Chemical reaction actual yield

The statements of Butlerov and his contemporaries allow us to infer reliably what meaning he attached to "molecular structure." For him it denotes the grouping of atoms within the molecule as determined by the valencies of those atoms, but not their actual spatial positions. (Distinguishing between "chemical atoms"—the smallest entities conserved in chemical processes—and "physical atoms" was standard in Butlerov s time.) It is worth remembering that in the nineteenth-century virtually all information about chemical structure was derived from chemical transformations. Given their inherently dynamic character, it seemed doubtful to contemporary chemists that chemical reactions could yield reliable information about the physical ("mechanical") structure of molecules (Rocke, 1981, pp. 30-38). Even more abstruse was the relationship between the "mechanical" and "chemical" structures of molecules. 

From the equation representing the chemical reaction involved, it is evident that 330 g. of silver maleate will theoretically react with 312 g. of ethyl iodide in ethereal solution to produce 172 g. of ethyl maleate. It follows, therefore, that 33 g. (0 1 mol) of silver maleate will react with 31-2 g. (0 2 mol) of ethyl iodide to give a theoretical yield of 17 2 g. (0-1 mol) of ethyl maleate. In practice, the actual yield found for these quantities is of the order of 16 0 g. the percentage yield is therefore (16 0/17-2) X 100 = 93 per cent. 

METHOD 5 Could something this easy really work Actually, yesi Well, ya wanna know something Actually Strike doesn t know if this really works or not (We ll discuss this in a bit. But first read the following which was what Strike wrote in the first edition of TS). Safrole is converted to MD-P-2-P in about 8 hours with no watched chemicals and a yield of 70%-80% [19]. The reaction is shown in the accompanying diagram. 

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

In working Example 18.8, we have in effect assumed that the electrolyses were 100% efficient in converting electrical energy into chemical energy. In practice, this is almost never the case. Some electrical energy is wasted in side reactions at the electrodes and in the form of heat This means that the actual yield of products is less than the theoretical yield. 

These results led us to reexamine the published work on nitrosation in the presence of bacteria (Table II) In two of the studies (, 10) the amount of NDMA formed is actually less than the amount that can be predicted from theoretical consideration of the uncatalyzed chemical reaction alone (15) In another study (12), the yield of NDMA is again slightly lower than the predicted chemical yield at the final pH (6 0) of the growing culture The work by Kunisaki and Hayashl (13), on the other hand, does indicate that resting cells of E. coli B catalyze... 

In the problem above, the amount of product calculated based upon the limiting reactant concept is the maximum amount of product that will form from the specified amounts of reactants. This maximum amount of product is the theoretical yield. However, rarely is the amount that is actually formed (the actual yield) the same as the theoretical yield. Normally it is less. There are many reasons for this, but the principal one is that most reactions do not go to completion they establish an equilibrium system (see Chapter 14 for a discussion on chemical equilibrium). For whatever reason, not as much product as expected is formed. We can judge the efficiency of the reaction by calculating the percent yield. The percent yield (% yield) is the actual yield divided by the theoretical yield and the resultant multiplied by 100 in order to generate a percentage ... 

Just as thermodynamic methods provide only a limiting value for the yield of a chemical reaction, so also do they provide only a limiting value for the work obtainable from a chemical or physical transformation. Thermodynamic functions predict the work that may be obtained if the reaction is carried out with infinite slowness, in a so-called reversible manner. However, it is impossible to specify the actual work obtained in a real or natural process in which the time interval is finite. We can state, nevertheless, that the real work will be less than the work obtainable in a reversible situation. 

The reason for constructing this rather complex model was that even though the mathematical equations may be easily set up using the dispersion model, the numerical solutions are quite involved and time consuming. Deans and Lapidus were actually concerned with the more complicated case of mass and heat dispersion with chemical reactions. For this case, the dispersion model yields a set of coupled nonlinear partial differential equations whose solution is quite formidable. The finite-stage model yields a set of differential-double-difference equations. These are ordinary differential equations, which are easier to solve than the partial differential equations of the dispersion model. The stirred-tank equations are of an initial-value type rather than the boundary-value type given by the dispersion model, and this fact also simplifies the numerical work. 

Many conditions are required for a chemical reaction to proceed. Conditions such as heat, light, and pressure must be just right for a reaction to take place. Furthermore, the reaction may proceed very slowly. Some reactions occur in a fraction of a second, while others occur very slowly. Consider the difference in the reaction times of gasoline igniting in a car s cylinder versus the oxidation of iron to form rust. The area of chemistry that deals with how fast reactions occur is known as kinetics (Chapter 12). Finally, not all reactions go to completion. The amount of product produced based on the chemical equation is known as the theoretical yield. The amount actually obtained expressed as a percent of the theoretical is the actual yield. In summary, it s best to think of a chemical equation as an ideal representation of a reaction. The equation provides a general picture of the reaction and enables us to do theoretical calculations, but in reality reactions deviate in many ways from that predicted by the equation. 

