Reagent limiting

A delicious treat known as a S more is constructed with the following ingredients and amounts  

At a particular store, these items can be obtained only in full boxes, each of which contains one gross of items. A gross is a specific number of items, analogous (but not equal) to one dozen. The boxes of items have the following net weights (the weight of the material inside the box)  

If you have a collection of 100 graham crackers, how many chocolate bars and how many marshmallows do you need to make S mores with all of the graham crackers  

If you have a collection of 1000 graham crackers, 800 chocolate bars, and 1000 marshmallows  

Chemists refer to the reactant which limits the amount of product that can be made from a given collection of original reagents as the limiting reagent or limiting reactant. 

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A procedure for determining the stoichiometry between two reactants by measuring the relative change in absorbance under conditions when each reactant is the limiting reagent. 

Ethyl acrylate is used in approximately 50% molar excess over the expensive, limiting reagent, iodine. Use of an equimolar amount results in a lower yield (85%). The checkers distilled the reagent at 20° (39 mm.) prior to use. 

Probably no reaction is truly zeroth-order. However, one might employ a very high concentration of the species on which the rate does depend to create pseudo-zeroth-order conditions. If the rate is independent of the limiting reagent, A, it is given by... 

Better yet, a least-squares analysis of k versus [B]av is carried out. The order with respect to [A], the limiting reagent, is established from the fit of the data to a chosen rate law. Experiments over a range of [A]o are a preferable way to show the order in LA], At constant [B], will be the same regardless of [A]o if the rate is first-order with respect to [A],... 

The time required to convert a given fraction of the limiting reagent is a characteristic of the rate equation. A comparison of successive half-times, or any other convenient fractional time, reveals whether a reaction follows any simple-order rate law. Thus, the ratio of the time to reach 75 percent completion to that for 50 percent is characteristic of the reaction order. Values of this ratio for different orders are as follows ... 

Fig. 8.9 A typical stoichiometry question on limiting reagents and amount of product formed in a reaction...

Determine a, b, c and d as functions of time. Continue your calculations until the limiting reagent is 90% consumed given uo=... 

Yield based on the limiting reagent Reactions carried out in inert atmosphere The corresponding anions were not mentioned... 

Yield and catalyst concentration based on the limiting reagent. 

If a balanced chemical reaction involves more than one reagent, one of them will be the reference limiting reagent which will define the scale of the entire reaction. The numerator in the stoichiometric factor term takes into account the sum of the masses of all excess reagents used as appropriate. For any balanced chemical reaction in which all byproducts are identified, equation (4.1) maybe used to determine RME under a variety of scenarios. 

Scheme 4.3 shows example reactions that produce more than one geometric or stereoiso-meric product. If we consider all isomers as the global target product then the reaction yield with respect to limiting reagent is designated as overall and the expression for RME as given in equation (4.1) is used with this modification. If we wish to evaluate the RME for any one specific isomer then we multiply the overall reaction yield for all isomeric products by the fraction of the total isomeric product that represents the specific isomer and substitute that value for e in equation (4.1). This fraction maybe easily obtained from the product ratio. 

Symbol definitions s Is reaction yield with respect to limiting reagent A for original reaction equal to y- /x s Is reaction yield with respect to limiting... 

In addition to enzyme activity, the concentration of an nonelectroactive substrate can be determined electrochemically by this technique. By keeping the substrate (analyte) the limiting reagent, the amount of product produced is directly related to the initial concentration of substrate. Either kinetic or equilibrium measurements can be used. Typically an enzyme which produces NADH is used because NADH is readily detected electrochemically. Lactate has been detected using lactate dehydrogenase, and ethanol and methanol detected using alcohol dehydrogenase... 

Lequea et al. used the activity of tyrosine apodecarboxylase to determine the concentration of the enzyme cofactor pyridoxal 5 -phosphate (vitamin B6). The inactive apoenzyme is converted to the active enzyme by pyridoxal 5 -phosphate. By keeping the cofactor the limiting reagent in the reaction by adding excess apoenzyme and substrate, the enzyme activity is a direct measure of cofactor concentration. The enzymatic reaction was followed by detecting tyramine formation by LCEC. The authors used this method to determine vitamin B6 concentrations in plasma samples. 

It is necessary to state clearly to which reagent the excess refers. This is often termed the limiting reagent. 

The variable / depends on the particular species chosen as a reference substance. In general, the initial mole numbers of the reactants do not constitute simple stoichiometric ratios, and the number of moles of product that may be formed is limited by the amount of one of the reactants present in the system. If the extent of reaction is not limited by thermodynamic equilibrium constraints, this limiting reagent is the one that determines the maximum possible value of the extent of reaction ( max). We should refer our fractional conversions to this stoichiometrically limiting reactant if / is to lie between zero and unity. Consequently, the treatment used in subsequent chapters will define fractional conversions in terms of the limiting reactant. 

One can relate the extent of reaction to the fraction conversion by solving equations 1.1.4 and 1.1.7 for the number of moles of the limiting reagent nlim and equating the resultant expressions. 

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