Equilibrium in acid-base reactions

If you understand this analogy and can make predictions of ice cream transfer using the table, then you can understand how to predict the direction of equilibrium in acid-base reactions. Turn the page. 

Use the approach we developed in Section 2.4 to predict the position of equilibrium in acid-base reactions. Equilibrium favors reaction of the stronger acid and stronger base to form the weaker acid and the weaker base. It is helpful to remember that even though ammonium ions are positively charged, they are much weaker acids than carboxylic acids. 

Ethanol has about the same acidity as water. Higher-molecular-weight, water-soluble alcohols are slightly weaker acids than water. Thus, although alcohols have some acidity, they are not strong enough acids to react with weak bases such as sodium bicarbonate or sodium carbonate. (At this point, it would be wise to review Section 4.4, which discusses the position of equilibrium in acid-base reactions.)... 

One specific application in the first category is to estimate the time required for a reaction to reach equilibrium in nature. If equilibrium is assumed in modeling a geochemical process, it is important to know the limitations (e.g., the timescale for the assumption to be valid). For example, in acid-base reactions, the reaction is rapid and the timescale to reach equilibrium is much less than one second. Hence, pH measurement of natural waters is usually meaningful and can be used to estimate species concentrations of various pH-related reactions. However, in redox reactions, the reaction is often slow and it may take days or years to reach equilibrium. Therefore, pe (or Eh) measurement of natural waters may not mean... 

On the other hand, if HA is an uncharged acid z = — V, e.g. CH3—CO2H), the right-hand side of Eq. (4-10) involves the sum of two reciprocal radii (zha = 0) and a strong influence of the relative permittivity on the ionization equilibrium is expected. Because in acid/base reactions of this charge type, neutral molecules are converted into anions and cations, which attract each other, reaction (4-5) will shift to the right with an increase in relative permittivity of the solvent in which HA is dissolved. Ionization increases when increases. This rule is qualitatively verifiable for water and alcohols as... 

The enthalpy change of some reactions can be measured directly, but for those that do not go to completion (as is common in acid-base reactions), thermodynamic data from reactions that do go to completion can be combined using Hess s law to obtain the needed data. For example, the enthalpy and entropy of ionization of a weak acid, HA, can be found by measming (1) the enthalpy of reaction of HA with NaOH, (2) the enthalpy of reaction of a strong acid (such as HCl) with NaOH, and (3) the equilibrium constant for dissociation of the acid (usually determined from the titration curve). 

Look at the table on p. 691 and notice how it looks just like the "children-with-ice-cream" table. We can use this table to make predictions about equilibrium position in acid-base reactions just as we did for the children with ice cream. 

First determine what type of reaction is occurring. When elemental sodium is used, an oxidation-reduction reaction takes place, producing a sodium alkoxide and hydrogen gas. In acid-base reactions, the position of the equilibrium resides on the side with the weaker acid and weaker base (i.e., the more stable species). 

In acid-base reactions, the equilibrium favors formation of the weaker acid. 

Several types of reactions are commonly used in analytical procedures, either in preparing samples for analysis or during the analysis itself. The most important of these are precipitation reactions, acid-base reactions, complexation reactions, and oxidation-reduction reactions. In this section we review these reactions and their equilibrium constant expressions. 

The most important types of reactions are precipitation reactions, acid-base reactions, metal-ligand complexation reactions, and redox reactions. In a precipitation reaction two or more soluble species combine to produce an insoluble product called a precipitate. The equilibrium properties of a precipitation reaction are described by a solubility product. 

In developing this treatment for determining equilibrium constants, we have considered a relatively simple system in which the absorbance of HIn and Im were easily measured, and for which it is easy to determine the concentration of H3O+. In addition to acid-base reactions, the same approach can be applied to any reaction of the general form... 

The position of equilibrium in an acid-base reaction lies to the side of the weaker acid. 

The sensitivity of the equilibrium constant to temperature, therefore, depends upon the enthalpy change AH . This is usually not a serious limitation, because most reaction enthalpies are sufficiently large and because we commonly require that the perturbation be a small one so that the linearization condition is valid. If AH is so small that the T-jump is ineffective, it may be possible to make use of an auxiliary reaction in the following way Suppose the reaction under study is an acid-base reaction with a small AH . We can add a buffer system having a large AH and apply the T-jump to the combined system. The T-jump will alter the Ka of the buffer reaction, resulting in a pH jump. The pH jump then acts as the forcing function on the reaction of interest. 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

The favorability of acid-base reactions is affected, in pa by electrostatic interactions between charged atoms a dipoles within the same molecule. The equilibrium w shift in the direction of an ion that is stabilized 1 intramolecular ion-dipole interactions. 

The acidic and basic properties of aqueous solutions are dependent on an equilibrium that involves the solvent, water. The reaction involved can be regarded as a Bransted-Lowry acid-base reaction in which the H20 molecule shows its amphiprotic nature ... 

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—... 

We can use this more general view to discuss the strengths of acids. In our generalized acid-base reaction (52), the proton transfer implies the chemical bond in HB, must be broken and the chemical bond in HB2 must be formed. If the HB, bond is easily broken, then HB, will be a strong acid. Then equilibrium will tend to favor a proton transfer from HB, to some other base, B2. If, on the other hand, the HB, bond is extremely stable, then this substance will be a weak acid. Equilibrium will tend to favor a proton transfer from some other acid, HB2, to base B, forming the stable HB, bond. 

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