Download IE EA

Chemistry 1311
1.
Problem Set 6
Which should be the most acidic ion and why? Ti3+, Ti2+, Fe3+, Fe2+
Fe3+ is most acidic because it has the largest charge to radius ratio, which results in the
strongest interaction with a base. One measure of the acidity of a metal ion is the pKa of
the hydrated ion; Fe(OH2)63+ is ionized to the greatest extent because the transfer of
electron density from oxygen to the metal results in increased polarization of the O-H bond
and increased ease of ionization.
2.
Which is the strongest acid in aqueous solution and why? HOCl, HOBr, HOI
HOCl is the strongest acid because of the greater electronegativity of chlorine.
3.
Describe the behavior of the oxides of the following elements. Al, S, Sr
Aluminum forms a largely ionic, amphoteric oxide, e.g., its oxide reacts with either acid or
base. Sulfur forms two stable oxides SO2 and SO3 ; both are acidic with the former
reacting with water to give sulfurous acid and the latter to give sulfuric acid. Sr as an
alkaline earth metal and forms an ionic, basic oxide.
4.
Which of the following has the highest proton affinity (is the strongest base)? Analyze the
reason for the difference in proton affinity in terms of a thermochemical cycle. NH21- or
PH21-.
Amide ion, NH2–, is significantly more basic than phosphide ion, PH2–. As shown by the
following thermochemical cycle the important factors that determine the relative proton
affinities are the E-H dissociation energy and the electron affinity of E.
Proton affinity, Ap, is defined as the energy associated with the
heterolytic cleavage of the E-H bond in the gas phase
H
+
+
E
-IE
H
+
-
-Ap
E-H
EA
E
-Bdiss
Ap = IE - EA + Bdiss
The more positive the bond dissociation energy, the greater the proton affinity and the
stronger the basicity of the anion. The more positive the electron affinity the smaller the
proton affinity and the weaker the basicity of the anion. To do the calculation exactly the
bond dissociation energies of the N-H and P-H bonds and the electron affinities of NH2
and PH2 are needed. However, if one assumes that the differences in these quantities are
about the same as the differences in the average bond dissociation energies (386 kJ mol-1
for NH3 and 322 kJ mol-1 for PH3) and electron affinities of the elements (-7 kJ mol-1 for N
and 72 kJ mol-1 for P) then it is possible to understand the reason for the stronger basicity
of NH2-1. (Note that these values are given in tables in the handouts.) From these data
one sees that since the difference in E-H bond strength is greater than the difference in
electron affinity, NH2 should be the stronger base.
5.
Of the following anions, which has the strongest conjugate acid? Why? ClO1-, ClO21-,
ClO31-, ClO41ClO4– is the weakest base because of the high formal oxidation state of the chlorine, which
withdraws electron density from the oxygens; therefore, it has the strongest conjugate acid.
6.
Which of the following anions is most basic, which is least basic? CH3–, C4-, F–, I–
The most basic ion is clearly C4-. Since methane is much less acidic than either HF or HI,
its conjugate base must necessarily be more basic. Obviously further deprotonation of
methane to give the methide anion produces an even more basic species. Iodide ion is the
least basic, which follows from the fact that its conjugate acid is more acidic than HF.
7.
Write a balanced chemical reaction that can account for the indicated observation or that
describes the chemical reaction that will occur when the indicated operation is performed.
Note that each of these reactions involves the reaction of an acid with a base.
a) Reaction of gaseous HF and gaseous SO3 combine to give a liquid that is a strong
Bronsted acid.
HF(g) + SO3 = HOSO2F
b) Iodine, a nonpolar substance is insoluble in water, but dissolves in aqueous I1- solution.
I–(aq) + I2(s) = I3–(aq)
c) Carbon dioxide dissolves in a large excess of aqueous OH1-.
CO2(g) + 2 OH–(aq) = CO32-(aq) + H2O
d) Addition of alumina, Al2O3, to a strongly basic aqueous solution results in a decrease in
the hydroxide concentration.
Al2O3(s) + 2 OH–(aq) + 3 H2O = 2 Al(OH)4–(aq)
e) Calcium oxide is treated with aqueous HCl.
CaO(s) + 2 H3O+(aq) = Ca2+ + 3 H2O(liq)
f) A gas is evolved when acetic acid (CH3CO2H) is added to a stirred suspension of lithium
deuteride (LiD) in THF (THF is an inert solvent)
LiD(s) + CH3CO2H = Li+(sol) + CH3CO2–(sol) + HD(g)
g) NaH, which is insoluble in THF (THF is an inert solvent) alone, dissolves in a solution of
triethylboron, B(CH2CH3)3, in THF.
NaH(s) + B(CH2CH3)3(sol) = Na+(sol) + HB(CH2CH3)3–(sol)
h) A gas is evolved when calcium phosphide (Ca3P2) is treated with water.
Ca3P2(s) + H2O = 3 Ca2+(aq) + 6 OH–(aq) + 2 PH3(g)
8.
Define solvent leveling of acid and base strengths and give an example of the leveling of
an acid or a base.
The strongest acid and base that can exist in a given solvent are the conjugate acid and
base of the solvent. For example in liquid ammonia the strongest acid that can exist is
NH4+ and the strongest base is NH2–. Although acetic acid is clearly a stronger acid than
NH4+ when acetic acid is dissolved in liquid ammonia the following reaction takes place:
CH3CO2H + NH3 = NH4+ + CH3CO2–. The acid that is present after this reaction is the
ammonium ion; the acidity of the stronger acetic acid has been “leveled” to that of the
ammonium ion.
9.
The anion [H3B-H-BH3]1- has an electron deficient B-H-B interaction. Draw the energy level
diagram for the three molecular orbitals that would be formed assuming interaction of two
B sp3 hybrid orbitals and a hydrogen 1s orbital and indicate the electron occupancy of each
orbital. Make a drawing of each molecular orbital and indicate whether it is bonding,
nonbonding or antibonding.
antibonding
B
nonbonding
H
bonding
10. Classify each of the following halides as (i) predominantly a Lewis acid, (ii) predominantly a
Lewis base, (iii) neither.
a) CF4 Neither; carbon is incapable of expanding its valence shell and fluorine is too
electronegative to effectively donate electron density to a base.
b) AgI Acidic; although halide is normally basic, when tied up with silver its basicity is
largely lost. Silver on the other hand will react with bases such as ammonia, which will
cause dissolution of the insoluble salt.
c) KF Basic; can act as a fluoride ion donor. Note the difference between this salt and AgI.
d) SiCl4 Acidic; silicon can expand its valence shell and bind a variety of bases including
chloride or pyridine.
e) PF3 Basic; however, because of the very electronegative fluorine substituents this
molecule is only a weak base toward protons but it is a strong ligand toward low-valent
metals.
f) SF6 Neither; the coordination number of six is rarely exceeded so that this molecule
does not act as a Lewis acid and the high electronegativity of fluorine does not allow for it
to act as a base.
g) PCl5 Acidic; this compound reacts with a wide variety of Lewis bases to form adducts.
h) (CH3)3N Basic; the lone pair on nitrogen is used to form bonds with a variety of Lewis
acids including PCl5 or SiCl4 above.
11. Dissect the following molecules or ions into the acid and base that could have been used in
their formation. H2CO3 (two answers), BH41-, (CH3)3NSO2, Ni(OH2)62+ (two answers)
H+, HCO3– or H2O, CO2; BH3, H–; SO2, N(CH3)3; H+, [Ni(OH2)5OH]+ or [Ni(OH2)5]2+, OH2