Stoichiometry of acid-base reactions

We will now deal with the stoichiometry of acid-base reactions in aqueous solutions. The procedure is fundamentally the same as that used previously. 

The evolution of carbon dioxide essentially follows the stoichiometry of acid—base reactions. Baking soda determines the amount of carbon dioxide evolved, whereas the type of acid controls the speed of liberation. The reaction equations for some acids with baking soda are as follows ... 

Attia reported a potentiometric investigation of the stoichiometry of acid-base reactions of phosphorus acids with oxide ions in molten KN03 at 350 °C... 

Recall the stoichiometry of acid-base reactions discussed in Chapter 4. 

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

Quantitative Calculations In acid-base titrimetry the quantitative relationship between the analyte and the titrant is determined by the stoichiometry of the relevant reactions. As outlined in Section 2C, stoichiometric calculations may be simplified by focusing on appropriate conservation principles. In an acid-base reaction the number of protons transferred between the acid and base is conserved thus... 

The discussion of acid-base titrations in Chapter 4 focused on stoichiometry. Here, the emphasis is on the equilibrium principles that apply to the acid-base reactions involved. It is convenient to distinguish between titrations involving—... 

The quantitative aspects of acid-base chemistry obey the principles Introduced earlier in this chapter. The common acid-base reactions that are important in general chemistry take place in aqueous solution, so acid-base stoichiometry uses molarities and volumes extensively. Example Illustrates the essential features of aqueous acid-base stoichiometry. 

By plotting i versus the ratio R = (CHX)t/(CB)t during the titration, they determined simultaneously the extent of acid-base interaction, the stoichiometry of that interaction and the degree of association of the acid-base adduct. Fig. 4.13 shows hypothetical titration curves line ABC corresponds to the interaction between B and HX as monomers without further reaction between BHX and HX, and the subsequent occurrence of the latter reaction to a small extent is indicated by the line ABC and to the full extent by line ABDE, when no more HX can react with BHX HX line AFDE arises when formation of BHX HX starts right away in the case of previous partial dimerization of B, the various lines will begin at A instead of A. 

In a related application, polyelectrolyte microgels based on crosslinked cationic poly(allyl amine) and anionic polyfmethacrylic acid-co-epoxypropyl methacrylate) were studied by potentiometry, conductometry and turbidimetry [349]. In their neutralized (salt) form, the microgels fully complexed with linear polyelectrolytes (poly(acrylic acid), poly(acrylic acid-co-acrylamide), and polystyrene sulfonate)) as if the gels were themselves linear. However, if an acid/base reaction occurs between the linear polymers and the gels, it appears that only the surfaces of the gels form complexes. Previous work has addressed the fundamental characteristics of these complexes [350, 351] and has shown preferential complexation of cationic polyelectrolytes with crosslinked car-boxymethyl cellulose versus linear CMC [350], The departure from the 1 1 stoichiometry with the non-neutralized microgels may be due to the collapsed nature of these networks which prevents penetration of water soluble polyelectrolyte. 

Two complications can prevent a simple determination or the concentration of each species from a measurement of absorbance at a chosen frequency. Although most of the acid-base reactions of interest result in a one-to-one stoichiometry, one cannot assume this a priori, and two-to-one and three-to-one adducts might also be present. Fortunately, this is usually an easy point to Tesolve. The presence of an isosbeslic point or point of constant absorbance (see Fig. 9.1) is usually a reliable criterion that only two absorbing species (the free acid or base and a single adduct) are present.26... 

When a strong acid or base is added to a buffered solution, it is best to deal with the stoichiometry of the resulting reaction first. After the stoichiometric calculations are completed, then consider the equilibrium calculations. This procedure can be represented as follows ... 

According to Chapter 11, an acid is a substance that upon dissolving in water increases the concentration of hydronium (H30 ) ions above the value found in pure water, and a base is a substance that increases the concentration of hydroxide (OH ) ions above its value in pure water. Despite the careful language, it is commonplace to view acids and bases as substances that dissociate to give protons (which upon hydration become hydronium ions) and hydroxide ions, respectively. If the dissociation is complete, we can easily calculate the concentration of hydronium and hydroxide ions in the solution and then calculate the yield of acid-base neutralization reactions, and acid-base titrations, by the methods of stoichiometry in solution. But experience shows that many acid-base reactions do not go to completion. So, to predict the amount (or concentration) of... 

Often the enthalpy of reaction is obtained by measuring the equilibrium constant of an acid-base reaction over a range of temperatures. If In K is plotted versus l/T. the slope will be equal to -AH/R. Thus various experimental methods have been devised to measure the equilibrium constant by spectrophotometric methods. Any absorption which differs between one of the reactants (either acid or base) and the acid-base adduct is a potential source of information on the magnitude of the equilibrium constant since it gives the concentration of two of the three species involved in the equilibrium directly and the third indirectly from a knowledge of the stoichiometry of the reaction. For example, consider the extensively studied reaction between organic carbonyl compounds and iodine. The infrared carbonyl absorption is shifted in frequency in the adduct with respect to the free carbonyl compound. Thus the equilibrium mixture exhibits two absorption bands in the carbonyl region of the spectrum (Fig. 9.1) and the relative concentrations of the free carbonyl and the adduct can be obtained.3- Alternatively, one can observe the absorption of the iodine molecule, U, in... 

Teachers report that students get a much better idea of the role of chemistry in society. Contexts are effective in engaging the students. Students are confronted with a lot of knowledge in a short time. They learn a lot. There are still problems however. Using the knowledge gained in other contexts needs to be established more firmly. There is a need for more practice with skills like calculations in stoichiometry, writing equations in precipitation reactions or acid-base reactions. 

The concept of eqnilibrinm constants is extremely important in chemistry. As you will soon see, equilibrinm constants are the key to solving a wide variety of stoichiometry problems involving eqnilibrium systems. For example, an industrial chemist who wants to maximize the yield of sulfuric acid, say, must have a clear understanding of the equilibrium constants for all the steps in the process, starting from the oxidation of sulfur and ending with the formation of the final product. A physician specializing in clinical cases of acid-base imbalance needs to know the equilibrium constants of weak acids and bases. And a knowledge of equilibrium constants of pertinent gas-phase reactions will help an atmospheric chemist better understand the process of ozone destraction in the stratosphere. 

An acid-base reaction is often called a neutralization reaction. When just enough strong base is added to react exactly with the strong acid in a solution, we say the acid has been neutralized. One product of this reaction is always water. The steps in dealing with the stoichiometry of any neutralization reaction are the same as those we followed previously. 

---