15.7: Problems
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1. Classify each of the substances below as an acid or base, and as strong or weak or as a spectator in the solvent listed. For each case, write out the dissociation reaction.
(a) H
3
AsO
4
in water
(b) HCl in glacial acetic acid
(c) CH 3 COOH in liquid ammonia
(d) H 2 O in molten NaOH
(e) Cl - in liquid ammonia
2. Write out the dissociation reactions for an acid HA and its conjugate base A
-
in water. Use the equilibrium expressions for K
a
and K
b
to prove that \(K_{a}K_{b}=K_{w}\).
3. Calculate the percent ionization of 1 M acetic acid (K
a
= 1.8 x 10
-5
) in water.
4. For each pair of compounds below, indicate which one is a stronger acid and explain your choice:
(a) [Fe(H 2 O) 6 ] 3+ or [Fe(H 2 O) 6 ] 2+
(b) H 3 BO 3 or H 3 PO 3
(c) [Ga(H 2 O) 6 ] 3+ or [Al(H 2 O) 6 ] 3+
(d) HIO 3 or HIO 4
(e) H 3 PO 4 or H 2 SeO 4
5. The Mg 2 + and Cu 2 + cations have similar ionic radii, but the acidities of aqueous nitrate solutions containing the two ions are different. Explain which ion is more acidic and why.
6. Cobalt reacts with excess fluorine to make CoF 3 , but higher oxidation states of cobalt do not exist in binary fluorides. However, Co(IV) is stable in oxides such as CoO 2 and BaCoO 3 . Explain why these oxides can stabilize a higher oxidation state of cobalt than fluorides.
7. (a) When solutions of the hydrosulfide (SH - ) salts of As 3 + , Sb 3 + , and Sn 4 + are reacted with aqueous solutions of ammonium hydrosulfide (NH 4 + SH - ), the sulfide salts As 2 S 3 , Sb 2 S 3 , and SnS 2 precipitate. When excess aqueous Na 2 S is added, however, these sulfides re-dissolve to form soluble anionic complexes. In contrast, solutions of Cu 2 + , Pb 2 + , Hg 2 + , Bi 3 + and Cd 2 + precipitate as solid sulfides but do not re-dissolve in solutions that contain excess sulfide. We may consider the first group of ions to be amphoteric for soft acid-base reactions that involve SH - instead of OH - . The second group is more basic (less acidic) in the sense that it does not react with excess soft base. Use this information to locate the amphoteric diagonal in the periodic table for sulfides. Compare this with the diagonal that defines the amphoteric oxides, which dissolve in either acidic or basic solutions. Does your analysis agree with the description of S 2- as a softer base than O 2- ?
(b) Au + is also precipitated by SH - ions, and the sulfide Au 2 S redissolves to form a soluble complex in excess SH - . Does this fit the trend you discovered in part (a)? Does it make sense in terms of the electronegativity of Au? Explain.
8. Calculate the enthalpy for the adduct (CH 3 ) 3 B-N(CH 3 ) 3 and compare it to the measured value of -73.6 kJ/mol. Parameters can be found at ECW Model.
9. Calculate the enthalpy for the adduct formation for the Lewis acid (CH 3 ) 3 SnCl with each of the following Lewis bases (C 2 H 5 ) 2 O, (CH 2 ) 4 O, and CH 3 CN. Their respective measured enthalpies are -9.2, -21.3 and -20.1 kJ/mol. Explain the any differences between the calculated and measured enthalpies. Parameters can be found at ECW Model.
10. Refer to the Cramer-Bopp plot found at ECW Model and indicate the order of Lewis base strength for
(a) An acid with Ra = -0.5
(b) An acid with Ra = 0.33
(c) An acid with Ra = -0.9
11. The Lewis acid [Rh(CO) 2 Cl] 2 has a W value of -43.47 kJ/mol
(a) What does W refer to?
(b) The E, C, and W parameters found at ECW Model give the enthalpy for forming one mole adduct, that is, \(\ce{B + \frac{1}{2}[RH(CO)2Cl]2 -> B-RhCl(CO)2}\)
(1) What is the heat of dissociation of one mole of [Rh(CO) 2 Cl] 2 ?
(2) What is the enthalpy of \(\ce{2B + [Rh(CO)2Cl]2 -> 2B-RhCl(CO)2}\)
(3) Calculate the enthalpy per mol of B- RhCl(CO) 2 for C 5 H 5 N, (CH 3 ) 2 CO