Chemical equations solubility products

STRATEGY First, we write the chemical equation for the equilibrium and the expression for the solubility product. To evaluate Ksp, we need to know the molarity of each type of ion formed by the salt. We determine the molarities from the molar solubility, the chemical equation for the equilibrium, and the stoichiometric relations between the species. We assume complete dissociation. 

STRATEGY First, we write the chemical equation for the equilibrium between the solid solute and the complex in solution as the sum of the equations for the solubility and complex formation equilibria. The equilibrium constant for the overall equilibrium is therefore the product of the equilibrium constants for the two processes. Then, we set up an equilibrium table and solve for the equilibrium concentrations of ions in solution. 

The equilibrium constant expression associated with systems of slightly soluble salts is the solubility product constant, Ksp. It is the product of the ionic concentrations, each one raised to the power of the coefficient in the balanced chemical equation. It contains no denominator since the concentration of a solid is, by convention, 1, and for this reason it does not appear in the equilibrium constant expression. The Ksp expression for the PbS04 system is ... 

The solubility product constant, Ksp, is the equilibrium constant expression for sparingly soluble salts. It is the product of the ionic concentration of the ions, each raised to the power of the coefficient of the balanced chemical equation. 

However, many important chemical problems cannot be described by explicit equations. They require iterative solutions. As an example, consider an apparently simple problem. The solubility product of calcium sulphate, gypsum, is defined as... 

Write the balanced chemical equation that represents the dissociation of each compound in water. Then write the corresponding solubility product expression. 

The ranges of Eh and pH over which a particular chemical species is thermodynamically expected to be dominant in a given aqueous system can be displayed graphically as stability fields in a Pourbaix diagram,10-14 These are constructed with the aid of the Nernst equation, together with the solubility products of any solid phases involved, for certain specified activities of the reactants. For example, the stability field of liquid water under standard conditions (partial pressures of H2 and 02 of 1 bar, at 25 °C) is delineated in Fig. 15.2 by... 

Dissolution of Sparingly Soluble Salts. Obtain precipitates of calcium carbonate and calcium oxalate in test tubes by reacting the relevant salts. Decant the solutions and pour an acetic acid solution onto the moist precipitates. What happens Repeat the experiment, but use hydrochloric acid instead of the acetic acid. Write the equations of the chemical reactions in the molecular and net ionic forms. Explain the results obtained on the basis of the dissociation constants of the acids and the solubility product. 

SOLUTION The chemical equation and the expression for the solubility product are... 

Quadratic equations arise frequently in the mathematical descriptions of common physical and chemical processes. For instance, silver chloride is only very slightly soluble in water. It has been determined experimentally that the solubility product Ksp of silver chloride at 25C is 1.56 x 10 12 M2, meaning that in a saturated solution the concentrations of silver ion and chloride ion satisfy the relationship... 

Consider the chemical equation for AgCl dissolved in water to make a saturated solution AgCl(s) <—> Ag1+(aq) + Cl1 (aq). At 298 K the solubility product constant is 1.8 x 10-10, which indicates that is a slightly soluble salt. There is a way of making AgCl even less soluble, via the common ion effect. Consider the following, when an ion that is already present is added to the solution, the equilibrium will shift to consume the increase in concentration of the ion. 

Net ionic equations (Chapter 9), like all other balanced chemical equations, give the mole ratios of reactants and products. Therefore, any calculations that require mole ratios may be done with net ionic equations as well as with total equations. However, a net ionic equation does not yield mass data directly because part of each soluble ionic compound is not given. For example, we can tell how many moles of silver ion are required to produce a certain number of moles of a product. 

The solubility product, ATgp, is equal to [Pb+ ] [Cl p. The chloride-ion concentration enters in this expression as the square becau two chloride ions are involved in the equation for the chemical reaction, as was discussed in Chapter 19. The numerical value of the solubility product is then found by use of Equation 2 ... 

The solubility product constant expression is the product of the concentrations of the ions each raised to the power equal to the coefficient of the ion in the chemical equation. The small value of indicates that products are not favored at equilibrium. Thus, few barium ions are present at equilibrium (1.0 X lO M) and a patient can safely ingest a barium sulfate solution to obtain a clear X ray like the one shown in Figure 18-14. 

Write the balanced chemical equation for the solubility equilibrium and the solubility product constant expression. 

Iron(III) sulfate, Fe2(S04)3, is a yellow compound that is used as a coagulant in water treatment. Write a balanced chemical equation and a solubility product expression for its dissolution in water. 

Write a chemical equation for the dissolution of mercury(I) chloride in water, and give its solubility product expression. 

