Stoichiometry of Solution Reactions

6 Stoichiometry of Solution Reactions 437 %/ Self-Check Exercise 14.7 

I AIM To understand the strategy for solving stoichiometric problems for solution reactions. 

Because so many important reactions occur in solution, it is important to be able to do stoichiometric calculations for solution reactions. The principles needed to perform these calculations are very similar to those developed in Chapter 9. It is helpful to think in terms of the following steps  

Steps for Solving Stoichiometric Problems Involving Solutions 

STEP 1 Write the balanced equation for the reaction. For reactions involving ions, it is best to write the net ionic equation. 

Step 4 Calculate the moles of other reactants or products, as required. Step 5 Convert to grams or other units, if required. 

What if all ionic solids were soluble in water  

It is always best to add concentrated acid to water, not water to the acid. That way, if any splashing occurs accidentally, it is dilute acid that splashes. 

EXERCISE 15.7 What volume of 12 M HCl must be taken to prepare 0.75 L of 0.25 MHCl  

---

Because a mixture, unlike a chemical compound, has a variable composition, the relative amounts of substances in a solution must be specified. The qualitative terms dilute (relatively little solute present) and concentrated (relatively large amount of solute) are often used to describe solution content. However, we need to define solution composition more precisely to perform calculations. For example, in dealing with the stoichiometry of solution reactions in Chapter 4, we found it useful to describe solution composition in terms of molarity, or the number of moles of solute per liter of solution. 

Stoichiometry of Solution Reactions 491 Neutralization Reactions 495 Solution Composition Normality 497 Chapter Review SOI... 

The approach followed in Chapter 3, with minor modifications, readily applies to the stoichiometry of solution reactions represented by net ionic equations. 

An important consideration when solving these problems is that data are given about the parent compounds and not about the particular ions in the net ionic equation. After all, you do not have reagent bottles with labels that say 3M OH but rather 3Af NaOH. Figure 4.6 (page 95) shows how the modifications fit into the flowchart for the stoichiometry of solution reactions. 

The numerical methods in this book can be applied to all components in the system, even inerts. When the reaction rates are formulated using Equation (2.8), the solutions automatically account for the stoichiometry of the reaction. We have not always followed this approach. For example, several of the examples have ignored product concentrations when they do not affect reaction rates and when they are easily found from the amount of reactants consumed. Also, some of the analytical solutions have used stoichiometry directly to ease the algebra. This section formalizes the use of stoichiometric constraints. 

Otsuka et al. (107) describe [Ni(CNBu )2], as a reddish brown microcrystalline substance, which is extremely air-sensitive. It can be recrystallized from ether at —78°C, and is soluble in benzene in the latter solution the infrared spectrum (2020s, br, 1603m, 1210m) and proton NMR (three peaks of equal intensity at t8.17, 8.81, and 8.94) were obtained. Neither analytical data nor molecular weight is available on this complex. The metal-ligand stoichiometry is presumably established by virtue of the molar ratio of reactants and by the stoichiometries of various reaction products. 

The first step In balancing a redox reaction is to divide the unbalanced equation into half-reactions. Identify the participants in each half-reaction by noting that each half-reaction must be balanced. That Is, each element In each half-reaction must be conserved. Consequently, any element that appears as a reactant In a half-reaction must also appear among the products. Hydrogen and oxygen frequently appear in both half-reactions, but other elements usually appear In just one of the half-reactions. Water, hydronium ions, and hydroxide ions often play roles In the overall stoichiometry of redox reactions occurring in aqueous solution. Chemists frequently omit these species in preliminary descriptions of such redox reactions. 

This method is primarily based on measurement of the electrical conductance of a solution from which, by previous calibration, the analyte concentration can be derived. The technique can be used if desired to follow a chemical reaction, e.g., for kinetic analysis or a reaction going to completion (e.g., a titration), as in the latter instance, which is a conductometric titration, the stoichiometry of the reaction forms the basis of the analysis and the conductometry, as a mere sensor, does not need calibration but is only required to be sufficiently selective. 

Material-balance problems are particular examples of the general design problem discussed in Chapter 1. The unknowns are compositions or flows, and the relating equations arise from the conservation law and the stoichiometry of the reactions. For any problem to have a unique solution it must be possible to write the same number of independent equations as there are unknowns. 

The stoichiometry of the reaction dictates that the final generated NO concentration will be equal to the concentration of SNAP in the solution. The method can be summarized as follows. Saturated cuprous chloride solution is first prepared by adding 150 mg CuCl to 500 mL distilled water. This solution is then deoxygenated by purging with pure nitrogen or argon gas for 15 min. The final, saturated CuCl solution will have a concentration of approximately 2.4 mM at room temperature. The solution is light sensitive and must therefore be kept in the dark prior to use. 

Kinetic studies in the physiological pH range (6.5 to 7.8) provided consistency with the above results in that the accumulation of H202 was also observed and the stoichiometry of the reaction depended on the conditions applied (66-69). However, a simple 2 1 stoichiometry was confirmed between cysteine consumed and hydrogen peroxide formed in dilute solution. The reaction followed Michaelis-Menten kinetics with... 

We need to deal with the stoichiometry of this reaction. For this reason, we need to know the moles of each of the reactants. We can find these from the concentration and the volume of each solution. 

The intervention of a metal ion in the stoichiometry of a reaction has been illustrated several times previously. Reaction is forced to completion in ester hydrolysis since the carboxylate grouping forms a more stable complex than the ester moiety does. A similar driving force underlies the formation of macrocycles and the completion of transamination by formation of the metal-Schiff base complex. The latter is particularly relevant in dilute solution and at low pH. For example, the extent of aldimine formation between pyridoxal and alanine is undetectable at the physiological pH but occurs to the extent of = 10% in the presence of zinc... 

Since the experimental kinetic data refer to a reaction rate and how this is affected by variables, such as concentration, temperature, nature of the solvent, presence of other solutes, structural variations of the reactants, and so forth, the assignment of a mechanism is always only indirectly derived from primary data. Therefore, it is not surprising that more than one mechanism has often been proposed to explain the same rate law and that reaction mechanisms, which were once consistent with all experimental information available on a system, have later on been considered erroneous and have been disregarded, or drastically modified, as long as new experimental evidence was accumulated. In general, the stoichiometry of the reaction, even when this is a simple one, cannot be directly related with its mechanism, and when the reaction occurs through a series of elementary steps, the possibility that the experimental rate law may be interpreted in terms of alternative mechanism increases. Therefore, to resolve ambiguities as much as possible, one must use aU the physicochemical information available on the system. Particularly useful here is information on the structural relations between the reactants, the intermediate, and the reaction products. 

By controlling the stoichiometry of the reaction between lanthanide trichlorides and sodium cyclopentadienide it is possible to replace the chloride ions stepwise. Equihbria are rapidly established, so the addition of Ln(C5H5)3 to one or two equivalents of LnCls will produce M(C5H5)2C1 and M(C5H5)Cl2, respectively. The dichlorides are known only for the lanthanides from samarium to lutetium and are obtained from THF solutions as tris-THF adducts. 

---