Weak electrolytes solutions

Fig. 11-1. A strong electrolyte solution conducts better than a weak electrolyte solution.

The solute in an aqueous strong electrolyte solution is present as ions that can conduct electricity through the solvent. The solutes in nonelectrolyte solutions are present as molecules. Only a small fraction of the solute molecules in weak electrolyte solutions are present as ions. 

Hence, the theory of electrolyte solutions subsequently developed in two directions (1) studies of weak electrolyte solutions in which a dissociation equilibrium exists and where because of the low degree of dissociation the concentration of ions and the electrostatic interaction between the ions are minor and (2) studies of strong electrolyte solutions, in which electrostatic interaction between the ions is observed. 

The first rigorous method for weak electrolyte solutions was that of Edwards et al. ( 5). Because comparisons with the models of other workers will be made, the thermodynamic framework will be outlined and the assumptions that were made stated. For a single solute which dissociates only in the aqueous solution, the model is based on four principles ... 

Unlike weak electrolytes, solutions of strong ones have a far higher specific conductance the rise of the latter with rising concentration is also much more rapid. There is another difference the anomalies ascertained in the colligative properties of strong electrolytes cannot be ascribed to partial dissociation of molecules to ions as in the case of weak electrolytes. 

This molality in can be calculated from the observed AT using Eq. (10-20). Thus freezing-point measurements on weak electrolyte solutions of known molality m enable the determination of a. 

Aromatic and basic substances are adsorbed on Sephadex columns. Aromatic sorption is usually completely reversible and if electrolyte concentration is high enough this is also the case for adsorption due to electrostatic interaction. Negative adsorption or ion exclusion is encountered for acidic substances in sufficiently weak electrolyte solutions. 

The three-dimensional, second-order, nonlinear, elliptic partial differential equation may be simplified in the limit of weak electrolyte solutions, where the hyperbolic sine of is well approximated by 4). This yields the linearized Poisson—Boltzmann equation... 

The water solutions of some substances conduct electricity, while the solutions of others do not. The conductivity of a solution depends on its solute. The more ions a solution contains, the greater its conductivity. Solutions that conduct electricity are called electrolytes. Solutions which are good conductors of electricity are known as strong electrolytes. Sodium chloride, hydrochloric acid, and potassium hydroxide solutions are examples of strong electrolytes. If solutions are poor conductors of electricity, they are called weak electrolytes. Vinegar, tap water, and lemon juice are examples of weak electrolytes. Solutions of substances such as sugar and alcohol solutions which do not conduct electricity are called nonelectrolytes. 

Weak electrolytes Solutes that are incompletely dissociated into ions in a particular solvent. 

Skill 16.6 Identifying properties of strong and weak electrolyte solutions... 

In this chapter we discuss some of the properties of electrolyte solutions. In Sec. 12-1, the chemical potential and activity coefficient of an electrolyte are expressed in terms of the chemical potentials and activity coefficients of its constituent ions. In addition, the zeroth-order approximation to the form of the chemical potential is discussed and the solubility product rule is derived. In Sec. 12-2, deviations from ideality in strong-electrolyte solutions are discussed and the results of the Debye-Hiickel theory are presented. In Sec. 12-3, the thermodynamic treatment of weak-electrolyte solutions is given and use of strong-electrolyte and nonelectrolyte conventions is discussed. 

In this section, we discuss the thermodynamic treatment of weak-electrolyte solutions. In weak-electrolyte solutions, both undissociated molecules and ions exist. Thus, it may seem possible to describe the properties of these solutions by using the conventions developed either for nonelectrolyte or for electrolyte solutions. Our discussion will center around the usefulness of these conventions and the regions of concentration in which one may be more applicable than the other. For simplicity, our discussion will be restricted to the case of a weak electrolyte with the formula HA. 

Application of the expression for the chemical potential of a strong electrolyte given by Eq. (12-10) to the system consisting of weak-electrolyte solute HA in a solvent yields the result... 

The elementary physical chemistry treatment of weak-electrolyte solutions follows from the zero approximation to Eq. (12-48). In zero approximation, Eq. (12-48) becomes... 

