Inorganic Chemistry
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Inorganic Chemistry


DisciplinaQuímica Inorgânica I3.902 materiais31.985 seguidores
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which requires only a small 
\u2022 vacancies or atoms missing from regular lattice positions; 
\u2022 interstitials or atoms in positions not normally occupied; 
\u2022 impurities either accidentally present or introduced as deliberate doping. 
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energy to escape into the CB of silicon. This is an n-type semiconductor. On the other hand, 
replacing an Si atom with Al gives a missing electron or \u2018hole\u2019, which may move in the VB giving a 
p-type semiconductor. Some other types of nonmetallic solid can be doped, especially compounds of 
transition metals, which have variable oxidation states. Thus slight reduction of TiO2 introduces 
electrons and gives n-type behavior. Similarly, oxidation of NiO removes some electrons and it 
becomes a p-type semiconductor. 
Instead of providing electrons, atoms in defect sites may themselves be mobile and thus provide 
ionic conduction in a solid. Ionic compounds such as NaCl have high conductivity in their molten 
form, and such conductivity is important for the manufacture of aluminum by electrolysis of molten 
cryolite (Na3AlF6). In most solids however, ionic conduction is much lower and arises largely from 
defects. Interstitial ions and vacancies in ionic compounds must occur in combinations that provide 
overall electrical neutrality. Two important combinations are Schottky defects where there is an 
equal concentration of anion and cation vacancies, and Frenkel defects where vacancies of one ion 
are balanced by interstitials of the same kind. For example NaCl has predominantly Schottky defects, 
and silver halides (AgCl and AgBr) mostly Ag+ Frenkel defects. Both interstitial ions and vacancies 
may be mobile and so contribute to ionic conduction. Doping with ions of different charge may 
change the defect concentrations and thus the conductivity. For example if AgBr is doped with a 
small concentration of CdBr2, each Cd
2+ replaces two Ag+ ions. The concentration of Ag+ vacancies 
is thereby increased and that of interstitials decreased. As the interstitials are more mobile than the 
vacancies in AgBr, the initial effect of doping is to decrease the ionic conductivity. However, as the 
concentration of Cd2+ is increased the vacancies become sufficiently numerous to dominate the 
conduction process, and so conductivity rises again. 
Some solids, known as fast ion conductors show a degree of ionic conduction which is 
comparable to that of the molten form, and which cannot be attributed to low concentrations of 
defects. For example above a transition temperature of 146°C, AgI adopts a structure with a body-
centered cubic array of I\u2212. The Ag+ ions move freely between a variety of sites where they have 
almost equal energy. One cannot think strictly of defects in a case like this, rather it is the absence of 
a unique ordered structure that gives rise to high ionic conductivity. Anions are mobile at 
temperatures well below the melting point in some compounds with the fluorite structure, such as 
PbF2 and ZrO2. The oxide ion conductivity of ZrO2 can be increased by doping with CaO or Y2O3. 
Thus, in Ca0.1Zr0.9O1.9 (consistent with the ionic charges Ca
2+, Zr4+ and O2\u2212) the ratio of anions to 
cations is less than the value 2:1 required for the normal ZrO2 lattice, so that oxygen vacancies are 
present. Doped ZrO2 is used as a \u2018solid electrolyte\u2019 in electrochemical sensors and in fuel cells. One 
important application is in sensors that measure the O2 concentration of exhaust gases from 
automobile engines, and is used in conjunction with \u2018catalytic converters\u2019 for removing pollutants 
(see Topic J5). Two platinum electrodes are placed on opposite faces of a sample. Oxygen gas reacts 
at one electrode according to 
 
Oxide ions pass through the solid and the reverse reaction occurs at the other electrode. A potential 
difference is developed between the two electrodes which depends on the ratio of O2 partial 
pressures on each side. 
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Section E\u2014 
Chemistry in solution 
E1 
SOLVENT TYPES AND PROPERTIES 
Polarity and solvation 
A solvent is a liquid medium in which dissolved substances are known as solutes. Solvents are 
useful for storing substances that would otherwise be in inconvenient states (e.g. gases) and for 
facilitating reactions that would otherwise be hard to carry out (e.g. ones involving solids, see Topic 
B6). The physical and chemical characteristics of a solvent are important in controlling what 
substances dissolve easily, and what types of reactions can be performed. The chemical as well as 
the physical state of solutes may be altered by interaction with the solvent. A list of useful solvents is 
given in Table 1. 
The most important physical property of a solvent is its polarity. Molecules with large dipole 
moments such as water and ammonia form polar solvents. The macroscopic manifestation is the 
dielectric constant (\u3b5r), the factor by which electrostatic forces are weakened in comparison with 
those in a vacuum (see Topic C10). For example, in water \u3b5r=82 at 25°C, and so attractive forces 
between anions and cations will be weaker by this factor. 
At a microscopic level, solutes in polar solvents undergo strong solvation. For example, the Born 
model predicts that the Gibbs free energy of an ion with charge q (in Coulombs) and radius r will be 
changed in the solvent compared with the gas phase by an amount 
 
(1) 
This estimate of the solvation energy is highly approximate, as it assumes that the solvent can be 
treated as a continuous dielectric medium on a microscopic scale. Nevertheless, it gives a rough 
Key Notes 
Polarity and 
solvation 
Strongly polar molecules form solvents with high dielectric constants that are good at 
solvating charged species. At a molecular level solvation involves specific donor-acceptor 
interactions and other types of intermolecular force. 
Donar and 
acceptor 
properties 
Most good solvents have donor (Lewis base) and acceptor (Lewis acid) properties, 
responsible for solvation and other chemical reactions. 
Ion-transfer 
solvents 
The solvent-system acid-base concept depends on the possibility of ion transfer from one 
solvent molecule to another. Protic solvents act as H+ donors and can support Brønsted acid-
base reactions. Oxide and halide ions may be transferred in other solvents. 
Related topics Inorganic reactions and synthesis (B6) Lewis acids and bases (C9) 
Molecules in condensed phases (C10) 
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guide that is useful in interpreting solubility trends (see Topic E4). 
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In reality, solvation involves donor-acceptor interactions, which may not be purely electrostatic in nature (see below), so that neutral molecules 
may also be strongly solvated. Solvent molecules are ordered round the solute, not only in the primary solvation sphere but (especially with ions) 
affecting more distant molecules. Solvation therefore produces a decrease in entropy, which can be substantial with small highly charged ions, and 
contributes to acid-base strength, complex formation and solubility trends (see Topics E2\u2013E4). 
Nonpolar solvents such as hexane have molecules with little or no dipole moment and low dielectric constants. They are generally better at 
dissolving nonpolar molecules and for carrying out reactions where no ions are involved. The molecules interact primarily through van der Waals\u2019 
forces (see Topic C10). Nonpolar media are generally poor solvents