Download Name: TF Name: 1

Name:
TF Name:
LS1a Fall 2014
Problem Set #1
Due Monday 9/22 at 6 pm in the drop boxes on the Science Center 2 nd Floor
Note: Adequate space is given for each answer. Questions that require a brief explanation
should be answered only in the provided space.
I.
Basic Concept Questions
1. (20 points) Consider a hypothetical reaction carried out at constant temperature and
pressure:
The table below shows five different hypothetical reactions with different thermodynamics. In
the empty spaces, fill in as much information as you can about:
1. Keq (e.g., >1, <1, or =1)
2. ΔG°rxn (e.g., >0, <0, or =0)
3. [C][D]/[A][B] at a particular state which is arbitrarily named “state A” (e.g., <Keq, >Keq,
or =Keq)
4. The sign of ΔG (proceeding from left to right) at the same “state A” as in bullet point #3.
Keq
Reaction 1
ΔG°rxn
>1
ΔG at a
particular state
(state A)
>0
Reaction 2
<0
Reaction 3
>0
<0
Reaction 4
<0
=0
Reaction 5
[C][D]/[A][B] at
a particular
state (state A)
=1
< Keq
> Keq
1
Name:
TF Name:
2. (16 points) The molecule shown below (Rodinal) is commonly used as a developer in blackand-white film.
a. (4 points) Draw the structure of Rodinal that is present in excess in a solution of constant
pH = 4.
b. (4 points) Draw the structure of Rodinal that is present in excess in a solution of constant
pH = 12.
c.
(8 points) Consider the ionizable group with a pKa of 10. For this group only, indicate the
relative ratio of protonated/deprotonated species at each pH value listed below.
Protonated (“H-A”)
Deprotonated (“A-”)
pH = 6
pH = 9
pH = 12
pH = 14
2
Name:
TF Name:
3. (10 points) Consider a hypothetical reaction in which molecules A and B interconvert.
Shown below are two graphs pertaining to this reaction. Graph 1 shows the energy of the
system versus the relative concentrations of molecules A and B. Graph 2 shows the
concentrations of molecules A and B as the reaction proceeds. The reaction described in
Graph 2 initially consisted of only one molecule, either A or B. The dotted and solid lines
show the concentrations of A or B, but these lines are left unlabeled for the purposes of this
question.
a. (4 points) Graph 2 has several numbered time points marked on the x-axis. Which time
point corresponds to the time at which ΔG is closest to zero? Briefly explain your answer.
b. (6 points) Does Graph 2 describe the forward reaction (A  B) or the reverse reaction (B
 A)? How can you tell?
3
Name:
TF Name:
II. Applied Concept Questions
4. (26 points) Ammonium (NH4+) and acetate (CH3COO-) can either exist as free ions or
associate to form an ionic interaction, as described by the following reaction:
Shown below are two Gibbs free energy diagrams describing the formation of the
ammonium:acetate ionic interaction in two different environments: either hexane
(CH3CH2CH2CH2CH2CH3) or water. The environment that each curve represents is
intentionally left unlabeled. The left side of the graph represents 100% free ions (i.e., no
ionic interactions are occurring), and the right side of the graph represents 100% ionic
interactions (i.e., all ammonium and acetate molecules are forming ionic interactions). For
clarity, both curves are arbitrarily shown to start at the same point on the left side of the
graph.
a. (4 points) Label ΔG°rxn and the position on the x-axis at which [products]/[reactants]
equal Keq for both curves.
b. (6 points) Match each of the curves to the environment it describes, either hexane or
water. Briefly explain your answer.
4
Name:
TF Name:
c.
(4 points) Would Keq for the formation of the ammonium:acetate interaction in salt water
(i.e., water with a large quantity of NaCl dissolved in it) be less than, greater than, or
equal to Keq for the formation of the ammonium:acetate interaction in pure water? Briefly
explain your answer.
Do not write below this line.
5
Name:
TF Name:
d. (6 points) A solution containing ammonium and acetic acid was analyzed using a titration
experiment in which small volumes of dilute NaOH were slowly added to the solution
while pH was monitored. (Please note that “acetate” is just the conjugate base of “acetic
acid.”) The resulting data are diagrammed below. Three points, A, B, and C, are
indicated (these points are only relevant to part e).
If the molar concentration of ammonium in the sample is double that of acetic acid, what
are the approximate pKa values of ammonium and acetic acid as indicated by these data?
Briefly explain your answer and specifically describe how the relative molar concentration
of ammonium and acetic acid can be used to determine your answer.
6
Name:
TF Name:
e. (6 points) Below are three statements that refer to points A, B, and C on the titration
curve shown in part (d). Circle the all of the points that are described by each statement,
keeping in mind that there may be more than one point described by each statement.
i.
A point at which pH equals the pKa
of one of the molecules in the solution.
Point A
Point B
Point C
ii.
A point at which ammonium
is predominately protonated.
Point A
Point B
Point C
Point A
Point B
Point C
iii. A point at which the system
is at equilibrium.
7
Name:
TF Name:
5. (14 points) Ethanol, the alcohol found in beer and wine, is metabolized into acetic acid by
two coupled reactions. Both reactions use NAD+/NADH (short for “nicotinamide adenine
dinucleotide,” a derivative of vitamin B3 that you will learn more about in later courses.
Nicotinamide adenine dinucleotide can exist in two forms: NAD+ or NADH). Both reactions
are described below, along with their ΔG°rxn values at 25°C. The net reaction, which
describes the sum of reactions 1 and 2, is also shown.
a. (6 points) Is the net reaction that converts ethanol into acetic acid, as shown above,
thermodynamically favorable? Briefly explain your answer.
Do not write below this line.
8
Name:
TF Name:
b. (4 points) On the axes below, draw a Gibbs free energy diagram for the net reaction that
graphs Gibbs free energy versus the ratio of products to reactants. Your curve must
correctly reflect the sign of ΔG°rxn and show the correct position of the equilibrium
relative to the point on the x-axis indicated as “50% reactants, 50% products.” The
magnitude of ΔG°rxn has be pre-drawn for you.
c.
(4 points) In much the same way that the cell keeps pH constant, NAD+ in the cell is
maintained at a much higher concentration than NADH. Briefly describe how this high
[NAD+]/[NADH] ratio affects the equilibrium ratio of ethanol to acetic acid in the cell.
Do not write below this line.
9
Name:
TF Name:
6. (14 points) Shown below is the next to last reaction in the series of reactions that the cell
uses to assemble the amino acid tryptophan. Keq at 25°C is shown.
a. (2 points) Write the expression for the equilibrium constant (K eq) in terms of the molar
concentrations of reactants and products.
b. (6 points) Calculate ΔG°rxn for this reaction. Show calculations to support your answer. (R
= 0.008314 kJ∙mol∙-1K-1 and T = 298.15 K)
c.
(6 points) Consider a state in which the concentrations of IGP, indole, and G3P are 3x10-5
M, 5x10-7 M, and 6x10-4 M, respectively. If the reaction were to proceed from this state,
would the concentration of indole increase, decrease, or stay the same? Briefly explain
your answer and show calculations to support your conclusion.
10