Thus it is useful to associate a with an activity coefficient at low concentrations. At low concentrations, it is useful to represent Pha by Eq. (12-59). Equation (12-59) is similar to Eq. (12-30) which expresses //ha in terms of strong-electrolyte conventions. This completes our discussion of weak-electrolyte solutions. [More details concerning computations involving Eq. (12-56) can be found in F. H. MacDougall, Thermodynamics and Chemistry, chap. XV, 3d ed., John Wiley, Sons, Inc., New York, 1939.]... 

Using X-ray scattering measurements, Toney and co-workers obtained experimental information about the density of water molecules in a weak electrolyte solution near charged Ag(lll) surfaces. These results were somewhat controversial, because they indicate the presence of three to four dense layers of water as well as distinct solution structures for negatively and positively charged surfaces. Nonetheless, the layering proposed by those authors is at least superficially similar to that derived by simulations. (We return to the discussion of these results later when simulations under electric fields are considered.) It would be useful to have additional experimental results for uncharged surfaces. 

Look at Figure 7.2 in the text. It is possible for a weak electrolyte solution to cause the bulb to glow brighter than a strong electrolyte. Explain how this is possible. 

Fig. 8 Limiting current density in a depleted DBL containing a weak electrolyte solution relative to the value corresponding to complete dissociation. The lower abcisa scale and the continuous lines correspond to a 1 —1 electrolyte. The upper abcisa and the dashed lines correspond to a 2 —1 electrolyte. The ratio D 2,u/Du has been given the values 0.5, 1, and 2, as shown on the curves.

The selected relationships given more suggest that the analysis of precise conductance data is an invaluable tool in studies on the structure and properties of both strong and weak electrolyte solutions. This field has been the focus of attention for several decades and the reader is referred to other more complete reviews on the subject. Here, we will merely list some analytical applications in order to given an introductory insight into the use of the conductance method. 

NaCI forms a strong electrolyte solution (Section 7.5). Weak electrolyte solutions are covered in Chapter 14. 

FIGURE 14.12 Conductivity of a weak electrolyte solution (a) Pure water will not conduct electricity, (b) An HF solution contains some ions, but most of the HF is intact. The light glows only dimly. 

Solutions such as these are called weak electrolyte solutions. 

Figure 4.1 An apparatus for distinguishing between electrolytes and nonelectrolytes, and between weak electrolytes and strong electrolytes. A solution s ability to conduct electricity depends on the number of ions it contains, (a) Pure water contains almost no ions and does not conduct electricity, therefore the lightbulb is not lit. (b) A weak electrolyte solution such as HFfa ) contains a small number of ions, and the lightbulb is dimly lit. (c) A strong electrolyte solution such as NaClfn ) contains a large number of ions, and the lightbulb is brightly lit. The molar amounts of dissolved substances in the beakers in (b) and (c) are equal.

Having dealt with activities and activity coefficients in solutions made up from strong electrolytes, we now turn to the determination of in weak electrolytic solutions. For this purpose we discuss some applications of the Debye-Huckel theory. 

Weak acids are classified as weak electrolytes, and the resulting solutions—called weak electrolyte solutions—conduct electricity only weakly. Figure 4.14 sununarizes the electrolytic properties of solutions. 

Grosse, C., Shilov, V. N., Electrophoretic Mobility of Colloidal Particles in Weak Electrolyte Solutions, J. Colloid Interface Sci, 1999,211, 160-170. 

Freezing point and osmotic pressure are both colligative properties. As we saw in Chapter 14, the values of these properties depend on the total concentrations of particles (molecules and ions) in a solution, but not on the identity of those particles. We can use the ICE method for equilibrium calculations (Chapter 15) to determine the total concentrations of particles (molecules and ions) in a weak electrolyte solution, as we learned to do in this chapter. Once we have those results we can turn to equations (14.4) and (14.5) to do the calculations required in parts (a) and (b). 

A decay constant is a first-order rate constant describing radioactive decay. Degree of ionization refers to the extent to which molecules of a weak acid or weak base ionize. The degree of ionization increases as the weak electrolyte solution is diluted. (See also percent ionization.)... 

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