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Chemistry Equations

  1. Nov 5, 2006 #1
    I have a very difficult quiz that I need help with a few questions on. I'll list them below and if you can help with any of the bunch that would be extremely helpful! Thank you so much!

    1. Consider two aqueous (HO)2SO2 solns, (i) & (ii). For (i), [H3O+] = 2/3Mi. For (ii), [H3O+] = 5/3Mi. WITHOUT calculation of [H3O+], explain why (i) is more acidic (has a higher [H3O+]) than (ii).

    2. Can a saturated, buffered, aqueous solution of CaCO3 be prepared at a pH for which all forms of "carbonate" are present at appreciable concentration? Answer this by calculating the pH for this system at which ["H2CO3"]EQ = [CO32-]EQ. Show work and state your conclusion.

    3. If the relative humidity is 100% at a T>0°C but <100°C, H20(l) --> H20(g) exists because, at these conditions, VP(H20(l)) = P(H2O(g)) since ?Gf(H20(l)) = ?Gf(H20(g)).
    * * a. But, at these conditions, how does ?Hf(H20(l)) compare to?Hf(H20(g)), and how does ?Sf(H20(l)) compare to ?Sf(H20(g)) ? Why? (Write the rxn eqn corresponding to subscript "f" and then decide).

    * * b. And, at these conditions, how does ?Gf°(H2O(l)) compare to ?Gf° (H2O(g))? Why?

    4. For NO2(g) at 25C, ?Gf° =+51.0 kJ/mol. Tell why ?Gf° is such a large, positive number. To answer, consider ?Hf° and ?Sf° and their impacts on ?Gf°. Also consider Lewis structures and appropriate data. Finally is nitrogen useful as*fuel? Why or why not?

    5. Consider: C(graphite) +O2(g) --> CO2 (g).
    * * a. Use date from Thermo Props Table to calculate ?Gr°and K for this rxn at 25°C. Show all work.

    * * b. Based on your K, what chance is had of finding CO2 molecules in a classroom (volume is ~107 L) producible by this reaction if we had 106g (~one ton) of graphie in an open vessel? Defend your answer (Remember that air is ~20% oxygen and atmospheric pressure is ~1.0 atm. So, is there enough O2 in the classroom to effect this conversion? And, should we expect the conversion to happen from a thermodynamic standpoint?) Show all pertinent work.

    * * c. Would we really get any CO2 molecules from this experiment if we did it? Why or why not?

    * * d. From a thermodynamic standpoint, if we did this experiment with diamond rather than graphite we'd have an ever better chance of producing CO2 molecules. Why?

    6. For each of the following redox systems, write correct equations for the 1/2- rxns. Then combine the 1/2-rxns to get the net redox rxn.
    * * a. Ga(s) does not react in 9 M Na2SO4 but reacts readily in 9 M (HO)2SO2 giving SO2(g) and a hydrated, octahedral complex ion among the products.
    * * b. Sr(s) reacts vigorously in 1 M Na2SO4 giving a precipitate and a highly flammable gas among the products.

    * *c. Ca(s) reacts vigorously in 1 M KHCO3 and, among the products, gives a precipitate and the same highly flammable gas as in b., above. {Note: For this rxn, the source of the gas is different from b.,above. Also, be aware that incompatible products cannot be produced in appreciable quantities by this (or any) rxn if the products can be in contact with each other. Keep this in mind on deciding on the equation for the net redox rxn for this system. Hint: Think about B/L A/B chem.}

    7. Based solely on information from Q 6.,above, answer the following.
    * * a. Would Sr(s) be expected to react violently in 9 M (HO)2SO4? Provide* meaningful comment.

    * * b. How would the pH's compare for the resulting system of each of these redox rxns? Why?

    8. a. Based on periodic trend considerations, how should the water-solubility of the precipitate formed in Q 6. b., above, compare to the water-solubility of gypsum? Why? (Hint: Are Epsom salts water soluble? How about barium sulfate? So?)

    * *b. Again, with respect to Q 6.b., above, compared to Sr(s), would Mg(s) be expected to react more vigorously or less vigorously in 1 M Na2SO4? Why.

    9. Imagine that an electochemical cell which operates spontaneously is constructed by coupling the*HO-(aq)|O2(g)|H2O(l) 1/2- cell at pH 14.00 with the Ag(s)|Ag+(aq) 1/2-cell.
    * *a. Write the equation for the net cell rxn and calculate E°cell.

    * *b. What would happen to E°cell if each 1/2-cell compartment is treated with excess 6 M* * HCl? Why? (Hint: Could this treatment make something happen to system polarity?)

    * *c. Note that the anode material for the O2(g)-containing 1/2-cell has not been specified.* * * * * * * * * * *Suggest a material which could be used as this electrode and tell why the suggested material would be suitable.
  2. jcsd
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