Equilibrium constant for selected substances

Appendix A Common Mathematical Operations in Chemistry A-1 Appendix B Standard Thermodynamic Values for Selected Substances A-5 Appendix C Equilibrium Constants for Selected Substances A-8 Appendix D Standard Electrode (Half-Cell) Potentials A-14 Appendix E Answers to Selected Problems A-15 Glossary G-1 Credits C-1 Index 1-1... 

Appendix A Common Mathematical Operations in Chemistry A-1 Appendix B Standard Thermodynamic Values for Selected Substances A-5 Appendix C Equilibrium Constants for Selected Substances A-8... 

Before a reaction-equilibrium calculation can be performed, we must select an appropriate standard state for each species. Moreover, we must clearly distinguish quantities, such as fugacities and activities, that depend on the final equilibrium state (T, P, x ), from those quantities, such as equilibrium constants, that depend only on the equilibrium temperature T, the standard-state pressures P , and the phase. Typically, the standard-state pressure and phase are chosen according to whether the real substance is gas, liquid, or solid at the equilibrium conditions. Those three possibilities are discussed, in turn, here, and each discussion culminates with a particular expression for the activity. Those expressions can be used either in the stoichiometric development, via (10.3.14), or in the nonstoichiometric development, via (10.3.38). We emphasize that when we use the stoichiometric approach, the standard states used for the fugacities must be consistent with those associated with the equilibrium constant. 

All methods of chromatography operate on the principle that the components of a mixture will distribute unequally between two immiscible phases, which is also the basis for separations by extraction (Chap. 5). The mobile phase is generally a liquid or a gas that flows continuously over the fixed stationary phase, which may be a solid or a liquid. The individual components of the mixture have different affinities for the mobile and stationary phases, so a dynamic equilibrium is established in which each component is selectively, but temporarily, removed from the mobile phase by binding to the stationary phase. When the equilibrium concentration of that substance in the mobile phase decreases, it is released from the stationary phase and the process continues. Since each component partitions between the two phases with a different equilibrium constant or partition coefficient, the components divide... 

Various amines find application for pH control. The most commonly used are ammonia, morpholine, cyclohexylamine, and, more recently AMP (2-amino-2-methyl-l-propanol). The amount of each needed to produce a given pH depends upon the basicity constant, and values of this are given in Table 17.4. The volatility also influences their utility and their selection for any particular application. Like other substances, amines tend towards equilibrium concentrations in each phase of the steam/water mixture, the equilibrium being temperature dependent. Values of the distribution coefficient, Kp, are also given in Table 17.4. These factors need to be taken into account when estimating the pH attainable at any given point in a circuit so as to provide appropriate protection for each location. 

In most cases the necessary material constant can be determined by direct measurement. In practice however, because of the time and cost required to measure the numerous types and combination possibilities of plastics and contacting media, only a limited selection of such experimental constants is available. Consequently in practice one cannot avoid using estimated values. Such estimations are possible within a degree of accuracy adequate for practical purposes, when the chemical structure of the migrating substance, the polymer and the contacting media are known. Thermodynamic terms are used to characterize the equilibrium distribution of a diffusant substance between plastic (P) and contacting media (e.g. a liquid L). The most important of these terms is the chemical potential i. 

An especially challenging task is maintaining the selectivity of the method for separation of compounds whose elution time is very short, close to the dead time. In such cases, it is necessary to perform a preliminary review of the planned chromatographic conditions, including the composition of the analyzed material. For example, a typical eluent employed in anion-exchange chromatography (with pH of 8.5) is intended to facilitate the dissociation of separated compounds. Neglecting the time necessary to achieve acid/base equilibrium of substances loaded into the column in a neutral solution can result in their elution in the dead volume. The phenomenon is observed, for example, for MMA(V), whose consecutive dissociation constants are p/sTi 3.6 and p/sT2 8.22 [164]. 

Extraction is seldom the sole method used to purify a compound, but it is a rapid and versatile technique that can be used to achieve a preliminary separation prior to a final purification step. Separation of components by extraction depends upon the difference in solubility of a compound in two mutually insoluble phases. Mathematical aspects of extraction are formulated in terms of a simple distribution law, K = CJC, which states that at equilibrium a solute will distribute itself between two immiscible phases, a and 6, such that the ratio of concentrations in the two phases is a constant at a given temperature. The constant K is called the partition or distribution coefficient. If a substance dissolved in solvent b is to be extracted into a second solvent a), it is obviously advantageous to choose solvent a such that the value of K will be as large as possible. Unfortunately, there is no sure way of predicting K, and the organic chemist relies on the rule that like dissolves like and his previous experience in selecting the best solvent system for an efficient extraction. 

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