CHEM 341
PHYSICAL CHEMISTRY
FINAL EXAM
Name_____________KEY____________________
ID#_______________________________________
Your name is worth 1 point, Merry Xmas.
Do not open this exam until told to do so.
The exam consists of 16 pages,
including this one. Count them to insure that they are all there. The last five pages of the exam are
the lists of constants and equations you were given previously. No additional notes are allowed. You
should only have your exam, writing implements and a calculator on your desk.
Do not write in the area below
Page
1
2-7
8
9
10
11
Total
Score
1/1
/99
/25
/25
/25
/25
/200
1
For each of the following problems, a right answer is worth 3 points, a
wrong answer results in -1 points and no answer is scored as a zero. You
do not need to justify your answer.
1) During the adiabatic expansion of an ideal gas against atmospheric pressure, the
internal energy will:
a) increase
b) decrease
c) stay the same
d) impossible to tell
2) During the isothermal expansion of an ideal gas, the internal energy will:
a) increase
b) decrease
c) stay the same
d) impossible to tell
3) During an exothermic chemical reaction inside a closed system the internal energy
will:
a) increase
b) decrease
c) stay the same
d) impossible to tell
4) The heat capacity at constant volume is ________ the heat capacity at constant
pressure for an ideal gas:
a) greater than
b) less than
c) the same as
d) cannot determine
5) The value of q for an isothermal ideal gas system when work is being done on that
system is:
a) positive
b) negative
c) zero
d) impossible to tell
6) The enthalpy change of a system at constant pressure when 1.00 J of heat is
transferred into it is:
a) 1.00 J
b) -1.00 J
c) 0.00 J
d) 8.31 J
2
7) When molecules which are used as the refrigerant in your refrigerator condense or
associate with each other, they will:
a) Give off heat
b) Take up heat
c) Neither give off nor take up heat
d) It depends on the temperature of the refrigerator.
8) The total entropy change for the spontaneous crystallization of sodium chloride from
a water/ethanol solution is:
a) positive
b) negative
c) zero
d) impossible to tell
9) For a constant pressure system, if the entropy change for the surroundings is negative,
then the process going on inside the system is:
a) isothermal
b) exothermic
c) endothermic
d) impossible to tell
10) When an open glass of water is cooled from 50 C to 20 C by putting the 50 C glass of
water in a bath that is at 20 C, the entropy change of the glass of water is:
a) positive
b) negative
c) zero
d) impossible to tell
11) For the last problem (10) the absolute value of the entropy change in the bath is
___________ the absolute value of the entropy change in the glass of water:
a) Greater than
b) Less than
c) The same as
d) impossible to tell
12) At constant pressure and temperature, if the total entropy change for the system and
surroundings during some process is positive then the free energy change for the
process is:
a) positive
b) negative
c) zero
d) impossible to tell
3
13) The total entropy change for the reversible expansion of an ideal gas is:
a) positive
b) negative
c) zero
d) impossible to tell
14) When I dissolve salt in liquid water, the chemical potential of the water will:
a) increase
b) decrease
c) stay the same
d) impossible to tell
15) At 0 C and 1 atm, the chemical potential of liquid water is ____________ the
chemical potential of ice:
a) greater than
b) less than
c) equal to
d) impossible to tell
16) Exerting pressure on ice causes it to melt because the molar volume of ice is
____________ the molar volume of liquid water.
a) greater than
b) less than
c) equal to
d) impossible to tell
17) Adding salt to a mixture of ice and liquid water causes the temperature of the mixture
to drop because:
a) it stabilizes the ice phase
b) it destabilizes the ice phase
c) it stabilizes the water phase
d) it destabilizes the water phase
18) A glass contains 1 mole of ice and 1 mole of liquid water at thermoequilibrium. After
adding 1 Joule of heat to the system the temperature will:
a) increase
b) decrease
c) stay the same
d) impossible to tell
19) The enthalpy change associated with mixing two different ideal gasses that start at the
same temperature and pressure is:
a) positive
b) negative
c) zero
d) impossible to tell
4
20) If I add salt to water, the vapor pressure of the water will:
a) increase
b) decrease
c) stay the same
d) impossible to tell
21) In a system with 5 phases that coexist at equilibrium, what is the minimum number of
components that must be present?
a) 3
b) 4
c) 5
d) cannot have 5 phases coexisting.
22) The reason why reactions come to an equilibrium point instead of going entirely to
products is:
a) The attractive and repulsive forces between molecules balance at exactly that
point.
b) The reaction is only endothermic at that point.
c) The reaction is only exothermic at that point.
d) The free energy change of mixing has a minimum when both reactant and
product are present.
23) When a system is at equilibrium, the reaction free energy is:
a) positive
b) negative
c) zero
d) impossible to tell
24) For a reaction AB with a standard reaction free energy of 1kJ/mole, the
equilibrium constant is.
a) greater than one
b) one
c) less than one
d) impossible to tell
25) For the same reaction as in problem 24, the standard reaction enthalpy is:
a) greater than zero
b) zero
c) less than zero
d) impossible to tell
5
26) One finds that the equilibrium constant for a particular reaction increases with
increasing temperature. Assuming that the standard reaction enthalpy and entropy are
temperature independent, this means that:
a) The standard reaction entropy is greater than zero
b) The standard reaction entropy is less than zero
c) The standard reaction enthalpy is greater than zero
d) The standard reaction enthalpy is less than zero
27) At 20 C, the concentration of OH- is 10-4 M. What is the pH?
a) 4
b) 10-4 M
c) 10-10M
d) 10
28) In a 0.1 M solution of NaCl in water, you expect the activity of Na+ to
be _________ 0.1
a) greater than
b) less than
c) equal to
d) impossible to tell
29) For the elementary reaction, A+BC, the units of the forward rate constant are:
a) S-1
b) MS-1
c) M-1S-1
d) M-2S-1
30) The energy per photon of red light is __________ that of blue light.
a) greater than
b) less than
c) equal to
d) impossible to tell
31) The most common reason why an absorbance spectrum of a molecule at room
temperature in solution has a width to it is:
a) The uncertainty principle
b) Light is not a single frequency -- it has some distribution of energies to it.
c) Absorbance spectrophotometers are not perfectly accurate.
d) Molecular motion in the ground and/or excited states.
6
32) The reason why metals are so shiny is:
a) Their hardness makes them easy to polish.
b) The electric field of the photon is forced to go to zero at the surface of the
metal, generating a wave in the opposite direction.
c) Because metals are so hard, photons undergo completely elastic collisions which
gives them an equal and opposite momentum.
d) Metals carry a large static charge at their surface that repels the photons.
33) Molecules generally fluoresce at wavelengths longer than they absorb because:
a) Relaxation among vibrational levels in the ground and excited states usually
results in loss of energy as heat.
b) Fluorescence normally occurs in two or more emission steps, each one with a
smaller energy than the original absorbance transition.
c) Fluorescence occurs from triplet states that are at lower energy than singlet states.
d) When the light bounces off a molecule it gives up some of its energy.
7
Each of the following numerical problems are worth 25 points. You
must show your reasoning for credit.
34) For a reversible, isothermal expansion of 1 mole of an ideal gas from 5 liters to 10
liters at a temperature of 25 C, what is the
a) internal energy change
Zero, the internal energy change of an ideal gas is zero if the temperature does not
change
b) enthalpy change for the system
Zero, the enthalpy change of an ideal gas is zero if the temperature does not change
c) work
w  nRT ln
VF
 1717 J
VI
d) heat
U  w  q  0
q   w  1717 J
e) entropy change for the system
S 
qrev
J
 5.763
T
K
8
35) A container with 10 moles of liquid water initially at 25 C is cooled to -25 C ice at
constant pressure. Calculate the
a) entropy change of the system
S sys  nC P ,liq ln
TF H fus
T
J

