Ideal chemical reaction, definition

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

Several comments need to be made concerning the state of aggregation of the substances. For gases, the standard state is the ideal gas at a pressure of 1 bar this definition is consistent with the standard state developed in Chapter 7. When a substance may exist in two allotropic solid states, one state must be chosen as the standard state for example, graphite is usually chosen as the standard form of carbon, rather than diamond. If the chemical reaction takes place in a solution, there is no added complication when the standard state of the components of the solution can be taken as the pure components, because the change of enthalpy on the formation of a compound in its standard state is identical whether we are concerned with the pure... 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

The definitions of the equilibrium parameters for nonideal systems involve the chemical potentials of the pure constituents that undergo the chemical reaction of interest. Thus, they are either exactly the same, or differ only slightly, from those adopted for ideal systems. For this reason the methodology and the results of Section 2.11 may be taken over (with appropriate minor modifications, as necessary) and need not be repeated here. 

The just-suspended state is defined as the condition where no particle remains on the bottom of the vessel (or upper surface of the liquid) for longer than 1 to 2 s. At just-suspended conditions, all solids are in motion, but their concentration in the vessel is not uniform. There is no solid buildup in comers or behind baffles. This condition is ideal for many mass- and heat-transfer operations, including chemical reactions and dissolution of solids. At jnst-snspended conditions, the slip velocity is high, and this leads to good mass/heat-transfer rates. The precise definition of the just-suspended condition coupled with the ability to observe movement using glass or transparent tank bottoms has enabled consistent data to be collected. These data have helped with the development of reliable, semi-empirical models for predicting the just-suspended speed. Complete suspension refers to nearly complete nniformity. Power requirement for the just-suspended condition is mnch lower than for complete snspension. 

The concept of free energy is useful in defining the possibility of a reaction and in determining its limiting or equilibrium conversion. The formal definition of the equilibrium state of a chemical reaction is the state for which the total free energy is a minimum. Thus the well-known rule reaction can occur if AG is negative it cannot occur if AG is positive. Now we shall present the main features of the equilibrium state for ideal and nonideal gases. 

Systems in which chemical reactions take place are called reactors. Special names, such as reaction vessel, will not be used in this book. In practice, the situation in a reactor is usually very different from the ideal requirement used in the definition (1.3.1) of reaction rates. Indeed, a reactor is generally not a closed system with uniform temperature, pressure and composition. These ideal conditions will be rarely met even in experimental reactors designed for the measurement of reaction rates. In fact, reaction rates cannot be measured directly in a closed system what is measured in a closed system is the composition of the system at various times and the rate is then inferred or calculated from these measurements. 

In this chapter, the fundamentals of chemical reaction engineering are presented. The basic definitions along with the material balance of different types of ideal reactors and their design equations are discussed. 

This shows the definition of the solubility product. Extensive tables of solubUity products reside in handbooks [5, p. 8—39]. The reported value there for AgCl is 1.77 x 10 . The fact that this value and the result of the above example are identical does not prove that they are right. Instead, it shows that a consistent data set of some kind was used to generate both the g° values in Table A.8 and the values in the corresponding table, calculated exactly as shown in this example. The point of this example is to show that solubility product and other quantities regularly used in dilute aqueous chemistry are computed exactly the same way as chemical reaction equifibria. (Here we have assumed ideal solutions of ions, yQ- = = 1.00. For dilute solutions this... 

Formation of concepts and models in chemistry is based on the assumption of distinct pure substances because chemical properties are relational. Characterising the entities in a chemical reaction (and giving an operational definition of the latter) requires pure substances as ideal limit reference points. The properties of a solution or conglomerate are described in terms of its constituents, understood... 

For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. 

Charge transfer resistance, 1056 Charge transfer overpotential, 1231 Charge transfer, partial. 922. 954 Charges in solution, 882 chemical interactions, 830 Charging current. 1056 Charging time, 1120 Chemical catalysis, 1252 Chemical and electrochemical reactions, differences, 937 Chemical equilibrium, 1459 Chemical kinetics, 1122 Chemical potential, 937, 1058 definition, 830 determination, 832 of ideal gas, 936 interactions, 835 of organic adsorption. 975 and work function, 835... 

In Chapter 1 it is pointed out that analyses comprise several stages or operations definition or recognition of the analytical aspects of a problem, sampling, separation, measurement, and evaluation of data. The preceding chapters have dealt mainly with the chemistry of the noninstrumental aspects of the measurement operation. Ideal methods of analysis would involve measurement techniques of such specificity that only the component of interest in a sample would give a reaction or response, without interference from other components. But in actual practice a sample may be composed of substances that are chemically similar, which undergo similar reactions and so are Ukely to interfere with the measurement operation. Therefore, analytical separations frequently are necessary to isolate or concentrate material before the measurement step can be carried out. The extent of such separation operations depends on the method of measurement selected or available, the nature and amount of sample, the time restrictions, and the precision required. 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

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