Chm 1316 Honors Freshman Chemistry II Spring 2000
Exam 3 due 9 AM on 5 April 2000

Open Book & Handouts. Solutions here Neither Give Nor Receive Help!
  1. Ball lightning has almost an element of myth about it; it is so rare. However, qualified observers have witnessed these free floating electrical discharges, and even been "chased" by them. (When you run, the vortices creating in the air by your movement can push floating objects after you!)

    One such observer watched a ball lighting for a couple of minutes until it happened to float over to a metal bucket filled with 2 gallons of water. Its internal fields could not support themselves in the dead electrical short of the metal, and the ball exploded with a terrible bang. The observer reported that the water in the bucket, which had been at room temperature (25°C), was now boiling!

    What can we say about the minimum energy content (kJ) of that lightning ball? We can safely ignore the heat capacity of the bucket's metal, since it's swamped by water's 1.0 cal gm-1. Water's density is 1.0 g cm-3.

  2. Lead melts at 327°C consuming 5.86 cal gm-1 in the process. But before it can melt, it must warm from room temperature (25°) to its melting point, consuming about 0.0327 cal gm-1 °C-1 in the process. How fast (m/s) would a lead bullet have to be going to melt when it strikes an immovable object?

    Watch your units! And it doesn't matter how much the bullet weighs, so pick a convenient weight. Ignore the immovable object's heat absorption; it happens so fast, the lead hasn't an opportunity to share the wealth. Now you know why most meteors (50,000 m/s) burn up even in the atmosphere.

  3. The enthalpy of combustion of cinnamic aldehyde, C6H5-CH=CH-CHO, is found (via bomb calorimetry) to be -1,112.3 kcal/gm. Since the enthalpies of combustion of H2 and graphite are -285.9 kJ/mol and -393.5 kJ/mol, respectively, what's the standard enthalpy of formation of cinnamic aldehyde? (essence of cinnamon)

  4. But enthalpy of combustion wasn't what was measured in the bomb calorimeter for cinnamic aldehyde!

    1. What was? Why?

    2. What value (kcal/gm for comparison) for the quantity in (a) would have been obtained from the bomb?

  5. Iron(III) can be chelated by either one (hexadentate) EDTA or three (bidentate) ethylene diamines. Which reaction does entropy favor? Why? If the energetics were about the same, in what way would the complex equilibrium constants differ?

  6. When a mole of manganese(II) nitrate (anhydrate*) dissolves in 300 mol of water, the process is endothermic by 53.09 kJ at 25°C. Since all nitrates are highly soluble, that dissolution is spontaneous. That means that the entropy of solution can be no lower than what value (J mol-1 K-1)?

    * means no water of hydration in the solid

  7. Acetobacter sours wine (oxidizes ethanol to acetic acid) in order to fuel its own existence. Like the rest of Life, acetobacter must make ATP from ADP, a reaction whose Gibbs Free Energy is +30.5 kJ/mol. If acetobacter were 100% efficient at its task, how many moles of ADP could it convert to ATP for every mole of ethanol it oxidizes?

    Molecule Ethanol, C2H5OH Acetic Acid, CH3CO2H H2O CO2
    DGf°, kJ/mol -174.8 -392 -237.2 -394.4

  8. The heat of fusion of sodium metal is 3.05 kJ/mol, and its entropy of fusion is 8.25 J/mol K. What is sodium's melting point (°C)? (Liquid sodium is used as a coolant in some nuclear reactors.)

  9. Consider crystals of benzene at zero degrees Kelvin.

    1. What is the absolute entropy of such crystals (per mole)?

    2. If the benzene were monodeuterated (C6H5D), and there was no crystal positional preference for deuterium vs. hydrogen, what would the absolute entropy be (per mole)? [HINT: the position of the deuterium around the benzene ring makes this an entropy of mixing question.]

  10. Le Châtlier would have loved the (thankfully correct) version of

    ln K = - (DH°/R) (1 / T) + (DS°/R)

    because it vindicates what he said about temperature effects on equilibrium.

    1. What did he say?

    2. How is this expression consistent with that?

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