 n  MW H 2O C s ,ice ln F  319.7
TI
T fus
TI
K
b) enthalpy change of the system
H sys  nC p,liq T1  nH fus  nMWH 2O Cs.ice T2  87750J
c) q for the process
q  H  87750 J
9
36) The initial rate of B formation in the elementary reaction AB is 0.10 M/s. The
initial concentrations of A and B were [A0] = 1.00 M and [B0] = 0.00 M . The final
concentration of A (after equilibration) is 0.20 M.
a) What is the equilibrium constant for this reaction?
After equilibration, A = 0.20 M and B = 0.80 M Thus Keq = 4
b) What is the forward rate constant for this reaction?
M dB0

 k f  A0   k b B0   k f 1.00M 
s
dt
k f  0.10s 1
0.10
c) What is the backward rate constant for this reaction?
kf
kb
 K eq  4
k b  0.025s 1
10
37) There are a number of dye molecules whose absorbance changes radically with pH.
Let's consider a dye that is a weak acid and only absorbs at 500 nm in the protonated
form (AH + H2O  A- + H3O+, only AH absorbs light) . The dye has a pKA of 6.00
and the extinction coefficient of the dye at 500 nm is 5.00 x 103 M-1cm-1.
a) If the total concentration of the dye is 1.00 mM (i.e., [A-] + [AH] = 0.00100 M),
what is the absorbance of the dye solution at pH 5.5 in a 1 cm cuvette at 500 nm?
A   AH   0.001
A 
pH  pK  log


A
A   0.5
AH 

log
AH 
A   0.3162

AH 
AH   7.598 x10 4 M
Abs   cl  5 x10 3 M 1cm 1 7.598 x10  4 M 1cm   3.80
If 1 microwatt (10-6 Joules/second) of 500 nm light entered the cuvette described
above, how many photons per second would be transmitted through the cuvette?
1W  10 6
E
hc

J
s
 3.973 x10 19 J
photons
10 6 J / s
photons

 2.517 x1012
19
s
s
3.973 x10 J
Abs   log T
T  10  A  1.585 x10  4


# photons
transmitted  T 2.517 x1012  4.00 x10 8
s
11