Yield of a reaction

The following data for a 2 factorial design were collected during a study of the effect of temperature, pressure, and residence time on the %yield of a reaction. " ... 

In the context of chemometrics, optimization refers to the use of estimated parameters to control and optimize the outcome of experiments. Given a model that relates input variables to the output of a system, it is possible to find the set of inputs that optimizes the output. The system to be optimized may pertain to any type of analytical process, such as increasing resolution in hplc separations, increasing sensitivity in atomic emission spectrometry by controlling fuel and oxidant flow rates (14), or even in industrial processes, to optimize yield of a reaction as a function of input variables, temperature, pressure, and reactant concentration. The outputs ate the dependent variables, usually quantities such as instmment response, yield of a reaction, and resolution, and the input, or independent, variables are typically quantities like instmment settings, reaction conditions, or experimental media. 

The theoretical yield of a reaction is the maximum quantity (amount, mass, or volume) of product that can be obtained from a given quantity of reactant. The quantities of products calculated from a given mass of reactant in Section L were all theoretical yields. The percentage yield is the fraction of the theoretical yield actually produced, expressed as a percentage ... 

Why Do We Need to Know This Material The dynamic equilibrium toward which every chemical reaction tends is such an important aspect of the study of chemistry that four chapters of this book deal with it. We need to know the composition of a reaction mixture at equilibrium because it tells us how much product we can expect. To control the yield of a reaction, we need to understand the thermodynamic basis of equilibrium and how the position of equilibrium is affected by conditions such as temperature and pressure. The response of equilibria to changes in conditions has considerable economic and biological significance the regulation of chemical equilibrium affects the yields of products in industrial processes, and living cells struggle to avoid sinking into equilibrium. 

If the percent yield of a reaction is already known, we can calculate how much of a product to expect from a synthesis that uses a known amount of starting material. For example, the Haber synthesis of ammonia stops when 13% of the starting materials have formed products. Knowing this, how much ammonia could an industrial producer expect to make from 2.0 metric tons of molecular hydrogen First, calculate the theoretical yield ... 

Operationally, a procedure may be based on measuring the yield of a reaction traceable to ionization, usually giving a lower limit to the ionization yield. Thus, in the radiation chemistry of hydrocarbon liquids, the product of an electron scavenging reaction (for example, C2H3- radical from the scavenger C2H5Br)... 

The result of a quantitative chemical measurement is not an end in itself. It has a cost and therefore it always has a purpose. It may be used, for example, in checking products against specifications or legal limits, to determine the yield of a reaction, or to estimate monetary value. 

Substrate and product inhibition. Few academic researchers are familiar with this phenomenon as they usually mn their hydrogenations at low substrate concentrations and low SCR. However, for industrial applications the space-time yield of a reaction - the amount of product per unit reactor volume per time unit - is quite important. Clearly, the higher the substrate concentration the higher the space-time yield and the more economic the process. More often than not, either substrate or product inhibition becomes a problem when the substrate concentration is increased to 10 wt% or more. 

It has been shown for many metal halides and monomers that binary mixtures of these can be prepared (usually in a solvent) without any polymerization taking place. Such a quiescent mixture can be made to react by the addition of a suitable third compound, which is called the co-catalyst. This term is preferable to the word promoter , because in certain contexts a substance is called promoter which enhances the rate or yield of a reaction that will also go in the absence of the promoter herein lies the true distinction between promoter and co-catalyst [28]. (For example, small quantities of epoxides or epichlorohydrin act as promoters in the cationic polymerization of tetrahydrofuran.) I will take it that in the above quotation the word promoter was inadvertently used in place of co-catalyst , for only thus does it become really meaningful. 

Humidification/Dehumidification Water often is added or removed in fuel cell systems to promote or prevent certain chemical reactions. For some reactions, excess water can help to drive the reaction, while too much requires larger equipment and can even reduce the yield of a reaction or decrease the performance of a fuel cell. Excess water often is utilized to increase the yield of reforming reactions and the water gas shift. 

The theoretical yield of a reaction can be calculated based on molecular weights. For example, the reaction of hydrogen with carbon dioxide to give methanol ... 

The ratio of the optical purity of the product of a reaction (involving chiral reagents and products) to the optical purity of the product s precursor. The optical yield of a reaction is not related to the chemical yield. 

