Stoichiometry of precipitation reactions

For each of the following reactions, write the formula equation, the complete ionic equation, and the net ionic equation. 

In Chapter 3 we covered the principles of chemical stoichiometry the procedures for calculating quantities of reactants and products involved in a chemical reaction. Recall that in performing these calculations we first convert all quantities to moles and then use the coefficients of the balanced equation to assemble the appropriate mole ratios. In cases where reactants are mixed we must determine which reactant is limiting, since the reactant that is consumed first will limit the amounts of products formed. These same principles apply to reactions that take place in solutions. However, two points about solution 

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We will introduce stoichiometric calculations for reactions in solution in Example 4.10. 

Calculate the mass of solid NaCI that must be added to 1.50 L of a O.IOOMAgNOs solution to precipitate all the Ag ions in the form of AgCl. 

In the cockroach identification study Carlson and Brenner found that the composition of the outer, waxy layer of a roach is distinct to a particular species. Thus, by dissolving this waxy coating and injecting it into the gas stream of a gas chromatograph, scientists can identify the cockroach unambiguously in less than half an hour. This technique is particularly useful for identifying hybrid Asian-German cockroaches, which have become a major problem for the food industry. 

The second special point about solution reactions is that to obtain the moles of reactants, we must use the volume of a particular solution and its molarity. This procedure was covered in Section 4.3. 

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Strong Electrolytes Weak Electrolytes 4.7 Stoichiometry of Precipitation Reactions Reduction Reactions 4.10 Balancing Oxidation-Reduction... 

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

Imbalance in the stoichiometry of polycondensation reactions of AA-BB-type monomers can be overcome by changing to heterofunctional AB-type monomers. Indeed, IIMU has been subjected to bulk polycondensation using lipases as catalyst in the presence of 4 A molecular sieves. At 70 °C, CALB showed 84% monomer conversion and a low molecular weight polymer (Mn 1.1 kDa, PDI 1.9). No significant polymerization was observed with other lipases (except R cepacia lipase, 47% conversion, oligomers only) and in reference reactions with thermally deactivated CALB or in the absence of enzyme. Further optimization of the reaction conditions (60wt% CALB, II0°C, 3 days, 4 A molecular sieves) gave a polymer with Mn of 14.8 kDa (PDI 2.3) in 86% yield after precipitation [42]. 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

The enzymatic synthesis approaches are discussed in more detail in Chapter 19. A protease can be used to catalyze the synthesis of a peptide bond (Scheme 31.21). When the stoichiometry of the reactions is such that two moles of phenylalanine methyl ester are used with one mole of Z-Asp, the Z-APM PM product precipitates and shifts the equilibrium to >95% conversion.232 This is the basis of the commercial TOSOH process operated by Holland Sweetener that uses thermolysin.233 One significant variation has been the use of racemic PM instead of the L-isomer. Because the enzyme will only recognize the l-PM isomer to form the peptide bond, the unreacted d-PM isomer forms a salt and then, after acidification, the d-PM can be chemically racemized and recycled. 

Titration — A process for quantitative analysis in which measured increments of a - titrant are added to a solution of an - analyte until the reaction between the analyte and titrant is considered as complete at the - end point [i]. The aim of this process is to determine the amount of an analyte in a -> sample. In addition, the determination can involve the measurement of one or several physical and/or chemical properties from which a relationship between the measured parameter/s and the concentration of the analyte is established. It is also feasible to measure the amount of a - titrand that is added to react with a fixed volume of titrant. In both cases, the -> stoichiometry of the reaction must be known. Additionally, there has to be a means such as a -> titration curve or an - indicator to recognize that the -> end point has been reached. The nature of the reaction between the titrant and the analyte is commonly indicated by terms like acid-base, complexometric, redox, precipitation, etc. [ii]. Titrations can be performed by addition of measured volume/mass increments of a solution,... 

Precipitation reactions have several applications in analysis in gravimetric methods, in precipitation titrations, and in separations. Gravimetry, which used to be a major l>art of analytical chemistry, has expanded less rapidly than other aspects of analysis and does not now occupy a prominent place. Precipitation titrimetry always has been restricted in application because most precipitation reactions fail to meet the requirements of rapid reaction rate and adequate stoichiometry. In separations, precipitation reactions are used in two ways in one the precipitate involved is of direct concern, and in the other it acts as a carrier for another substance of interest. The application of precipitation reactions to separations is described in Chapter 22. 

Metallic thallium has been suggested as an intermediate in a reaction reported by Gilman and Jones.78,79). The addition of an alkyl or aryllithium compound to a mixture of the corresponding alkyl or aryliodide and thallous halide give a dark precipitate of the finely divided metal. This disappeared during the course of the reaction to give good yields of the trialkyl or aryl thallium compounds. The overall stoichiometry of these reactions was as in Eq. (1). 

I n volumetric analysis, the volume of a known reagent required for complete reaction with analyte by a known reaction is measured. From this volume and the stoichiometry of the reaction, we calculate how much analyte is in an unknown substance. In this chapter we discuss general principles that apply to any volumetric procedure, and then we illustrate some analyses based on precipitation reactions. Along the way, we introduce the solubility product as a means of understanding precipitation reactions. 

The lithium salt precipitates out quantitatively from pentane solution. Consequently, the pure B-R-9-BBN is isolated by decantation from the salt and removal of the solvent. It is necessary that the stoichiometry of the reaction is closely controlled in order to achieve high yields. The excess organolithium reagent reacts to give the insoluble ate complex. 

Certain aqueous reactions are useful for determining how much of a particular substance is present in a sample. For example, if we want to know the concentration of lead in a sample of water, or if we need to know the concentration of an acid, knowledge of precipitation reactions, acid-base reactions, and solution stoichiometry will be useful. Two common types of such quantitative analyses are gravimetric analysis and acid-base titration. 

The stoichiometry between two reactants in a precipitation reaction is governed by a conservation of charge, requiring that the total cation charge and the total anion charge in the precipitate be equal. The reaction units in a precipitation reaction, therefore, are the absolute values of the charges on the cation and anion that make up the precipitate. Applying equation 2.3 to a precipitate of Ca3(P04)2 formed from the reaction of Ca and P04 , we write... 

Quantitative Calculations In precipitation gravimetry the relationship between the analyte and the precipitate is determined by the stoichiometry of the relevant reactions. As discussed in Section 2C, gravimetric calculations can be simplified by applying the principle of conservation of mass. The following example demonstrates the application of this approach to the direct analysis of a single analyte. 

In a gravimetric analysis a measurement of mass or change in mass provides quantitative information about the amount of analyte in a sample. The most common form of gravimetry uses a precipitation reaction to generate a product whose mass is proportional to the analyte. In many cases the precipitate includes the analyte however, an indirect analysis in which the analyte causes the precipitation of another compound also is possible. Precipitation gravimetric procedures must be carefully controlled to produce precipitates that are easily filterable, free from impurities, and of known stoichiometry. 

Quantitative Calculations The stoichiometry of a precipitation reaction is given by the conservation of charge between the titrant and analyte (see Section 2C) thus... 

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