Lattice hydroxide ions

The exciton reacts further instantaneously with a surface hydroxide ion and a Ti4+ ion of the lattice, forming a hydroxyl radical and a (formal) Ti3+ ion. 

A weakening of the critical metal-oxygen bonds occurs as a consequence of the protonation of the oxide ions neighboring a surface metal center and imparting charge to the surface of the mineral lattice. The concentration (activity) of D should reflect that three of such oxide or hydroxide ions have to be protonated. If there is a certain numer of surface-adsorbed (bound) protons whose concentration (mol nr2) is much lower than the density of surface sites, S (mol 2), the probability of finding a metal center surrounded with three protonated oxide or hydroxide ions is proportional to (CJ/S)3. Thus, as has been derived from lattice statistics by Wieland et al. (1988), the activity of D is related to (C )3, and the rate of proton-promoted dissolution, Rh (mol nrr2 lr1), is proportional to the third power of the surface protonation ... 

Estimate the standard enthalpy of hydration of the hydroxide ion. given that the lattice enthalpy of potassium hydroxide is —802 kJ mol 1 and its standard enthalpy of formation is —424.6 kJ mol . ... 

Although there is no space to develop a detailed discussion of the solubilities of compounds of the transition elements, the general insolubility of their + 2 and + 3 hydroxides is important. The rationale underlying their insolubility can be summarized (i) the hydroxide ion is relatively small (152 pm ionic radius) and the ions of the +2 and +3 transition metals assume a similar size if their radii are increased by 60-80 pm, and (ii) the enthalpy of hydration of the hydroxide ion (—519 kJ mol ) is sufficiently negative to represent a reasonable degree of competition with the metal ions for the available water molecules, thus preventing the metal ions from becoming fully hydrated. Such effects combine to allow the lattice enthalpies of the hydroxides to become dominant. 

By contrast, the chlorides of the metal ions are soluble because the chloride ion (181 pm ionic radius) is considerably larger than the hydroxide ion, and its enthalpy of hydration ( — 359 kJ mol l) is less negative than that of OH. This allows the metal cations to exert more nearly their full effect on the solvent molecules, thus overcoming the lattice enthalpy terms, and this leads to their general solubility as chlorides. 

Ultrafast proton transfer. The diffusion-controlled limit for second-order rate constants (Section A3) is 1010 M 1 s 1. In 1956, Eigen, who had developed new methods for studying very fast reactions, discovered that protons and hydroxide ions react much more rapidly when present in a lattice of ice than when in solution.138 He observed second-order rate constants of 1013 to 1014 M 1 s These represent rates almost as great as those of molecular vibration. For example, the frequency of vibration of the OH bond in water is about 1014 s . The latter can be deduced directly from the frequency of infrared light absorbed in exciting this vibration Frequency v equals wave number (3710 cm-1 for -OH stretching) times c, the velocity of light (3 x 1010 cm s ). 

In practice, a single crystal of europium-doped lanthanum fluoride (K,p = 2 X 10" ) responds in a matter of seconds according to (13-4) to fluoride ion activity in solutions containing fluoride from about 1 Af to 10" M. Europium doping increases fluoride mobility by introducing lattice disorder. Hydroxide ion, beginning at about 10" Af, is the principal interference, probably the result of competition due to the low solubility of lanthanum hydroxide. 

Chemically these catalysts contain metallic nickel along with 1-8% aluminum and up to 20% aluminum oxide or hydroxide. The surface is 50-100% metallic nickel which is present in an fee crystalline lattice, the same crystal orientation found for the bulk nickel. The use of low hydroxide ion concentrations and reaction temperatures in the reaction with the alloy gives catalysts containing more aluminum and aluminum oxide. The alumina in these preparations is occluded in the metallic skeleton and is difficult to remove even when later exposed to higher hydroxide concentrations. High temperature digestion is needed to remove all of the alumina from these catalysts. 

Although each of these ion-dipole bonds (Sec. 1.21) is weak, in the aggregate they supply a great deal of energy. (Wc should recall that the ion dipole bonds in hydrated sodium and chloride ions provide the energy for the breaking down of the sodium chloride crystalline lattice, a process which in the absence of water requires a temperature of 801. ) Just as a hydrogen ion is pulled out oj the molecule by a hydroxide ion, so a halide ion is pulled out by solvent molecules. 

For specificity, let us consider the application of these ideas to a-CraOa. Its crystal structure can be represented as a hexagonal close-packed lattice of oxide ions (the closed-packed layers of oxide ions alternate ababab. ..) in which two-thirds of the octahedral holes are filled with Cr3+ ions in a systematic fashion. Suppose the crystal to be cleaved in a close-packed plane in the presence of water. To preserve electrical neutrality, the oxide ions in this plane must be equally divided between the surfaces of the faces being formed. As a result, each Cr3+ in the layer below these oxide ions would be five-coordinate and in a square pyramidal configuration. Each ion would react with a molecule of water following which a proton would move from each adsorbed water molecule to an adjacent oxide ion. Thus, the outer face would consist of a close-packed layer of hydroxide ions. This is shown in Fig. 2. The basic point is that electrical neutrality and six-coordination can both be preserved by replacing what would be a plane of oxide ions in bulk by an equivalent plane of hydroxide ions at the surface. Similar ideas obtain on alumina. 

Phenolphthalein is thus colored red. However, the indicator cannot enter the crystal lattice of the solid acetate. Only when the salt is fused, do the ions become mobile and undergo the hydrolysis described by eqn. (1). Since the equilibrium constant is temperature-dependent, the hydroxide ion concentration increases with increasing temperature, so that the red coloration of the indicator increases. On cooling the original lattice is re-formed and the residue is colorless. 

Problem Saturated barium hydroxide solution ( baryta water ) reacts with sulfuric acid in two ways on the one hand, hydronium ions react with hydroxide ions to form water molecules. On the other hand, Ba2+(aq) ions combine with S042 (aq) ions to an ion lattice solid barium sulfate is deposited. If one follows this precipitation reaction with a conductivity tester, in this particular case the current strength reverts to a value of almost zero neither the formed water nor the deposited solid barium sulfate conducts the electricity. 

Even though students knew the formula aluminum hydroxide, some had no mental model of an ionic lattice of aluminum ions or hydroxide ions (see Fig. 

FIGURE 1.13 Schematic model for the active surface of the perovskite with an electrochemically active metal, M , anion lanthanide , and the lattice oxide o ion with hydroxide ions in solution and OH adsorbates at M. This figure is a simplified scheme of that in [23]. 

The physical basis of this change in bonding and structure is due to formation of hydrogen bonds, interaction with adjacent metal ions, and the influence of other lattice forces, especially those of a repulsive nature. In this context, hydrogen bonding, which is discussed in Sect. 6, is most relevant. The other features of importance are most appropriately studied on hydroxide ions, which do not act as hydrogen-bond donor groups. 

The crystal structure of Ni Ses quenched from 420 °C has been determined. " The selenium atoms form a zig-zag pattern, with the nickel atoms in deformed tetrahedral, octahedral, or pyramidal positions in the selenium lattice. AgaSe is formed during heating a suspension of Ag powder and selenium in alkaline solution. The temperature of reaction was found to decrease with increasing alkali concentration because of activation of selenium by hydroxide ion. The ternary selenides CsaPd3Se4, Rb,Pd3Se4, and KaPd3Se4 have been prepared by fusion of alkali-metal carbonates with palladium and selenium at 850 °C. 

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