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Questions found on previous Second Hour Exams; the answers are in curly brackets.
D stands for delta; for example, DH represents delta H.
1(15)- When the temperature is increased from 298 K to 310 K, the equilibrium constant for a particular reaction doubles. Calculate the value of DHo for the reaction. {44.4 kJ/mol}
2(15)- Calculate DG for the the compression of 2.00 mol of CO2(g) from 0.100 atm to 200. atm at 400 K,
a) assuming CO2 behaves ideally. {50.6 kJ}
b) using the fact that, for CO2, Pc = 72.9 atm and Tc = 304 K. {(gamma = .72, 48.4 kJ}
3(25)- The value of the Henry's Law constant for CO2 in water at 298 K is k' = 1670 bar.
a) Calculate the mole fraction of CO2 in water if the pressure of CO2 is 0.10 bar.
{.0000599}
b) Calculate D_Gmixing for the solution in a). If you couldn't do (a), assume the mole fraction of CO2 was 1.0x10-3. {-1.59, or -19.6 J}
c) What assumptions did you have to make about the solution to do problem 3?
{Ideal dilute solution}
4(20)- Consider the reaction
H2(g, P = 0.1 bar) + Cl2(g, P = 100 bar) = 2 HCl(aq, 0.010 mol/kg) Eo = 1.615 volts
If the standard reduction potential of Cl2 is 1.360 volts, and (gamma(0.010 molal HCl) = 0.905, calculate the fugacity coefficient of Cl2. If necessary, I'll sell you the standard reduction potential of H+aq). {gamma = 0.276}
5(25)- Consider the following set of concentrations vs. time, for the reaction
H2O2(g) = 2 OH(g)
time, minutes 0 10 20 30 40 50 60 70
[A], moles/L 1.8 .98 .68 .51 .42 .35 .30 .26
a) Predict the order of the reaction. Explain. {second order}
b) Confirm your prediction. {k = 0.046 M-1min-1 (twice)}
c) What is the rate constant for the reaction? { " }
d) Propose a rate determining step for the reaction. { 2 H2O2 = H2O2 + H2O2*}
e) Propose an entire mechanism for the reaction, using your RDS in d). {RDS, then
{H2O*2 = 2 OH}
Spring, 1992
R = 8.315 J/mol-K F = 96,487 coulombs/mole kB = 1.38x10-23 J/K
1(32) - At 25oC, the vapor pressure of pure bromine is 67.9 torr, and the vapor pressure of pure CCl4 is 33.9 torr. A solution with XBr2 = 0.100 has PBr2 = 1.35 torr and PCCl4 = 33.0 torr.
a) Calculate DG when 10.0 moles of bromine are mixed with 90.0 moles of CCl4. {-103 kJ}
b) Calculate both activity coefficients for the solution in a). {1.08, 0.199}
c) Would you expect DH for the process in a) to be positive, negative, or zero? Why? {damfino}
d) If, for CCl4, the cryoscopic constant is -4.0 oC/molal solution, and the normal freezing point is -23.0 oC, calculate the freezing point of the solution in a).{-25.9 oC}
e) Estimate the molar heat of fusion of CCl4. {20.0 kJ/mol}
Kf = -R(To)2(MWA)/(DHfus.1000g/kg)
2(20) - Consider the reaction Cu2S = 2 Cu+(aq) + S2-(aq) Ksp = 1.6x10-48
a) Calculate the solubility of Cu2S in water. {7.4x10-17 M}
b) Calculate the solubility of Cu2S in 0.10 M NaNO3. {1.6x10-16 M}
c) Calculate DGo for the reaction at 25oC. {273 kJ/mol}
3(16) - Two compartments are separated by a membrane that is permeable to Na+ and Cl-, but
not to a protein, P+10. Initially, the right compartment contains 0.0100 M NaCl, and the left
compartment contains 0.0100 M NaCl and 0.0010 M PCl10, which ionizes completely in water.
a) Calculate the concentration of Cl- in each compartment at equilibrium.{.0080,.012}
b) Use the equilibrium concentrations of Cl- to calculate the Nernst potential,
Mright - Mleft, at 25oC. {-10.4 mV}
4(10) - A cell has Eo = 0.200 volts at 25.00oC and Eo = 0.210 volts at 30.00oC.
