Chm 1316 Honors Freshman Chemistry II
Spring 2001
Final 11 AM on 7 May 2001

Link to Solutions. Closed book; open mind. Open cheat sheets & handouts.
  1. (Dedicated to Miles and his disturbing fascination with weaponry.) In a "fuel-air" explosion, a combustible vapor (might be fuel droplets or a gas) is mixed rapidly and stoichiometrically with the oxygen of the air and detonated. To a first approximation, the reaction is over before the volume has changed (the hallmark of an explosion). The exothermicity has instantaneously changed the volume's temperature from its initial Ti to its final Tf while the stoichiometry has changed the number of moles of gas from ni to nf .

    The destructivity of such devices comes from the blast "overpressure" which is DP = Pf - Pi where the initial pressure is conveniently taken to be 1 atm (Pi = 1).

    Assuming ideal gases, derive an expression for overpressure in terms of the initial and final values of gas moles and temperatures.

  2. Dinitrogen diffuses 1.254 times faster than a gaseous alkane at the same T and P.

    1. Which alkane is it?

    2. How much faster would it diffuse than would its gaseous combustion product(s) at STP?

  3. In the equilibrium between nitrogen dioxide and its dimer, the Gibbs' Free Energies are DG°f [NO2(g)] = +52 kJ/mol and DG°f [N2O4(g)] = +98 kJ/mol.

    1. At what total pressure would the partial pressures of NO2(g) and N2O4(g) be the same?

    2. If that total pressure is increased, which molecule is favored by Le Châtlier?

    4. Lava; Microsoft Encarta 2001
  4. Hawaiian lava is molten basalt flowing freely to the sea at temperatures around 1000°C. If we assume that its specific heat is about that of sand, viz., 1.2 J g-1 K-1, how many tons of 25°C seawater is brought to its boiling point by every ton of lava?

    [Presume that seawater has the same specific heat (4.2 J g-1 K-1) and boiling point as pure water, even though we'll see that's not so later.]

  5. Strontium has a density of 2.6 g cm-3. It crystallizes as Fm3m, i.e., face-centered cubic. What would be the dimensions of its unit cell (in Å)?

  6. An alkaline battery runs a flashlight for 5.2 hours. The anode reaction is the oxidation of Zn to ZnO. If the battery begins with about 48 g of zinc (and it is the limiting reactant), how much current (amps) must the lamp be drawing?

  7. We use the Centigrade scale, dividing the temperature span between the freezing and boiling points of water into 100°C. But if technology had developed in the ocean (dolphin scientists?), that standard would have referred to seawater not fresh water! The ocean is 0.6 m in [NaCl] whose van't Hoff factor is about 1.3. Water's Kb = 0.51 °C/molal and Kf = 1.86 °C/molal. We can safely ignore all of seawater's other components.

    1. How many of our Centigrade degrees would have fit on the dolphin's thermometer (i.e., between the freezing and boiling points of seawater)?

    2. Find a simple formula to convert from °C to °D ("degrees Dolphin").
      [HINT: like °F = 1.8(°C) + 32.]

    8. Dolphin; Microsoft Encarta 2001
  8. Dolphins wouldn't measure humidity as we do either. They'd relate it to the saturation of air above seawater! On a typical 100% humidity (people measured) day in the Houston ship channel:

    1. Would the dolphins think the air's (dolphin measured) % humidity was higher or lower than 100%? Why?

    2. What value would it have numerically?

    Seawater is still 0.6 molal NaCl as it was in the last problem.
     

  9. At 15.0°C, the concentration of aqueous CO beneath one atmosphere of the gas is 1.16×10-3 M. But [CO(aq)] drops to 8.68×10-4 M at 35.0°C.

    1. Therefore is CO's enthalpy of solution exo- or endothermic, Le Châtlier? Why?

    2. If R = 8.314 J mol-1 K-1, what's the 25°C value of DH°solution[CO] in kJ/mol?

  10. That sacrificial Mg anode we talked about in class is so effective that it can even protect an active metal like Zn by the reaction:

    Mg(s) + Zn2+(aq) ® Mg2+(aq) + Zn(s)

    The thermodynamic properties of all those species are:

    Species DH°f kJ S° J/K DG°f kJ
    Zn(s) 0420
    Zn2+(aq) -153-110-108
    Mg(s) 0330
    Mg2+(aq) -467-137-416

    1. Calculate E° in Volts from the values in the table. [F = 96,485 C/mol.]

    2. What would E be if the concentrations were instead [Mg2+(aq)]=0.8 M and [Zn2+(aq)] = 1.2 M?

    3. What temperature for the cell in (b) would make its E return to the value of E°?

  11. The results of a rate experiment with the alkaline hydrolysis of ethyl nitrobenzoate (called A hereafter for brevity) are reported below. Find the order of the reaction with respect to [A] and the value of the rate constant (with proper units).

    t (s)0100200300400
    [A] M 0.050.03550.02750.02240.0190

  12. One of the fastest solution reactions known is the old standby:

    H3O+(aq) + OH-(aq) ® 2 H2O(liq)

    where k = 1.4×1011 M-1 s-1.

    Imagine an experiment where a strong monoprotic acid HA is instantaneously introduced into a neutral solution at a concentration of [HA]=10-4. Therefore, at time zero, [H+] is 10-4 M but [OH-] hasn't yet come to equilibrium and is still at 10-7 M.

    Since [H+] is in great excess, we can safely assume that it's fixed at that initial value, but we know that [OH-] will soon fall to it's correct equilibrium of 10-10 M. About how many microseconds will that take? (See? I told you it was fast!)

Comments to Chris Parr Return to CHM 1316 HomePage Last modified 7 May 2001.