Download H,mR,cn,nii,f^{aq) + H,O(1)

83
7.1
(a)pir ^-logif.
K. = 10rPK.
or
For the first dissociation step: p ^ / 1 ) = 0.74
For the second dissociation step: ^Kp) = 6-49
.-. is: a ) = 10-0-^'*-0.18
.-. ii:(2)==l(H-''9=:3.24.;X^10"''
(b)
^flxO.iny^^m)
—
[HCrO4}-(aq) + H p ( l ) ^
7.2
[li^OJ-^Caq) + [HCrO,]-(aq)
[HgOJ^Caq) + [CrO,]2-(aq)
The structure of H / A i^ shown in 7.1. The 4 dissociation steps with associated
x)K^ values are:
H4P2O7(aq) + H2O(i)
[H3Pp,]iaq)+H2O(l)
[H,P2O,]2iaq) + H2O(l)
HO^^^^^
[HgOrCaq) + [H3PA]"(aq)
[H30]^(aq) + m^Vp,f-{^<\)
[H30r(aq) + [HPAP"(aq)
P^a(i)
vKJ2)
P^a(3)
OH
OH
OH
pK^l) = 1.0
(7.1)
vKj:Z) = 2.0
piC,(3) = 7.0
pit/4) = 9.0
These can be assigned on the basis that generally:
KSI)>KJ2)>KS3)>K14)
removal of H^ being increasingly more difficult as the negative charge on the anion
increases. The larger piiC,, the smaller K^. e.g. pif,(4) = 9.0 corresponds to K, - 1 x
10-^ while pKJil) = 1.0 corresponds to K^ = 0.1.
7.3
\
//
o- - H
5-F
Overall inductive effect
(7.2)
74
For CH3C0,H, pK, = 4.75. For CF3CO2H, pK^ = 0.23. A smaller pK^ corresponds
to a larger K^. since if, - 10-='^'. The greater acid strength of CF3CO2H can be
explained in terms of the inductive effect (7.2). A physicochemical interpretation
follows from studies of the temperature dependence of pii:, which show that for
dissociation ofHCO,H,CH3CO,H and Ca3C0,H,AH<' « 0. The variation in pi^,
arises from the variation in entropy of dissociation which becomes less negative
along the series CH3C0,H, HCO.H, CCl3C0,H, CF3C0,H. The withdrawal of
electrons away from the CO^- group of the anion results in less orientation of
surrounding solvent molecules.
(a) H2NCH2CH2NH2 is a Br0nsted base. The question asks about piC, values of the
conjugate add of H^NCH^CH^NH^, i.e. equilibria involving H^ loss from the
protonated base:
[H,mR,cn,nii,f^{aq) + H,O(1)
[H2NCH2CH2NH3]-^(aq) + [H30]naq)
p i t / i ) = 10.71
[H,NCH,CH,NH3]naq) + H p ( l ) - - H,NCH,CH,NH,(aq) + [H30]Xaq)^^
(b) The relationship between pK^ and pK^, or K^ and K^ is:
piC, + pir,-14.00
or
K,xK, = lO-^'-''^
96
Acids, bases and ions in aqueous solution
7.31
(a) The Li+ ion has the highest charge density of the group 1 M* ions because it is
the smallest metal ion in the group. Therefore the second coordination sphere is
heavily solvated; it contains « 20 Kp molecules even though the first coordination
sphere contains only 4 H2O molecules.
(b) The ligand in the question is drawn here as ligand 7.26. It is tridentate but the
cyclic structure restricts the ligand to binding to an octahedral metal centre in a
/(zc-arrangement. Thus, the expected structure for [Ru(7.26)2]^"^ is 7.27. Each
coordinated ligand in 7.27 forms 3 chelate rings. Therefore the complex cont.ains 6
chelate rings.
(c) The stability constant for the formation of [Au(CN)2]" refers to the equilibrium:
Au-^(aq) + 2[CN3-(aq)
[Au(CN),
K ^ 103^at298K
= ~(8.314xlO"^)(298)lnl0^^
(7.27)
= -222 kJ mol~^
This large, negative value of AG°(298 K) indicates that the formation of [AuCCN)^]from Au(I) and cyanide ion is thermodynamically very favourable.
The extraction of Au from its ores takes place using the reactions:
4Au + 8[CN]- + O2 + 2H2O ^ 4[Au(CN)2]- + 4[OH32[Au{CN)2]- -f Zn -^ [Zn(CN)J^- + 2Au
In the first reaction, Au(O) is oxidized to Au(I) and is stabilized in the form of the
Au(I) complex [AuCCN)^]". In this form, it is extracted from the ore. In the second
reaction, Zn is used to reduce Au(I) to Au(O); Zn is oxidized to Zn(n), the latter as
a cyano complex.
7.32
I
Deprotonated desfenioxamine can be represented as L^~ and has six hard O~donor
sites that are ideally suited to binding hard Fe^-^. The ligand has a flexible organic
backbone and can wrap around the metal ion forming a complex with three 5membered chelate rings involving three ONCO-unite. This can be represented below
and explains the origin of the term chelation therapy.
\
°
PeC"^
0
,
xr
IN
" connection into the backbone-of the ligand
W.
98
Acids, bases and ions in aqueous solution
bases' (HSAB) is useful for rationalizing the observed trends in eomplex
stabilities. In aqueous solution, complexes fonned between class (a), or
hard, metal ions (metal ions are Lewis acids) and ligands containing donor
atoms from groups 15 and 16 show the following trends in stabilities:
N » P > A s > S b
•
,.,.,.•
O » S > Se > Te
Class (b), or soft, metal ions prefer soft donor atoms, e.g. S rather than O,
and P rather than N. Of the metal ions given in the question, Gd^^ ajid_ Fe^-^
are hard aeids while Ag"^ is a soft metal ion. The pattern in values of log JTis
consistent with a favourable hard metal ion-hard ligand match, but a less
favourable soft metal ion-hard donor atom combination.
• [DTPA]5- is a chelating ligand, and thus is able to form particuarly stable
complexes with metal ions. The values of log K of 22.5 and 27.3 are large,
and illustrate the 'chelate effect'.
7.34
(a) Use of ^^ corresponds to an overall stability constant, in constrast to K^ which
refers to an indiviual step. This, log ^^ for [Pd(CN)j2~ refers to the process:
Pd2^(aq)+4[CN]-(aq) = = - EPd(CN)j2-(aq)
(b) Data available:
Pd2-(aq) + 4[CN]-(aq) = -
[Pd(CN)j2-(aq)
Pd(CN)2{s) + 2[CN]-(aq) ^ = - [Pd(CN)J2-(aq)
log ^, - 62.3
log K^ 20.%
The equilibrium that corresponds to K is:
Pd(CN)2(s) = - Pd2^(aq) + 2ECN]-(aq)
Constx-uct a Hess-type cycle that combines these reactions. In the cycle shown
below, single an'ows rather than equilibrium signs are used to give the cycle a
sense of'direction':
Pd2+(aq) + 4[CN]-(aq)
AG»,
pPd(CN)4]^-(a.q)
AC?,
Pd(CN)2(s)^-2[CNr(aq)
AG" -AG^-AG^
-RT In K^ = ~RT in K^ + RT In K^
]nK^ = \nK^~\nK^
log iCj-20.8 - 6 2 . 3 = -^1.5