Ionic bond dissociation

The electrochemical behaviour of the compounds containing bonds between silicon and other group-14-metals is also interesting. Mochida et al. reported the electrochemical oxidation potentials of group-14-dimetals [66], As shown in Table 8, there is a good correlation between the oxidation potentials and the ionization potentials which decrease in the order Si-Si > Si-Ge > Ge-Ge > Si-Sn > Ge-Sn > Sn-Sn in accord with the metal-metal ionic bond dissociation energy. 

We have seen that sodium chloride crystals consist of sodium and chloride ions dissociation held together by ionic bonds. Dissociation is the process by which the ions of a salt separate as the salt dissolves. When placed in water, the sodium and chloride ions are attracted by the polar water molecules, which surround each ion as it dissolves. In water, the salt dissociates, forming hydrated sodium and chloride ions (see Figure 15.2). The sodium and chloride ions in solution are surrounded by a specific number of water molecules and have less attraction for each other than they had in the crystalline state. The equation representing this dissociation is... 

Consider now the behaviour of the HF wave function 0 (eq. (4.18)) as the distance between the two nuclei is increased toward infinity. Since the HF wave function is an equal mixture of ionic and covalent terms, the dissociation limit is 50% H+H " and 50% H H. In the gas phase all bonds dissociate homolytically, and the ionic contribution should be 0%. The HF dissociation energy is therefore much too high. This is a general problem of RHF type wave functions, the constraint of doubly occupied MOs is inconsistent with breaking bonds to produce radicals. In order for an RHF wave function to dissociate correctly, an even-electron molecule must break into two even-electron fragments, each being in the lowest electronic state. Furthermore, the orbital symmetries must match. There are only a few covalently bonded systems which obey these requirements (the simplest example is HHe+). The wrong dissociation limit for RHF wave functions has several consequences. 

For a given molecule and a given intemuclear separation a would have a definite value, such as to make the energy level for P+ lie as low as possible. If a happens to be nearly 1 for the equilibrium state of the molecule, it would be convenient to say that the bond is an electron-pair bond if a is nearly zero, it could be called an ionic bond. This definition is somewhat unsatisfactory in that it does not depend on easily observable quantities. For example, a compound which is ionic by the above definition might dissociate adiabatically into neutral atoms, the value of a changing from nearly zero to unity as the nuclei separate, and it would do this in case the electron affinity of X were less than the ionization potential of M. HF is an example of such a compound. There is evidence, given bdow, that the normal molecule approximates an ionic compound yet it would dissociate adiabatically into neutral F and H.13... 

Polyelectrolyte complexes composed of various weight ratios of chitosan and hyaluronic acid were found to swell rapidly, reaching equilibrium within 30 min, and exhibited relatively high swelling ratios of 250-325% at room temperature. The swelling ratio increased when the pH of the buffer was below pH 6, as a result of the dissociation of the ionic bonds, and with increments of temperature. Therefore, the swelling ratios of the films were pH-and temperature-dependent. The amount of free water in the complex films increased with increasing chitosan content up to 64% free water, with an additional bound-water content of over 12% [29]. 

The ions that conduct the electrical current can result from a couple of sources. They may result from the dissociation of an ionically bonded substance (a salt). If sodium chloride (NaCl) is dissolved in water, it dissociates into the sodium cation (Na+) and the chloride anion (CL). But certain covalently bonded substances may also produce ions if dissolved in water, a process called ionization. For example, acids, both inorganic and organic, will produce ions when dissolved in water. Some acids, such as hydrochloric acid (HC1), will essentially completely ionize. Others, such as acetic acid (CH3COOH), will only partially ionize. They establish an equilibrium with the ions and the unionized species (see Chapter 13 for more on chemical equilibrium). 

The potential energy curves of a neutral molecule AB and the potential ionic products from processes 7.18-7.20 are compared below (Fig. 7.11). These graphs reveal that the formation of negative molecular ions, AB, is energetically much more favorable than homolytic bond dissociation of AB and that the AB " ions have internal energies close to the activation energy for dissociation. [65,73,75]... 

One of the early efforts to evaluate quantitatively the bond dissociation energy of particular bonds in a compound was the work initiated by Mulliken (-3) in his so-called Magic Formula. Although this formula contains five terms, the two most important for the evaluation of a bond dissociation energy, Dq (uncorrected for zero-point vibrational energy), between two atoms i and j, are the covalent bond energy, Xjj, and the ionic resonance energy, IRE. The evaluation of Ay takes the form ... 

When a compound containing ionic bonds is placed in water, the polar water molecules separate some or all of the substance into its cations and anions. The separation is referred to as ionic dissociation. 

Substances that dissociate completely into ions when placed in water are referred to as strong electrolytes because the high ionic concentration allows an electric current to pass through the solution. Most compounds with ionic bonds behave in this manner sodium chloride is an example. 

Addition of a second crown produces the loose ion pair A, Cr,K, Cr. However, the complexation constant for adding the second crown is 1800 M 1 for the fluorenyl carbanion and only 200 M 1 for the picrate salt. The lower value for picrate may in part be due to less charge delocalization, e.g., the free ion dissociation constant for potassium fluorenyl in TEF is 1.6 x 10 7M (18) as compared to 9.2 x 10 M for potassium picrate (17). The two N02 substituents close to the 0 bond in picrate may also hinder the enlargement of this ionic bond and the insertion of a crown ether molecule because of electronic or sterlc effects. 

Electroplating. When ionically bonded molecules are dissolved in a solvent, some of the molecules dissociate into ions, whether the solvent is water, organic solvent, or a fused salt. A simple example is that of sulfuric acid or copper sulfate in water, giving... 

In general, in ICP-MS, metal argide ions (MAr+) are observed at lower intensities compared to the dimeric metal oxide ions (MO+). Both ionic species correlate with the bond dissociation energies in the ICP or by the expansion of plasma in the vacuum. A correlation of measured oxide ion intensities (MO+) and experimentally determined or theoretically calculated bond dissociation energies of oxides has been found in laser mass spectra using a LAMMA 500 (laser microprobe mass analyzer, Leybold Hereaus AG, Cologne) by Michiels and Gijbels.52... 

Often the color of compound is different from that of the ions into which it might be dissociated thus lead iodide is yellow, although both plumbous ion and iodide ion are colorless. In 1918 Bichowsky,41 in a paper dealing with valence colors of atoms, suggested that this change in color is the result of the sharing of electrons between bonded atoms, and this idea has been extended by Pitzer and Hildebrand,48 who proposed the postulate that the extent of deviation of the color of a compound from that of the ions into which it might dissociate may be taken as a measure of the deviation of the bonds from pure ionic bonds. 

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