Reversibility of reactions

The reverse of reaction 6.15 is called a dissociation reaction and is characterized by a dissociation constant, Ka, which is the reciprocal of K. ... 

The electron can be trapped, for example by an interstitial which is converted to an H atom. The AlO is the hole color center which absorbs light and gives the color to smoky quart2. Bleaching is the result of thermal energy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quart2 is exactly like the smoky, except that Fe " replaces. ... 

The Fe(III)0 4 hole color center gives the purple color. On being heated, the trapped electron is released and the reverse of reaction 2 occurs producing Fe(III)0 4, which provides the pale yellow color of citrine. 

As the temperature approaches the NTC zone, the reversibility of reaction 2 comes into play and the steady-state concentration of alkyl radicals rises. There is a competing irreversible reaction of oxygen with radicals containing an alpha hydrogen which produces a conjugate olefin (eq. 23). 

Now we wish to obtain reaction (5) by combining reactions (6) and (7). Since NO is a reactant in reaction (5), we need the reverse of reaction (6). We obtain the heat of the reverse reaction merely by changing the algebraic sign of AHe. If 21.6 kcal of heat are absorbed when one mole of NO is formed, then 21.6 kcal of heat will be released when one mole of NO is decomposed in the reverse reaction ... 

Comparing equations (9), (10), and (11), we see that is just the ionization energy of F (g). By usual practice, however, the reverse of reaction (11) is usually considered. Of course the heat of reaction (12) is just the negative of that of reaction (II) ... 

When acid is added to a solution such as in equation (4d), the above set of reactions is progressively reversed, first causing precipitation of chromium hydroxide by the reverse of reaction (4d) and then its subsequent dissolving by the reverse of reaction (4c). 

Conversely, for an unreactive XH, or one present in small conch, or for a highly aq medium, the reverse of reaction (8) prevails over reaction (16), and the rate of nitration now depends on both the nature and concn of XH, ie, it is first-order in XH. The comments made above about H2S04 or N03 additives are also applicable to this situation... 

Differentiation between the two forms of Ag2C03 is not easy and, from the many methods used, electron spin resonance spectroscopy and thermal analysis have been most successfully applied [757]. The imperfections mentioned above occur in the low temperature decomposition product and are identified as being responsible for enhanced activity in readsorbing C02. Annealing of the residue removes these defects and reduces the reversibility of reaction. 

The same experimental procedure used in Fig. 4.15 is followed here. The Pt surface is initially (t < - 1 min) cleaned from Na via application of a positive potential (Uwr=0.2 V) using the reverse of reaction (4.23). The potentiostat is then disconnected (1=0, t=-lmin) andUWR relaxes to 0 V, i.e. to the value imposed by the gaseous composition and corresponding surface coverages of NO and H. Similar to the steady-state results depicted in Fig. 4.18 this decrease in catalyst potential from 0.2 to 0 V causes a sixfold enhancement in the rate, rN2, of N2 production and a 50% increase in the rate of N20 production. Then at t=0 the galvanostat is used to impose a constant current I=-20 pA Na+ is now pumped to the Pt catalyst surface at a... 

We saw in Section 6.11 that the first law of thermodynamics implies that, because enthalpy is a state function, the enthalpy change for the reverse of a process is the negative of the enthalpy change of the forward process. The same relation applies to forward and reverse chemical reactions. For the reverse of reaction A, for instance, we can write... 

Much has been learned in recent years about the 00 dimer , O2O2, produced in reaction 17. It is actually dichlorine peroxide, OOOCl its geometry is now well established from submillimeter wave spectroscopy (15). Photolysis of OOOO around 310 nm the atmospherically important wavelengths -- yields chlorine atoms and ClOO radicals (16), as given in reaction 18, rather than two OO radicals, even though QO-OQ is the weakest bond (it has a strength of about 17 Kcal/mol (17)). Thermal decomposition of QOOQ (the reverse of reaction 17) occurs very fast at room temperature, but more slowly at polar stratospheric temperatures. Hence, photolysis is the predominant destruction path for CIOOQ in the polar stratosphere and two Q atoms are produced for each ultraviolet photon absorbed. 

