Reaction without chemical change

Electron Transfer Reactions without Chemical Change 150... 

Electron transfer reactions without chemical change... 

Another method used to vary the AG° of the recombination reaction without chemical modification of the centers, consists of placing the system in an electric field whose orientation and intensity are well defined [141]. However, the energy level shifts induced by the field also change the electronic factors, so that the interpretation of the experimental results is not straightforward. Bixon and Jortner have proposed using electric field effects to elucidate the nature of the primary electron step in bacterial photosystems [142], a problem that will be discussed in Sect. 3.5. One basic difficulty encountered in this method is the evaluation of the internal field effectively seen by the redox centers in the membrane. 

Catalysts are substances that accelerate chemical reactions without themselves being consumed in the process. Since catalysts emerge from the catalyzed reaction without being changed, even small amounts are usually suf dent to cause a powerful acceleration of the reaction. In the cell, enzymes (see p. 88) generally serve as catalysts. A few chemical changes are catalyzed by special RNA molecules, known as ribozymes (see p. 246). 

Selenium oxychloride absorbs all light up to a wave-length of 4050pp. It is miscible with chloroform, carbon disulphide and benzene without chemical change. It is also soluble in carbon tetrachloride, but after a time reaction takes place with formation of selenium tetrachloride and carbonyl chloride.10 At the ordinary temperatures selenium oxychloride is not miscible with the paraffin hydrocarbons,... 

Unlike chemical compounds, whose constituent units are identical, mixtures are simply blends of two or more substances added together in some random proportion without chemically changing the individual substances themselves. Thus, hydrogen gas and oxygen gas can be mixed in any ratio without changing them (as long as there is no flame nearby to initiate reaction), just as a spoonful of sugar and a spoonful of salt can be mixed. 

Catalyst Any matter that is used to change or speed up a chemical reaction without being changed itself or destroyed in the reaction. 

Enzymes as catalysts Enzymes are catalysts that increase the rate of a chemical reaction without being changed themselves in the process. In the absence of an enzyme, the reaction may hardly proceed at all, whereas in its presence the rate can be increased up to 107-fold. Enzyme catalyzed reactions usually take place under relatively mild conditions (temperatures well below 100°C, atmospheric pressure and neutral pH) as compared to the corresponding chemical reactions. Enzymes are also highly specific with respect to the substrates that they act on and the products that they form. In addition, enzyme activity can be regulated, varying in response to the concentration of substrates or other molecules (see Topic C5). Nearly all enzymes are proteins, although a few catalytically active RNA molecules have been identified. 

Enzymes are specialized proteins that act as biological catalysts. They can accelerate chemical reactions without being changed themselves. Most enzymatic reactions occur within a narrow temperature range, from 30 to 40°C. Each enzyme usually only reacts with only a small number of closely related compounds. Some require the presence of additional small non-protein molecules (coenzymes). 

In addition to the stereochemical changes that accompany chemical reaction, these systems can be rearranged without chemical change. Several such examples of geometrical and of optical rearrangements are known for metal complexes. A brief account is given of some examples of geometrical (or cis-trans) isomerization in this section, and the next section contains a few examples of optical isomerization (or racemization). 

Most of the reactions discussed in this chapter involve the removal of a stoichiometric proportion of the water of crystallization without chemical change, other than concurrent or consecutive recrystallization. At higher temperatures, there may be a reaction between the evolved water and other constituents of the solid, particularly where the product water is not rapidly removed. The possible occurrence of hydrolysis is not discussed, because complex reactions of this type are difficult to characterize in detail and little mechanistic information is available,... 

The interaction between some organic contaminants and mineral surfaces has recently attracted attention as a way of cleaning up contaminants in natural waters. The large cation exchange capacity of smectite clay minerals (Section 4.5.2), in particular, has prompted research into their use as a catalyst, i.e. a substance that alters the rate of a chemical reaction without itself changing. Clay catalysts have potential applications as adsorbents to treat contaminated natural waters or soils. 

Enzymes are the body s catalysts. Without them, the cell s chemical reactions would be too slow to be useful, and many would not occur at all. A catalyst is an agent which speeds up a chemical reaction without being changed itself. 

Heterogeneous catalysts are defined as solids or mixture of solids that are used to accelerate a chemical reaction without undergoing change themselves. The types of solids used in industry as heterogeneous catalysts include simple oxides, mixed oxides, metal salts, solid acids and bases, metals, and dispersed metals. Catalysts are used in a wide variety of chemical and environmental processes worldwide. The global value of fuels and chemicals produced by catalytic routes is about US 2.4-3 trillion per year. About 20% of all products produced in the United States are derived from a catalytic process of some form. As important as catalysis is to the world economy, the number of various chemicals used as a catalyst as well as the form and shape of the material vary as much as the number of processes that use catalysts. Fig. 1 is a picture of a number of various types of catalysts and illustrates the numerous possibilities of shapes and sizes. Naturally, the preparation processes of such a wide variety of products is also numerous. 

There are no fundamental differences between chemical catalysts and biological enzyme catalysts. Enzymes accelerate the rates of chemical reaction without being changed in the overall transformation they reduce the activation energies of chemical... 

The substances for which this phenomenon has been observed are invariably polycyclic aromatic hydrocarbon structures. No exciplex formation has been reported in the literature to involve drug molecules, but this remains a possibility in concentrated solution or perhaps in solid-state mixtures. The consequences of exciplex formation are a radiative or nonradiative return to the ground state without chemical change, or electron transfer leading to chemical reaction of the drug, the quencher, or both. Many photoaddition processes are postulated to proceed via exciplex formation with the quencher molecule becoming chemically bound. 

Excited molecules can revert to their ground state without chemical change (Reaction 4) or by energy transfer to other molecules. This latter process is ineffective for pure hydrocarbons. 

Even as Lavoisier demolished phlogiston, he postulated a new gaseous simple substance or element called caloric—the element of heat (see Lavoisier s Table of Elements, Figure 202). Lavoisier had fully explained mass transformation in chemical reactions. The nature of energy transformations remained a mystery. Caloric could be transferred from a warmer body to a cooler body without chemical change. However, Lavoisier also posited that oxygen the element was a com-... 

Catalyst Substance that increases the rate of a chemical reaction without being changed in identity... 

Because it is used frequently to study photochemical reactions that lead to bond cleavage, this procedure is often called flash photolysis. The term photolysis literally means breaking apart by light," however, so technically it would not apply to the measurement of excited states that decay back to starting molecules without chemical change. 

From the isotopic labelling and nuclear magnetic resonance (NMR) studies, it has been observed that the reaction has a rate constant consistent with the second-order reaction and occurs rapidly at 25 °C. Since the reaction is without chemical change, there is no change in heat or reactants. 

---