Repulsion, between ions

The Orientation of Water Molecules Adjacent to an Ion. Order and Disorder in the Vicinity of Solute Particles. Coulomb Attraction and Repulsion between Ions. Activity Coefficients. The Distance of Closest Approach. Activity Coefficients of Various Solutes. Forces Superimposed on the Coulomb Forces. 

Sulfides play an important role in hydrotreating catalysis. Whereas oxides are ionic structures, in which cations and anions preferably surround each other to minimize the repulsion between ions of the same charge, sulfides have largely covalent bonds as a consequence there is no repulsion which prevents sulfur atoms forming mutual bonds and hence the crystal structures of sulfides differ, in general, greatly from those of oxides. 

In ionic solids, the lattice arrangements of cations and anions are more rigid. When an ionic solid is distorted, it is possible for cation-cation and anion-anion alignments to occur. However, this will cause the solid to shatter due to electrostatic repulsions between ions of like charge. 

The solubility of an ionic dye in water normally increases with temperature, since the enhanced mobility favours electrostatic repulsion between ions rather than closer approach to form aggregates by means of the short-range attractive forces. Addition of a simple inorganic electrolyte, on the other hand, normally lowers the solubility limit at a given temperature. Such additions enhance the ionic character of the aqueous phase and help to stabilise the structure of dye aggregates by forming an electrical double layer within the sheath of clustered water molecules around them. 

If the reactants are ionic with opposite charges, the rate constant can be greater than 1010 L mol -1 s-1 due to the favorable attractive forces. For example, the rate constant for the reaction of H+ with OH- in aqueous solutions at 25°C is 10n L mol-1 s-1. On the other hand, the electrostatic repulsion between ions of like sign can significantly slow their reaction. Similarly, if the reactants are polar molecules, electrostatic forces between them and the solvent may come into play. 

The hydrodynamic repulsion between ions is stronger as ions approach each other closely. Recombination is therefore appreciably less likely compared with the case of no hydrodynamic repulsion. Escape becomes slightly less probable compared with the no hydrodynamic repulsion case... 

Trends in Lattice Energy. We have seen that the lattice energy of ionic crystals is affected to some extent by the coordination numbers of the ions (Table 3.4) and by repulsion between ions in contact with each other (Eq. 3.14). These factors are, however, of minor importance when compared to the effect of ionic charge and ionic size. 

The reader is probably familiar with a simple picture of metallic bonding in which we imagine a lattice of cations M"+ studded in a sea of delocalised electrons, smeared out over the whole crystal. This model can rationalise such properties as malleability and ductility these require that layers of atoms can slide over one another without-undue repulsion. The sea of electrons acts like a lubricating fluid to shield the M"+ ions from each other. In contrast, distortion of an ionic structure will necessarily lead to increased repulsion between ions of like charge while deformation of a molecular crystal disrupts the Van der Waals forces that hold it together. It is also easy to visualise the electrical properties of metals in... 

For ionic crystals there is strong attraction between cations and anions, and strong repulsion between ions having the same charge. These interactions determine structures because ions must be shielded from those with the same charge. The relative sizes of the ions are important in determining the CN. Removal of electron(s) decreases the size of a cation relative to the atom and addition of electron(s) increases the size of an anion relative to the atom. Commonly, for an MX compound the anion is larger than the cation and the anions are close packed in crystals with cations in octahedral or tetrahedral sites. 

The simplest improvement is the mean spherical approximation model (Section 3.12), but a somewhat better version of this is what can be pictorially called the mound model, because instead of having an abrupt change from simple Coulomb attraction to total repulsion between ions of opposite sign when they meet, this model (Rasaiah and Friedman, 1968) allows for a softer collision before the plus infinity of the hard wall is met (Fig. 3.55). 

Ionic solids Remember that each ion in an ionic solid is surrounded by ions of opposite charge. The type of ions and the ratio of ions determine the structure of the lattice and the shape of the crystal. The network of attractions that extends throughout an ionic crystal gives these compounds their high melting points and hardness. Ionic crystals are strong, but brittle. When ionic crystals are struck, the cations and anions are shifted from their fixed positions. Repulsions between ions of like charge cause the crystal to shatter. 

The ions are assumed to be on their lattice sites with their formal charges, so that in NaCl, for example, we have an array of Na+ and Cl" ions. The net interaction can be obtained by summing the interactions over all the pairs of ions, including not only the attraction between Na+ and Cl but also the repulsion between ions of the same sign. The net interaction decreases with distance but slowly so that it is difficult to obtain an accurate value. 

Combining equations 5.13 and 5.15 gives an expression for the lattice energy that is based on an electrostatic model and takes into account Coulombic attractions, Coulombic repulsions and Born repulsions between ions in the crystal lattice. Equation 5.16 is the Born-Lande equation. 

When both reactants in a redox reaction are kinetically inert, electron transfer must take place by a tunnelling or outer-sphere mechanism. For a reaction such as 25.46, AG° 0, but activation energy is needed to overcome electrostatic repulsion between ions of like charge, to stretch or shorten bonds so that they are equivalent in the transition state (see below), and to alter the solvent sphere around each complex. 

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