Estimate DS for the cell reaction if three moles of electrons are transferred. {580J/K}
5(14) - The first vibrational excited state of Br2(g) is 400 cm-1 (8.0x10-21 J) above the ground
state. Calculate the ratio of the number of molecules in the first excited state, compared to the
number of molecules in the ground state, (N1/N0), at 25oC.{0.143}
Second Hour Exam Spring, 1994 F = 96,487 coulombs/mol
1(18) - As everyone who has had general chemistry knows, at 25 oC,
H2O(l) W H+(aq) + OH-(aq), Kw = 1.0x10-14
We also showed that, at 100 oC, Kw = 9.2x10-13.
a) Calculate DHorxn. {55.8 kJ/mol}
b) Calculate DSorxn. {-81.0 J/mol}
c) Is the sign of DSorxn intuitively correct? That is, did it have the sign you expected? Why or why
not? (If you couldn't do (b), what you think the sign should be, and why?)
2(20) - a) Show that (dS/dV)T = (dP/dT)V. Start with the appropriate state function, not its
differential.
b) For a Van der Waals gas, (P + an2/V2)(V - nb) = nRT.
Derive the expression for (dS/dV)T for this gas. {nR/(V - nb)}
c) Show that DS = nRln{(V2 - nb)/(V1 - nb)}
3(18) - a) Why are we justified in using mu = muo + RTln(PA/PoA) for solvents, rather than mu =
muA + RTln(fA/foA)?
b) Consider EtOH, dissolved in acetone. If the mole fraction of EtOH is 0.20, and gammaEtOH =
1.20,
i) calculate the activity of ETOH. {0.24}
ii) calculate the vapor pressure of EtOH, if the vapor pressure of pure EtOH is 32.0 torr. {7.68
torr}
4(14) - a) The ratio of the concentration of K+ inside a cell to that outside a cell is 20/1. Calculate
Einside - Eoutside at 37 oC. {- 0.0800 V}
b) If Ein - Eout = - 0.020 volts, was K+ actively transported into or out of the cell? Explain. {into}
5(10) - Derive the Nernst equation, starting with two equations for DGrxn.
Chemistry 142 Second Hour Exam Spring, 1995
1(23)- Consider one of my favorite reactions:
H2O(l) = H+(aq) + OH-(aq) Kw = 1.0x10-14 at 298 K.
Also, DHo = 55.8 kJ/mol, and can be presumed to be constant.
(6) a) Calculate the equilibrium constant at 310 K. {2.4x10-14}
(4) b) Calculate the pH at 310 K. {6.81}
(8) c) Calculate DSo for the reaction. {-80.8 J/K}
(5) d) Rationalize the sign of DSo. (Explain why it has the sign that it does. If you couldn't do c),
assume that DSo is negative.)
2(23) The vapor pressures of pure methanol and pure water at 21 oC are 100 and 18.7 torr,
respectively. The vapor over a solution containing 48.0 g CH3OH and 32.4 g H2O has
P(CH3OH) = 40.2 torr and P(H2O) = 9.21 torr.
(10) a) Calculate the activity and activity coefficient for each component in the solution.
{.493,.402,.904,.884}
(8) b) Calculate DGmixing for this (non-ideal!) solution. {- 6542 J}
(5) c) Do you expect the mixing process for this solution to be exothermic or endothermic?
Explain. {Exo}
3(10) - The molar conductivity at infinite dilution, (Lambda)o, for potassium chloride is 149.9
cm2/mol-ohm. If the ionic mobilities of K+ and Cl- are 0.000762 and 0.000791 cm2/volt-sec
respectively, calculate the individual molar conductivity at infinite dilution of each ion.
{for K+, 73.6; for Cl-, 76.3 cm2/mol-ohm}
4(25) - Consider the reduction of acetaldehyde to ethanol (yay!) by NADH:
CH3CHO(aq) + NADH(aq) + H+(aq) = C2H5OH(aq) + NAD+(aq)
At 298 K, Eo = 0.330 volts, and at 310 K, Eo = 0.333 volts.
(10) a) Calculate an approximate value of DSo for the reaction. {50 J/K}
(8) b) If all solutes except H+ are at a concentration of 0.0010 molar, and the pH is 7.0,
calculate E at 298 K. {0.123 V}
(6) c) Technically, the constant "0.0592 volts" in the Nernst equation is incorrect at temperatures
differing from 298 K. What would be a more accurate constant at 310K? {0.0616 volts}
5(20) - Consider the isomerization of cyclopropane: cyclo-C3H6(g) 6 CH3CH=CH2(g)
time, minutes 0 10 20 30 40 50 60
[cyclo-C3H6]/10-3M 1.00 0.67 0.50 0.40 0.33 0.28 0.25
(6) a) Determine the order of the reaction with respect to cyclopropane. {second}
(4) b) What is the rate constant? {50 L/mol-min}
(5) c) Propose a Rate Determining Step for the reaction. {2 C3H6 = C3H6* + C3H6}
(5) d) Propose a mechanism for the reaction. {The RDS, followed by C3H6* = C3-CH=CH2
Second Hour Exam Spring, 1996 F = 96,487 coulombs/mol
1(25) - Let's see how water likes having some sucrose mixed in with it. At 20oC, the vapor
pressure of pure water is 17.54 torr. After we toss 0.400 moles of sugar into 100 grams of water,
the vapor pressure of the solution is 15.89 torr.
