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Acid and Base Concepts
Exercises and solutions
Prof
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A
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An F- ion is small and singly charged
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Therefore, an I- ion is likely to be the more polarizable
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Cs+ is isoelectronic to Xe and Na+ is
isoelectronic to Ne
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According to Fajans’ rules,
small highly charged cations have polarizing ability
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Covalent vs
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Explain
each of the following observations:
(a) Ag2S is much less soluble than Ag2O
(b) Fe(OH)3 is much less soluble than Fe(OH)2
(c) FeS is much less soluble than Fe(OH)2
(d) Ag2S is much less soluble than AgCl
(a) S2- is much larger and softer than O2(b) Fe3+ has a larger charge (is harder) than Fe2+
(c) Fe2+ is borderline acid, S2- is much larger and softer than OH-  OH- interacts
more strongly with acids harder than Fe2+; the match between the soft S2- and the
borderline Fe2+ is more favourable
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Covalent vs
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Covalent vs
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Consider only the HSAB characteristics in your answers
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In the same fashion, it will react more readily with
S2- than with O2-, forming CuS in basic solutions of sulfide
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The product in
basic ammonia is Fe(OH)3 (approximately; the product is an ill-defined hydrated
Fe(III) oxide and hydroxide mixture)
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(c) Ag+ is a soft acid and is likely to combine more readily with PH3 than NH3
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Accounting for trends in aqua acid strength
Account for the following trend in acidity:
[Fe(OH2)6]2+ < [Fe(OH2)6]3+ < [Al(OH2)6]3+ ≈ [Hg(OH2)6]2+
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The weakest acid is the Fe2+
complex on account of its relatively large ionic radius and low charge
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The greater acidity of Al3+ can be
explained by its smaller radius
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This complex reflects the failure of an ionic model, because in the complex there is a
large transfer of positive charge to oxygen as a result of covalent bonding
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The order of increasing acidity is [Na(H2O)6]+ < [Mn(H2O)6]2+ < [Ni(H2O)6]2+ <
[Sc(H2O)6]3+
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Acidity
Which member of the following pairs is the stronger acid?
Give reasons for your choice
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(a) The Fe(III) complex, [Fe(OH2)6]3+, is the stronger acid
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(c) Si(OH)4, is more acidic
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(e) HMnO4 is the stronger acid
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Oxides
Arrange the oxides Al2O3, B2O3, BaO, CO2, Cl2O7, SO3 in order from the most acidic
through amphoteric to the most basic
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Acidity
The ions Na+ and Ag+ have similar radii
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Polyoxo species
When a pair of aqua cations forms an M–O–M bridge with the elimination of water,
what is the general rule for the change in charge per M atom on the ion?
Polycations formation reduces the average positive charge per central M atom by +1
per M
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Mercury(II) is a soft Lewis acid, and so it is found in nature only combined with soft
Lewis bases, the most common of which is S2–
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What does
this indicate about their hardness?
They are all hard acids
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We need to identify the electron pair acceptor (the acid) and the electron pair donor
(the base)
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Therefore BrF3
is a Lewis acid and F- is a Lewis base
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The net reaction is:
H– + H2O → H2 + OH–
If we think of this reaction as:
H– + H+:OH– → HH + :OH–
we see that H provides a lone pair and is therefore a Lewis base
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Identifying Lewis acids and bases
Identify the Lewis acids and bases in the reactions:
(a) FeCl3 + Cl-
→ FeCl4(b) I- + I2 → I3We need to identify the electron pair acceptor (the acid) and the electron pair donor
(the base)
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(b) acid is I2, base is I–
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The ions Fe3+, Ce3+, Al3+, Cr3+,
and V3+ precipitate as hydrous oxides
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Discuss these steps in terms of the
acidities of oxides
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Aluminium(III) oxide is amphoteric and
redissolves in alkaline solution to give aluminate ions, [Al(OH)4]-
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HSAB theory
For each metal ion listed, predict which of the ligands would give the more stable
complex and explain your choice:
(a) Cr3+ with NH3 or CO
(b) Hg2+ with (C2H5)2S or (C2H5)2O
(c) Zn2+ with H2NCH2CH2NH2 or H2NCH2CH2CH2NH2
(d) Ni0 with (C2H5)2O or PCl3
(e) Cd2+ with Br- or F(a) Cr3+ hard  more stable with harder NH3
(b) Hg2+ soft  more stable with softer (C2H5)2S
(c) Zn2+ intermediate  more stable with H2NCH2CH2NH2 due to 5-membered ring
formation, more stable than 6-membered ring with H2NCH2CH2CH2NH2
(d) Ni0 soft  more stable with PCl3
(e) Cd2+ soft  more stable with Br-

HSAB theory
For the following pairs of ligands, predict which would form the more stable complex
with the metal indicated:
(a) Cr3+ with C6H5-O- or ortho-O--C6H4-CHO
(b) Cu+ with (C2H5)3N or (C2H5)3P
(c) Zn2+ with CH3C(O)CH2C(O)CH3 or CH3C(O)CH2CH2C(O)CH3
(d) Cr3+ with NR3 or PR3 (R is an alkyl group)
(e) Ni0 with C5H5N or PR3
(a) Cr3+ hard  more stable with ortho-O--C6H4-CHO due to its chelating effect
(b) Cu+ soft  more stable with softer (C2H5)3P
(c) Zn2+ intermediate  more stable with CH3C(O)CH2C(O)CH3 due to 6-membered ring
formation, more stable than 7-membered ring with CH3C(O)CH2CH2C(O)CH3
(d) Cr3+ hard  more stable with NR3
(e) Ni0 soft  more stable with PR3

HSAB theory
Tell which of the following pairs of ligands form more stable complexes with a firstrow metal ion such as Cr3+:
(a) H2NCH2CH2NH2 or H2NCH2CH2CH2NH2
(b) (CH3)3N or H2NCH2CH2NH2
(c) H2PCH2CH2PH2 or H2NCH2CH2NH2
(d) CH3C(O)CH2C(O)CH3 or CH3C(O)CH2CH2C(O)CH3
(a)
(b)
(c)
(d)

H2NCH2CH2NH2
H2NCH2CH2NH2
H2NCH2CH2NH2
CH3C(O)CH2C(O)CH3



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