Figure 28-8. Biosynthesis of proline from glutamate by reversal of reactions of proline catabolism.

Photosynthesis is the reverse of reaction (30.1) the formation of carbohydrates and oxygen from water and carbon dioxide with solar energy. Photosynthesis occurs in the chloroplasts contained in the cells of green plants. The chloroplasts hold two types of photosynthetic systems, which are called PSl and PS 11. These systems... 

Here, the sign of equality (=) has been replaced by the double oppositely directed arrows (s=) called a sign of reversibility. Such a reaction is called a reversible reaction. The reversibility of reactions can be detected when both the forward and the reverse reactions occur to a noticeable extent. Generally, such reactions are described as reversible reactions. The most important criterion of a reaction of this type is that none of the reactants will become exhausted. When the reaction is allowed to take place in a closed system from where none of the substances involved in the reaction can escape, one obtains a mixture of the reactants and the products in the reaction vessel. Every reversible reaction, depending on its nature, will after some time reach a stage when the reactants and the products coexist in a state of balance, and their amounts will remain unaltered for unlimited time. Such a state of a chemical reaction is called chemical equilibrium, and the point of such an equilibrium varies only with temperature. 

The Structure of Quinone Methides and Their Precursors Modulate the Reversibility of Reaction... 

Sometimes besides direct pulses, reverse pulses are also applied (Tacussel PRG 5 polarograph), because by tracing out the polarogram using first positive-, then negative-going pulses, one may study the reversibility of reactions. 

The reversibility of reactions is another important characteristic in assessing the fate of deep-well-injected wastes. Depending on environmental conditions, reversible reactions readily proceed in either or both directions. Most acid-base reactions exemplify reversible processes. In aqueous solutions, relatively minor changes in such factors as pH or concentration can change the direction of these reactions. Irreversible reactions, typified by hydrolysis, have a strong tendency to go in one direction only. 

Water was produced through the reduction of stored NOx and was detected at the reactor exit with a time delay of about 50 s, that compared well with the characteristic time of C02desorption. Likewise, the consumption of C02 was ascribed to the reverse of reaction (10), which implied readsorption of C02 on BaO/Ba(OH)2 once NO had been reduced. 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

Acetylene also plays an important role as intermediate in soot formation in flames, because the reverse of reaction (la) forms singlet methylene,... 

In reaction 35, activation energy has to be provided to the precursor ion by collisions or other means and charge reduction will occur when the activation energy is lower than that for the desolvation reaction. In reaction 36, the solvation of the ion by B, i.e. reaction a, provides the activation energy and proton transfer and charge reduction will occur if the activation energy for reaction b is less than that for the reverse of reaction a. 

For the second mechanism, equation 4.1.18 is the rate controlling step, and the arrows in parentheses indicate the assumptions made regarding the reversibility of reactions 4.1.17 and 4.1.18. The overall reaction rate is now... 

If the reverse of Reaction 1 is slow compared to 2 ( the colli sional stabilization step) then overall cluster growth will not depend strongly upon the total helium pressure. This is found to be the case using RRK estimates for k n and hard sphere collision cross sections for ksn for all clusters larger than the tetramer. The absence of a dependence on the total pressure implies that the product of [M] and residence time should govern cluster growth. Therefore, a lower pressure can be compensated for by increasing the residence time (slower flow velocities). 

The reverse of reactions (3) and (4) in the high pressure limit is the desorption process described by... 

Since the overall cell potential is positive, this step is spontaneous. The next step involves oxidation of Fe2+(aq) back to Fe3+(aq), (i.e. the reverse of reaction (i) and the reduction of H202(aq) to H20(l) in acidic solution, for which the half-reaction is... 

It can be obtained by adding reaction (b) to the reverse of reaction (c) (both appear in the provided figure)... 

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