Chemical change irreversible

Chemical changes are not irreversible unless tliere is some fonn of dissipation in tire system. That is, tire reaction free energy must be dispersed to a number of degrees of freedom distinct from tire reaction coordinate. Models tliat include... 

Though formazans can be protonated, there are no reports of isolation of formazan cations. The study of the basicity of formazans is complicated by the fact that exposure to acid can lead to irreversible chemical changes. Recently, the protonation of the triaryl formazan 194 with perchloric acid in aprotic solvents has been studied spectroscopically. In this a hypso-chromic shift is observed and is more pronounced when X is an electron-donating substituent. Thus, the shift is 33, 14, and 73nm when X is hydrogen, /Mnethoxy, and w-nitro, respectively.337... 

Many other mechanisms lead to energy dissipation, although they may be less universal than those related to boundary lubricant-induced geometric frustration. Chemical changes in lubricant molecules, reversible or irreversible, produce heat. Examples are configurational changes in hydrogen-terminated... 

Certain constituents when added to the reaction mixture, slow down the rate of reaction. This phenomena is called inhibition and constituent called inhibitor. Such an effect is similar to the negative catalysis. But the constituent usually undergoes chemical change, inhibition is the preferred term. Inhibition may occur in chain reactions, enzyme catalysed reactions, surface reactions or many reversible or irreversible reactions. A trace amount of an inhibitor may cause a marked decrease in the rate of reaction. The inhibitor sometimes combines with a catalyst and prevents it from catalyzing the reaction. 

With the exception of Buchner s yeast extract and some comparable muscle preparations (Chapter 4), disrupting tissues often caused such damage to cells that normal metabolism was irreversibly affected. A further obstacle was that classical methods of analysis were neither sufficiently sensitive, rapid nor simple enough for the multiple measurements required to follow chemical changes in small samples of tissue. 

There are precautions that must be taken when attempting to separate heavy feedstocks or polar feedstocks into constituent fractions. The disadvantages in using ill-defined adsorbents are that adsorbent performance differs with the same feed and in certain instances may even cause chemical and physical modification of the feed constituents. The use of a chemical reactant such as sulfuric acid should only be advocated with caution since feeds react differently and may even cause irreversible chemical changes and/or emulsion formation. These advantages may be of little consequence when it is not, for various reasons, the intention to recover the various product fractions in toto or in the original state, but in terms of the compositional evaluation of different feedstocks, the disadvantages are very real. 

Corrosive The term refers to a chemical that causes visible destruction of, or irreversible alterations in, living tissue by chemical action at the site of contact. For example, a chemical is considered to be corrosive if, when tested on the intact skin of albino rabbits by the method described by the U.S. Department of Transportation in Appendix A to 49 CFR Part 173, it destroys or changes irreversibly the structure of the tissue at the site of contact following an exposure period of four hours. 

Deactivation is brought about primarily by the continual and irreversible buildup of deposits within the catalyst pores. Loss of activity can be attributed to both the obstruction of catalyst pores by deposits and the chemical changes that occur when the deposited metal interacts with the original active sites on the catalyst. The former restricts the access of... 

When an antagonist binds irreversibly to the receptor site, thus producing either permanent chemical changes or inactivation of the receptor site, or both, this is referred to as noncompetitive... 

Alumina High sorption capacity, no organic bleed Irreversible sorption and chemical change of sorptive possible... 

When groundwater is brought to the surface and stored in sample containers, the exposure to oxygen in air, the effects of daylight, and the differences in temperature and barometric pressure will cause irreversible chemical changes in a water sample, which in turn will change some of the water quality parameters. That is why water quality parameters should be measured in situ and with as little disturbance to the groundwater as possible. 

The biggest decrease is observed in the azo-bond sensitive vibration at 1500 cm-1, suggesting that the molecule is being cleaved at the azo bond. Nearly all of the aromatic skeletal vibrations also disappear, suggesting that the molecule is undergoing irreversible chemical changes. The spectra in Fig. 11 also show a new... 

Thermoset A polymeric material that undergoes irreversible chemical changes when cured with heat, catalysts or ultraviolet light. 

If a physical system is isolated, its state changes irreversibly to a time-invariant state in which no physical or chemical change occurs, and a state of equilibrium is reached in a finite time. Some conditions of equilibrium are (i) for a system thermally insulated with an infinitesimal change at constant volume dS 0, dV = 0, dU = 0, (ii) for a system thermally insulated with an infinitesimal change at constant pressure dS = 0,dP = 0, dH = 0, (iii) for a system thermally insulated with an infinitesimal change at constant volume and temperature dA = 0, dV = 0, dT = 0, and (iv) for a system thermally insulated with an infinitesimal change at constant pressure and temperature dG = 0,dT= 0, dP = 0. 

The feasibility of light-promoted valence isomerization as a route to bicyclooctanes has been demonstrated as illustrated for 12118s) and 122,186) but the possible synthetic utility of such chemical changes have not been exploited. Photo-isomerization studies of the four geometrical isomers of 1,5,9-cyclononatriene have been reported.187) The predominant process is cis-trans isomerization of the olefinic bonds which transforms each of the isomers into the same photostationary mixture of all four trienes. In addition, however, there occurs an irreversible isomerization to 123. 

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