Potassium chloride as a component in fertilizers is specified in terms of its K2O equivalent. On this basis, 100% pure potassium chloride is equated to 63.18% K2O equivalent. The approximately 96% pure potassium chloride product of the flotation separation is thus equivalent to 60.7% K2O (see Chap. 9). Chemical grades correspond to the once-crystallized, soluble product of about 99.5% KCl, and 99.95% KCl, which is the Refined, twice crystallized material. The price differential, 60-62.4% K2O equivalent at 108-112 US /tonne, and 99.95% KCl at 115-137 US /tonne (1995 prices [30]) is sufficient to cover the cost of the additional processing for Chemical grade potash. 

For part (a), we write the appropriate chemical equations and solubility product expression, designate the equilibrium concentrations, and then substitute into the solubility product expression. For part (b), we recognize that NaF is a soluble ionic compound that is completely dissociated into its ions. MgF2 is a slightly soluble compound. Both compounds produce F ions so this is a common ion effect problem. We write the appropriate chemical equations and solubility product expression, represent the equilibrium concentrations, and substitute into the solubility product expression. For part (c), we compare the molar solubilities by calculating their ratio. 

For aqueous systems, a unit activity is expected for the solid species (i.e., we assume that the chemical reactivity of a solid in water is unchanging as long as there is solid in equilibrium with the solution). Also, for dilute concentrations, we assume that the activities are equal to the concentrations of the species. With these assumptions, we can reduce the solubility product constant equation to... 

The solubility of a solid salt can be expressed through the use of a solubility product constant Kgp, as illustrated in the equation for a general salt MX that dissociates in solution. The chemical equation for this process is also given below. 

Solubility Products. The activity expression of equation 1,36 can be applied in general to any chemical reaction. In particular, it can be used to describe dissolution reactions for sohds such as aluminum hydroxide ... 

Consider, for example, a trace metal cation, B, isomorphously substituted into a solid composed of metal cations. A, and anions, Y. The chemical formula, A B Y is variable because x can range from 0.0 to 1.0 if AY and BY, the solid-solution end members, form a continuous series. Unlike ionic compounds of fixed composition, solid solutions do not have constant solubility products. Instead, equations for both components, AY and BY, must be specified ... 

Given the problems inherent to the electrode measurement of in soil solution, it would seem desirable to consider other methods of estimating Eh. Theoretically, one could analyze the soil solution for the reduced and oxidized species of a redox couple, say dissolved Fe " and Fe, and use the Nernst relation (equation 7.2) to calculate Eh. Usually, however, there are difficulties with this approach. In the case of the Fe couple, Fe solubility in all but very acid soil solutions is extremely low (below detection), so that an assumption must be made about the activity of the free Fe " ion. It might reasonably be assumed that the solubility product of Fe oxide limits this activity, but Fe oxides have a rather wide range of solubilities depending on oxide crystallinity, structure, and purity. A better approach to measuring E would be to use an indicator chemical that undergoes reversible electron transfer with natural redox couples in soil solution, that is,... 

As mentioned in Section 3.7.1.2, there is a considerable scatter of solubility product values obtained in the molten KCl-NaCl eutectic using different methods of solubility determination. This disagreement in the solubility parameters may be explained by differences in the sizes of oxide particles whose solubility is to be determined. The difference in size causes the scatter of the solubility data according to the Ostwald-Freundlich equation and the employment of the isothermal saturation method, which implies the use of commercial powders (often pressed and sintered), leads to values which are considerably lower than those obtained by the potentiometric titration technique where the metal-oxides are formed in situ. Owing to this fact, the regularities connected with the effect of physico-chemical parameters of the oxides or the oxide cations should be derived only from solubility data obtained under the same or similar experimental conditions. However, this does not concern the dissociation constants of the oxides, since homogeneous acid-base equilibria are not sensitive to the properties of the solid phase of... 

If the the basis species formed by dissolution of solid Pjt are in chemical equilibrium with the solid, then the activity product, Q,, is equal to the thermodynamic solubility product. A, . Residue equations for the solids are formed in the following manner. At each finite-difference node the activity product, Qsk is computed and compared to the theoretical solubility product, Ksk, for the solid. If the solid is present at the node, or if the solid is not present and Qsk > Kski then the residue for the solid at the node is set equal to the algebraic difference, Q k — A ajt. On the other hand, if Qsk < Kak and the solid is not present at the node, the residue is set equal to zero. This procedure provides a residue equation for each solid at each node and eliminates the need to change the number of unknowns at nodes where solids have precipitated or dissolved. 

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