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A Química Inorgânica Ácidos e Bases

Por:   •  3/8/2021  •  Exam  •  2.029 Palavras (9 Páginas)  •  266 Visualizações

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Acids and Bases

5.2 Identify the conjugate bases of the following acids:

(a) [Co(NH3)5(OH2)]3+?

A conjugate base is a species with one fewer proton than the parent acid. Therefore, the conjugate base in this case is [Co(NH3)5(OH)]2+,

(b) HSO4-?

The conjugate base is S0 2-.

(c) CH3OH?

The conjugate base is CH30-.

(d) H2P04-?

The conjugate base is HP042-.

(e) Si(OH)4?

The conjugate base is SiO(OH)3.

(f) HS-?

The conjugate base is S2-.

5.3 Identify the conjugate acids of the following bases?

(a) C5H5N (pyridine)?

A conjugate acid is a species with one more proton than the parent base. Therefore, the conjugate acid in this case is the pyridinium ion, C5H6N+, shown below.

(b) HP042-?

The conjugate acid is H2P04-.

(c) 02-?

The conjugate acid is OH-.

(d) CH3COOH?

The conjugate acid is CH3C(OH)2+ shown above.

(e) [Co(CO)4]?

The conjugate acid is HCo(CO)4, shown below. A drawing of the HCo(CO) 4 molecule,

the conjugate acid of the tetrahedral Co(CO) - anion. The C atoms of the CO ligands are bound

to the Co atom. The O atoms are unshaded.

(f) CN-?

The conjugate acid is HCN.

5.4 List the bases HS-, F-, I-, and NH2- in order of increasing proton affinity?

You should make use of Table 5.1 to answer this question. The species with the greatest proton affinity will be the strongest base, and its conjugate acid will be the weakest acid. The weakest acid will have the smallest value of Ka (or the most positive value of pKa). Since Table 5.1 shows that HI is a stronger acid than HF which is a stronger acid than H2S, a partial order of proton affinity is I- <F- < HS-. Since NH3 is a very weak acid, NH - must be a very strong base.

Therefore, our final list, in order of increasing proton affinity, is I- <F- < HS- <NH2-.

5.5 Which bases are too strong or too weak to be studied experimentally?

(a) C032-, 02-, ClO4-, and N03- in water?

You can interpret the term “studied experimentally” to mean that the base in question exists in water (i.e. it is not completely protonated to its conjugate acid) and that the base in question can be partially protonated (i.e. it is not so weak that the strongest acid possible in water, H30+, will fail to produce a measurable amount of the conjugate acid). Using these criteria, the base C0 2- is of directly measurable base strength, since the equilibrium C032- + H20 ? HC0 - + OH- produces measurable amounts of reactants and products. The base 02-, on the other hand, is completely protonated in water to produce OH-, so the oxide ion is too strong to be studied experimentally in water. The bases Cl0 - and N0 - are conjugate bases of very strong acids, which are completely

4 3

deprotonated in water. Therefore, since it is not possible to protonate either perchlorate or nitrate ion in water, they are too weak to be studied experimentally.

(b) HSO4-, NO3-, and ClO4-, in H2SO4?

The hydrogen sulfate ion, HSO4-, is the strongest base possible in liquid sulfuric acid. However, since acids can protonate it, it is not too strong to be studied experimentally. Nitrate ion is a

weaker base than HSO4-, a consequence of the fact that its conjugate acid, HNO3, is a stronger acid than H2SO4. However, nitrate is not so weak that it cannot be protonated in sulfuric acid, so NO - is of directly measurable base strength in liquid H SO . On the other hand, ClO -, the

3 2 4 4

conjugate base of one of the strongest known acids, is so weak that it cannot be protonated in sulfuric acid, and hence cannot be studied in sulfuric acid.

5.7 Is the pKa for HAsO42- consistent with Pauling’s rules?

Pauling’s first rule for predicting the pKa of a mononuclear oxoacid is pKa approximately equals 8 - 5p (where p represents the number of oxo groups attached to the central element).

Since p = 1, the predicted value of pKa(1)for H3AsO4is: 8-(5x1)=3.

Pauling’s second rule for predicting the pKa of a mononuclear oxoacid is that successive pKa values for polyprotic acids increase by five units for each successive proton transfer. Since pKa(l) for H3AsO4 was predicted to be 3, the predicted value of pKa for HAsO 2- which is pKa(3) for H3AsO4, is 3 + (2 x 5) = 13. The actual value, which differs by 1.5 pKa units, is 11.5. This illustrates that Pauling’s rules are only approximate.

5.9 Which of the following is the stronger acid:

(a) [Fe(OH2)6]3+ or [Fe(OH2)62+?

The Fe(III) complex, [Fe(OH2)63+, is the stronger acid by virtue of the higher charge. The electrostatic parameter will be considerably higher for z = 3 than for z = 2. The minor decrease in the radius on going from the Fe(II) to the Fe(III) species will enhance the differences in for the two species.

(b) [Al(OH2)6]3+ or [Ga(OH2)6]3+?

In this case, z is the same but r + d is different. Since the ionic radius, r, is smaller for period 3 Al3+ than for period 4 Ga3+, r + d for [Al(OH2)6]3+ is smaller than r + d for [Ga(OH2)6]3+ and the aluminum-containing species is more acidic.

(c) Si(OH)4 or Ge(OH)4?

As in part (b), above, z is the same but the r + d parameter is different for these two compounds. The comparison here is also between species containing period 3 and period

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