NH2 an increase of the yield of a reaction of the Kemp acid derivative 117 owed to template action ofthe product 118a [24] (Figure 5.4). The... 

In an experiment set up to determine the effect of time and catalyst concentration on the yield of a reaction, the following results were obtained (results shown are averages of two determinations) ... 

The quantum yield of a reaction can be related to reactivity of a given state only after the rates of other competing processes are identified and measured. Consider the reaction... 

In organic chemistry, the yield of a reaction A — B can be determined by 1H NMR. This is done by recording the spectrum of the coarse material after reaction and by identifying a peak specific to the remaining reagent (A) and one specific to the product formed (B). The yield of B relative to A can be obtained by the following equation ... 

We have mentioned how chlorine molecules dissociate to chlorine atoms on absorption of near-ultraviolet light and thereby cause radical-chain chlorination of saturated hydrocarbons (Section 4-4D). Photochemical chlorination is an example of a photochemical reaction that can have a high quantum yield— that is, many molecules of chlorination product can be generated per quantum of light absorbed. The quantum yield of a reaction is said to be unity when 1 mole of reactant is converted to product(s) per einstein1 of light absorbed. The symbol for quantum yield is usually 4>. 

The variety of manifestations in time of coherent development of molecular dynamics also includes such phenomena as mono- and bimolecular chemical reactions. Thus, Seideman et al [342] suggest the idea of governing the yield of a reaction by suddenly creating coherent superposition of two states of the transient complex and applying a second pulse with fixed delay for the dissociation of the complex. The appearance of coherent beats in femtochemistry , in particular, at photodissociation, has been analyzed by Zewail (review [404]). 

The theoretical yield of a reaction is the amount of product that is predicted using the stoichiometric ratios of moles from the balanced chemical reaction. 

Typical yields have been provided for many synthetic reactions, although I hope students will not misuse these numbers. Too often students consider the yield of a reaction to be a fixed characteristic just as the melting point of a compound is fixed. In practice, many factors affect product yields, and literature values for apparently similar reactions often differ by a factor of 2 or more. The yields given in this book are typical yields that a good student with excellent technique might obtain. 

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 percentage yield of chemical reactions is extremely important in industrial chemistry and the pharmaceutical industry. For example, the synthesis of certain drugs involves many sequential chemical reactions. Often each reaction has a low percentage yield. This results in a tiny overall yield. Research chemists, who generally work with small quantities of reactants, may be satisfied with a poor yield. Chemical engineers, on the other hand, work with very large quantities. They may use hundreds or even thousands of kilograms of reactants A difference of 1% in the yield of a reaction can translate into thousands of dollars. 

The percentage yield of a reaction is determined by numerous factors The nature of the reaction itself, the conditions under which the reaction was carried out, and the nature of the reactants used. 

Q lifiLF When calculating the percentage yield of a reaction, what units should you use grams, moles, or number of particles Explain. 

Determine the percentage purity of a reactant based on the actual yield of a reaction. 

Chemists need to know the percentage yield of a reaction. Why is this true, particularly for industrial reactions ... 

An example of where systematic experimental design is valuable is the optimisation of die yield of a reaction as a function of reagent concentration. A true representation is given in Figure 2.1. In reality this contour plot is unknown in advance, but die experimenter wishes to determine die pH and concentration (in mM) diat provides die best reaction conditions. To witiiin 0.2 of a pH and concentration unit, titis optimum happens to be pH 4.4 and 1.0 mM. Many experimentalists will start by guessing one of die factors, say concentration, then finding die best pH at diat concentration. 

The size of the coefficients can inform the experimenter how significant the coefficient is. For example, does pH significantly improve the yield of a reaction Or is the interaction between pH and temperature significant In other words, does the temperature at which the reaction has a maximum yield differ at pH 5 and at pH 7 ... 

The coefficients can be used to construct a model of the response, for example die yield of a reaction as a function of pH and temperature, and so establish the optimum conditions for obtaining the best yield. In this case, the experimenter is not so interested in the precise equation for the yield but is very interested in the best pH and temperature. 

It is not necessary, of course, to have replicates to perform this type of analysis. If the yield of a reaction varies between 50 and 90 % over a range of experimental conditions, then a factor that contributes, on average, only 1 % of this increase is unlikely to be too important. However, it is vital in all senses that the factors are coded for meaningful comparison. In addition, certain important properties of the design (namely orthogonality) which will be discussed in detail in later sections are equally important. 

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