The Concept of Limiting Reagent

To find the moles of NaHC03, we need to know the molar mass (84.01 g/mol). 

3 What is the mole ratio between HCl and NaHCO3 in the balanced equation  

Suppose you have a part-time job in a sandwich shop. One very popular sandwich is always made as follows  

So far we have been considering reactions in which all the reactants exist in adequate quantities. In this section, we will consider what happens when the amount of one of the reactants available is less than the amount required to complete the reaction. When such a condition exists, we call that reactant or reagent the limiting reagent. 

We will further explore this scenario through the following examples. Example 2-6 

A reaction mixture contains 60.75 g magnesium and 146 g hydrogen chloride. Predict the limiting reagent if the reaction occurs as shown below. 

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The concept of immobilizing reagents or probes onto polymer supports for use in chemistry and biology has received a great deal of attention. Since the activity of supported reagents depends on the accessibility of the active sites and is often limited by diffusion, considerable efforts are made to develop new polymer supports with improved capacity, accessibility and selectivity [88, 89]. In this context, the technique of polymerisation in microemulsion, developed in the early 1980s, offers new opportunities [90]. Indeed, the polymerisation of oil or water-soluble component in oil-in-water or water-in-oil microemulsions allows one to produce stable suspensions of ultrafine particles in the nanosize range (diameter smaller than... 

The concept of selectivity is most commonly encountered (and most useful in mechanistic investigations, see Chapter 2) when a reactant or a reactive intermediate has alternative bimolecular routes it is then also very useful in yield optimisation in chemical process development [12]. The reaction in Scheme 4.3 involves an electrophilic intermediate (X) which is captured by nucleophilic reagent C (which could be solvent). If another nucleophilic species (D) is added to the reaction mixture, the additional product D—X is formed in competition with C—X. If Ap is known (e.g. if D is known to react with electrophiles at the diffusion limit), then values for [D], [C] and the product ratio [C—X]/[D—X] allow determination of kc, i.e. quantitative information about the reactivity of X with C, and information about the selectivity of X in reactions with nucleophiles. 

The concept of the limiting reagent is analogous to the relationship between men and women in a dance contest at a club. If there are fourteen men and only nine women, then only nine female/male pairs can compete. Five men will be left without partners. The number of women thus limits the number of men that can dance in the contest, and there is an excess of men. 

In the problems we have worked thus far, the presence of an excess of one reactant was stated or implied. The calculations were based on the substance that was used up first, called the Umiting reactant or reagent. Before we study the concept of the limiting reactant in stoichiometry, let s develop the basic idea by considering a simple but analogous nonchemical example. 

During the 2007-2008 acaderttic year, we intend to pilot a ferrofluid lab in several sections of the first semester general cherrristry laboratory 30). The procedure will be rewritten to emphasize stoichiometric concepts inclnding limiting reagent and yield. Eventnally, we anticipate nsing this lab in all first semester sectiorrs and the modified version of the Color My Nanoworld (77) in all second semester sections. 

In conclusion we see that if we take into accoimt the consirmption of reagents, the concept of a chain reaction allows us to accoimt for all the curves in Figure 12.1, particularly the shapes of curves 1 and 2 of this figure, which reactions with limited sequences could not predict. We have also reported an effect of the shape and dimensions of the reactors. Finally, we have accounted for the three explosion threshold pressure limits at constant temperature. 

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