Problem Set #1 – Chem 391
Name
th
Due in class on Thursday Sept. 8
A note: To help me read quickly, I structure the problem set like an exam. I have made small spaces for answers
where I’d like short answers. You won’t win friends by waxing eloquent. Pithy answers that get to the point quickly
are hugely preferred. As is legible handwriting.
1.
For the following processes, identify whether ∆G˚, ∆H˚ and ∆S˚ are positive (+), negative (-)
or about zero (~0) at the standard state and 298 K. Offer brief explanations.
Reaction
∆G˚
∆H˚
∆S˚
Explanation
N2(g) → 2 N(g)
2 NH3(aq) → NH3•NH3(aq)*
C6H12(aq) → C6H12(l)
* The formation of an H-bonded dimer of ammonia in water
2.
Ethanol (C2H5OH) is highly soluble in water but propane (C3H8) is fairly insoluble, even
though the molecules are about the same size. Why? Compare:
ΔH°
(kcal/mol)
ΔS°
(cal/molK)
∆G˚
(kcal/mol)
C2H5OH(l) → C2H5OH(aq)
-2.9
+3.8
-4.0
C3H8(l) → C3H8(aq)
-1.5
-13.3
+2.6
a.
Does ethanol dissolve spontaneously in water (at the standard state)? How do you know?
Explain the values of ∆H˚ and ∆S˚ associated with dissolution.
b.
Does propane dissolve spontaneously in water (at the standard state)? Why? Explain the
values of ∆H˚ and ∆S˚ associated with dissolution.
3.
Calculate K and ∆G˚ at 25˚C for the following, make-believe equilibrium conditions in a
reaction where A(aq) + B(aq) ⇔ C(aq). See if you can avoid using a calculator.
[A]
[B]
[C]
0.5 mM
2 mM
1 mM
1 µM
1 nM
1 mM
1 mM
1 µM
1 pM
4.
K
∆G˚
(kcal/mol)
Consider molecules A & B below.
A.
H
B.
N
H
O
HO
O
N
N
H
H
H
N
N
H
N
N
a.
Identify heteroatoms (N & O) that can act as H-bond donors by circling them. If they can
act as acceptors, draw a square around them. If they can both donate and accept, draw a diamond.
b.
Redraw A & B and orient them so
that each molecule is both donating an Hbond to and accepting an H-bond from the
other. Be careful! Make sure that the H-bond
has a roughly 180˚ angle about the hydrogen
atom. Note that there are multiple ways to
achieve this.
c.
Sketch a scheme in which molecule B
accepts two hydrogen bonds from A. Explain
why it is a bad H-bonding scheme. (new for
next year)
5. Draw the structure of the peptide WHISPER, in the dominant ionization state for pH 8.0.
6.
Fatty acids are amphiphilic molecules possessing a polar “head group” and a non-polar
“tail”. Most are sparingly soluble in water as individual molecules and above a particular
concentration (the cmc, critical micelle concentration) they aggregate to form micelles – spherical
aggregates of a few dozen to a few hundred molecules where the tails are buried from solution and
the heads are exposed (see picture).
a.
The cmc’s of several fatty acids are given above. Given the values of cmc listed above, how
does the free energy of micelle formation change with increasing number of carbons in the tail?
Explain briefly.
b.
Consider the enthalpic issues attached to aggregation and briefly state whether you expect
them to be favorable or unfavorable:
i.
∆H for transfer of tail from water to micelle interior:
ii.
∆H for transfer of head group from water to micelle surface.
c.
What entropic considerations are responsible for that trend? Briefly discuss each of the
following concerns and ID the one that dominates.
i.
Conformational entropy of the chain:
ii.
Entropy of aggregation
iii.
Solvent entropy
d.
Given you analysis of enthalpic and entropic contributions to micelle formation, briefly
identify the contribution that you think dominates the trend in free energy of micelle formation.
8.
Rebecca performed her senior thesis on a molecule that would use a hydrogen bond
between carbon and oxygen (that is C-H•••O; carbon is the donor, oxygen the acceptor). I used to
argue with Alan about this a lot. The following plots show the geometry of O-H•••O vs. C-H•••O
interactions. http://www.icmr.ucsb.edu/programs/archive/documents/Desiraju2.pdf
a.
Why might C-H typically be considered a poor H-bond donor (no reference to above plots
necessary)?
b.
Assume a simple vdW contact between a O-H group and a carbonyl oxygen (see handout
from lecture 1). What would the predicted O to O distance be? Compare to the plots above. Is
anything unusual happening?
c.
Assume a simple vdW contact between a C-H group and a carbonyl oxygen. What would
the predicted C to O distance be? Compare to the plots above. Is anything unusual happening?
d.
As I stressed in lecture, H-bonds between O/N donors/acceptors impose significant
stereochemical restraints on the geometry of intermolecular interactions. How is that made visible in
the OH•••O=C plot? Would that be true for C-H•••O=C bonds?