The actual yield will always be less than the theoretical yield because no chemical reaction ever reaches 100 percent completion. In a lab setting, there s always some cimount of error, whether it s big or small. 

Now I would like to turn to some of the issues of operations within the manufacturing process itself and speak to certain process controls that are expected. In a chemical synthesis sequence, as I mentioned above, intermediates will need to be fully characterized. That characterization will then lead to a set of specifications for the intermediate, that is, its level of purity, its form, etc. Test procedures that demonstrate that the intermediate meets specifications must be established. Some intermediates are deemed to be more important than others and are given specific designation, such as pivotal, key, and final intermediates. In those cases, it is necessary to demonstrate that the specific and appropriate structure is obtained from the chemical reaction and that the yield of the intermediate is documented and meets the expected yield to demonstrate process reproducibility and control. Purity of the substance is to be appropriately documented. And, finally, in reactions which produce pivotal, key, and final intermediates, side products or undesirable impurities are identified and their concentrations measured and reduced by appropriate purification procedures so that the intermediate meets in-process specifications. Thus, those important intermediates become focuses of the process to demonstrate that the process is "under control" and functioning in a reproducible and expected manner. All of these activities ultimately are designed to lead to the production of the actual active ingredient which is referred to then as a "bulk pharmaceutical agent." That final product will need to be completely characterized which then will document that it meets a set of specifications ("Final Product Specifications") for qualification as suitable for pharmaceutical use. 

The actual final products of radiolysis are the result of a complex set of chemical reactions. Detailed quantitative estimates of product yields are, therefore, more complicated and beyond the scope of this book. The reader is referred to other textbooks that discuss how these estimates are made (Woods et al., 1999 Mozumder, 1999). 

In the previous sections concerning reference and standard states we have developed expressions for the thermodynamic functions in terms of the components of the solution. The equations derived and the definitions of the reference and standard states for components are the same in terms of species when reactions take place in the system so that other species, in addition to the components, are present. Experimental studies of such systems and the thermodynamic treatment of the data in terms of the components yield the values of the excess thermodynamic quantities as functions of the temperature, pressure, and composition variables. However, no information is obtained concerning the equilibrium constants for the chemical reactions, and no correlations of the observed quantities with theoretical concepts are possible. Such information can be obtained and correlations made when the thermodynamic functions are expressed in terms of the species actually present or assumed to be present. The methods that are used are discussed in Chapter 11. Here, general relations concerning the expressions for the thermodynamic functions in terms of species and certain problems concerning the reference states are discussed. 

The amounts of products calculated in the stoichiometric problems so far represent theoretical yields. The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. In most chemical reactions, the amount of product obtained from a reaction is less than the theoretical yield. The measured amount of a product that is obtained from a reaction is called the actual yield of that product. 

Asymmetrical nitrido Tc(V) complexes (simply defined as heterocomplexes) are defined as coordination compounds in which two different bidentate ligands are bound to the same Tc=N group, and are represented by the general formula [Tc(N)(L)(L )]"+/0/". The attempt to develop a high-yield synthesis of these types of complexes may first appear to be prevented by basic chemical considerations. Actually, it is reasonable to expect that the reaction of two different bidentate ligands, A and B, with the same Tc=N group would always yield a statistical mixture of symmetrical and asymmetrical complexes, namely [Tc(N)(A)2], [Tc(N)(B)2] and [Tc(N)(A)(B)]. However, the peculiar properties of mixed 7r-acceptor-7r-donor ligands offered the route to the solution of this synthetic problem. The key approach can be outlined as follows. 

Chemists use stoichiometry to predict the amount of product that can be expected from a chemical reaction. The amount of product that is predicted by stoichiometry is called the theoretical yield. This predicted yield, however, is not always the same as the amount of product that is actually obtained from a chemical reaction. The amount of product that is obtained in an experiment is called the actual yield. 

The actual yield of chemical reactions is usually less than the theoretical yield. This is caused by a variety of factors. For example, sometimes less than perfect collection techniques contribute to a lower than expected yield. 

The percentage yield of a chemical reaction compares the mass of product obtained by experiment (the actual yield) with the mass of product determined by stoichiometric calculations (the theoretical yield). It is calculated as follows ... 

Sometimes chemists know what percentage yield to expect from a chemical reaction. This is especially true of an industrial reaction, where a lot of experimental data are available. As well, the reaction has usually been carried out many times, with large amounts of reactants. Examine the next Sample Problem to learn how to predict the actual yield of a reaction from a known percentage yield. 

The actual yield of most chemical reactions is less than 100%. 

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