a) Calculate the activity of water in the solution. {0.9059}
b) Calculate the activity coefficient of water in the solution. {0.971}
c) Calculate DGmixing for water. {-13.5 kJ}
d) Would you guess that the heat of solution of sugar in water would be positive or negative?
Why? {negative}
e) Should you have any reservations about your answer in d)? Why? {Sure!}
2(14) - a) What fraction of the total current is carried by Na+ when a current flows through an
aqueous solution of NaBr at 25oC? The limiting molar ionic conductivity of Na+(aq) and Br-(aq)
are 50.1 S cm2/mol and 78.1 S cm2/mol, respectively.{.391}
b) Would the conductivity of a NaBr solution go up or down if the concentration of the salt were
increased? Why? {up}
c) Would the molar conducivity of the solution increase or decrease if the concentration were
increased? What evidence do you have/remember? {decrease}
4(46) - Consider the reaction, at 25oC.
2 Fe(s) + 3 Co2+(aq) = 2 Fe3+(aq) + 3 Co(s)
If the standard potential for the reaction is Eo = 0.1955 + 0.00015(T) volts,
a) Calculate the standard potential for the cell. {0.240 V}
b) Calculate the Standard Reduction Potential (SRP) for cobalt if the SRP
for iron(III) is -0.0402 volts. {0.200 V}
c) Calculate DGo for the reaction. {-139 kJ}
d) Calculate DSo for the reaction. {86.9 J/K}
e) Calculate E for the reaction in a solution of 0.10 molar Fe(NO3)3 and 0.010 molar
Co(NO3)2, assuming ideality. {0.279 V}
f) Calculate the ionic strength of the solution. {0.063}
g) Using the DHLL equation, calculate the ionic activity coefficient of each metal ion.
{0.414 for Fe(III), 0.444 for Co(II)}
h) Assuming the results from (g) were (gamma = 0.50, calculate the voltage of the rxn. {.243V}
1(15)- The change in Gibbs Free Energy when 2.00 moles of O2 is compressed from 0.100 bar
to 200. bar at 298 K is 36.6 kJ.
(10) a) Calculate the fugacity of oxygen at 200 bar, 298 K. {161 atm}
(5) b) Calculate the fugacity coefficient of oxygen at these conditions. {.806}
2(27)- The vapor pressure of pure CCl4 is 39.345 kPa at 20 oC. When 1.00 mol CCl4 is mixed
with 4.00 mol C3H8O (acetone), the activity coefficient of CCl4 is 1.10.
(9) a) Calculate the activity of CCl4. {.220}
(9) b) Calculate the vapor pressure of CCl4. {8.66kPa}
(9) c) Calculate the change in Gibbs Free Energy of CCl4 when it was mixed with acetone.
{-3690 J}
3(58)- Consider the reaction at 298 K:
2 AgCl(s) + Cu(s) = 2 Ag(s) + Cu2+(aq) + 2 Cl-(aq) DHorxn = - 16.41 kJ/mol
Here are some useful data:
AgCl(s) + e- = Ag(s) + Cl-(aq) Eo = 0.2224 Volts
Cu2+(aq) + 2 e- = Cu(s) Eo = 0.337 Volts
(8) a) Calculate Eorxn {- 0.115 V}
(8) b) Calculate the equilibrium constant for the reaction. {.000129}
(8) c) Calculate DSo for the reaction. {-130J/mol-K}
(10) d) Use your answer in (c) to estimate the value of Eorxn at 350 K. {-0.150 V}
(If you couldn't do (c), pretend )So = - 120 J/K)
e) Let [CuCl2] = 0.010 M
(8) i) Calculate Erxn at 298 K, assuming all activity coefficients are unity. {0.045 V}
(8) ii) Calculate the mean ionic activity coefficient of the 0.010 M CuCl2 solution.(.666)
(8) iii) Calculate Erxn at 298 K, using your answer in (ii). {0.061 V}
As usual, if you need a specific equation, just ask. Also, if you need an answer to an early question
in order to proceed, either give a good guess (and indicate that's what you're doing), or ask for
one.
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