h = 6.626 × 10−34 J ∙ s
1 eV = 1.602 × 10−19 J
K a × K b = K w = 10−14 at 25 ℃
9
℉ = (℃ ∙ 5) + 32; ℃ = K − 273.15
moles of i
Mole Fraction (Χi ) = total moles
PV = nRT
Let ratex be the rate of effusion of a gas x:
rate1
rate2
Molar Mass 2
= √Molar Mass 1
∗
For any volatile liquid, Psolution = Χ i ∙ Ppure
liquid
pH = −log[H + ]; pK a = − log(K a ) ; pX = −logX
1
For an amphiprotic salt with two K a ′s, pH = 2 (pK a1 + pK a2 )
a
a−
b+
a− b
Given Aa Bb(s) ⇄ aAb+
(aq) + bB(aq) , K sp = [A ] [B ]
Ephoton =
hc
λ
Zero Order Reaction: [A] = [A]0 − kt
= λν
c = fλ
First Order Reaction: [A] = [A]0 e−kt
PTotal = P1 + P2 + P3 + ⋯ + Pn
Second Order Reaction: [A] = kt + [A
pH = pK a + log
1
[conjugate base]
[acid]
1
0]
1
Third Order Reaction: [A]2 = 2kt + [A
[C]c [D]d
Given aA + bB ⇄ cC + dD, K eq = [A]a[B]b
∆G° = −RT ∙ lnK eq
∆G = ∆H − T∆S
k = Ae
1
−Ea
RT
For aA + bB → products, rate = k[A]m [B]n
2
0]
Half-life of a first order reaction: t 1 =
2
E=
ln(2)
k
RT
E0 − nF lnQ
Q = mC∆T
Q Phase Change = mL
n
n!
( ) = r!(n−r)!
r
P2
∆Hsub 1
1
ln ( ) =
( − )
P1
R
T1 T2
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Name (Last, First):_________________________ ID Number: __________________________
Credit: http://schoolbag.info/chemistry/ap_chemistry/ap_chemistry.files/image004.jpg
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Name (Last, First):_________________________ ID Number: __________________________
Scoring Sheet
Do not write on this page, this sheet is for the grader’s use only.
Question 1: _______________ / 20
Question 5: _______________ / 19
a) _______________ / 4
a) _______________ / 3
b) _______________ / 16
b) _______________ / 2
Question 2: _______________ / 6
c) _______________ / 5
d) _______________ / 6
Question 3: _______________ / 24
a) _______________ / 5
e) _______________ / 3
Question 6: _______________ / 18
b) _______________ / 8
c) _______________ / 6
d) _______________ / 5
a) _______________ / 2
b) _______________ / 2
c) _______________ / 8
Question 4: _______________ / 18
a) _______________ / 6
d) _______________ / 6
Question 7: _______________ / 23
b) _______________ / 4
c) _______________ / 8
a) _______________ / 6
b) _______________ / 1
c) _______________ / 4
d) _______________ / 5
e) _______________ / 7
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