Soluble ionic compounds

A precipitation reaction occurs when two or more soluble species combine to form an insoluble product that we call a precipitate. The most common precipitation reaction is a metathesis reaction, in which two soluble ionic compounds exchange parts. When a solution of lead nitrate is added to a solution of potassium chloride, for example, a precipitate of lead chloride forms. We usually write the balanced reaction as a net ionic equation, in which only the precipitate and those ions involved in the reaction are included. Thus, the precipitation of PbCl2 is written as... 

Sulfide minerals and K. Sulfides are among the least soluble ionic compounds. Their AQ, values are often smaller than 10-25. For this reason, many sulfides are found as minerals, for example (clockwise from the left), iron pyrite (FeS2), yellow orpiment (AS2S3). and black galena (PbS). 

For each of the following reactions, suggest two soluble ionic compounds that, when mixed together in water, result in the net ionic equation given ... 

A1P04, used in cements and as an antacid (c) iron(II) hydroxide, Fe(OH)2 (d) Select two soluble ionic compounds that, when mixed in solution, form each of the insoluble compounds in parts (a), (b), and (c). Identify the spectator ions. 

Sodium bromide and silver nitrate are both soluble ionic compounds, so the major species are Na , Ag, Br, NO3, and H3 O molecules. The solubility guidelines presented in Section 4- identify the... 

For soluble ionic compounds, ions are written separately using the state symbol (aq). 

Finally, we will separate all soluble ionic compounds. We need only consider those compounds not already separated or that do not have a dash beneath the formula. 

B—A (nitrous acid) and D (acetic acid) are weak acids, and E (ammonia) is a weak base. Weak acids and bases are weak electrolytes. C (ethanol) is a nonelectrolyte. Potassium nitrate (B) is a water-soluble ionic compound. 

C—A is a soluble ionic compound all the others are acids or bases. C is a neutral molecular compound. 

Infer the relative solubilities of two sparingly soluble ionic compounds. 

Sodium chloride and other soluble ionic solids dissolve in polar solvents such as water because of ion-dipole forces. An ion-dipole force is the force of attraction between an ion and a polar molecule (a dipole). For example, NaCl dissolves in water because the attractions between the Na and Cl ions and the water molecules provide enough energy to overcome the forces that bind the ions together. Figure 4.14 shows how ion-dipole forces dissolve any type of soluble ionic compound. 

Barium sulfate, BaS04, is a sparingly soluble ionic compound that is used to enhance X-ray imaging. 

As more ions enter the solution, the rate of the reverse change, recrystallisation, increases. Eventually, the rate of recrystallisation becomes equal to the rate of dissolving. As you know, when the forward rate and the backward rate of a process are equal, the system is at equilibrium. Because the reactants and the products are in different phases, the reaction is said to have reached heterogeneous equilibrium. For solubility systems of sparingly soluble ionic compounds, equilibrium exists between the solid ionic compound and its dissociated ions in solution. 

The Ksp expressions are based on balanced equations for saturated solutions of slightly soluble ionic compounds. The exponents in the K p expressions match the corresponding coefficients in the chemical equation. The coefficient 1 is not written, following chemical convention. 

Q A saturated solution of a slightly soluble ionic compound does not contain any of the solute in solid form. Is this system at equilibrium Explain your answer briefly. 

You calculate Qp by substituting the concentration of each ion into the expression. If Qp is larger than K p, the product of the concentrations of the ions is greater than it would be at equilibrium. For the system to attain equilibrium, some of the ions must leave the solution by precipitation. Conversely, if Qp is less than IQp, the product of the concentration of the ions is smaller than it is at equilibrium. Therefore, the solution is not yet saturated and more ions can be added to the solution without any precipitation. The relationship between Qsp and K p for the dissociation of a slightly soluble ionic compound is summarized on the next page. Use the following general equation as a reference. 

You can use the relationship between the ion product expression and the solubility product expression to predict whether a precipitate will form in a given system. One common system involves mixing solutions of two soluble ionic compounds, which react to form an ionic compound with a very low solubility. If Qsp > Kp. based on the initial concentrations of the ions in solution, the sparingly soluble compound will form a precipitate. 

How do you know which ionic compounds are soluble and which are not In your previous chemistry course, you learned a set of solubility guidelines. Table 9.3 summarizes these guidelines. Remember the higher guideline takes precedence. (For instance, guideline 3 says that carbonates have very low solubility. Sodium carbonate is soluble, however, because guideline 1 says that ionic compounds containing sodium are soluble.) Chemists do not usually work with solubility products for soluble compounds. Thus, you will not find soluble ionic compounds listed in tables. If you see a compound in a A p table, you know that it has a low solubility relative to compounds such as sodium chloride. 

For example, silver nitrate and sodium chloride are both soluble ionic compounds. When they are mixed in solution, however, they react to form silver chloride, as shown in Figure 9.7. The equation for the reaction is ... 

In the previous Sample Problems, you saw that combining solutions of soluble ionic compounds can cause the precipitation of a slightly soluble ionic compound. Analytical chemists use precipitation reactions to remove ions from solution or to identify ions in an unknown solution. 

For example, consider a solution that contains three halide ions Cl , Br , and l. Since these halides all come from the same group on the periodic table, they share many properties. When they are the anions in slightly soluble ionic compounds, however, they have different solubilities. (See Table 9.4.) Therefore, chemists can use fractional precipitation to separate them from solution. 

In Chapter 9, as in most of Unit 4, you learned about equilibrium reactions. In this section, you analyzed precipitation reactions. You mainly examined double-displacement reactions—reactions in which two soluble ionic compounds react to form a precipitate. You used the solubility product constant, Ksp, to predict whether or not a precipitate would form for given concentrations of ions. In Unit 5, you will learn about a class of reactions that will probably be new to you. You will see how these reactions interconvert chemical and electrical energy. 

Soluble ionic compounds tend to be strong electrolytes, while alcohols and organic compounds are nonelectrolytes. Remember that classification as a strong electrolyte, weak electrolyte, or nonelectrolyte is somewhat subjective. Freshwater can be either a weak electrolyte or a nonelectrolyte depending on its purity. The important consideration in classifying a substance is to what extent an aqueous solution of the substance will conduct electricity. 

Chemistry is often conducted in aqueous solutions. Soluble ionic compounds dissolve into their component ions, and these ions can react to form new products. In these kinds of reactions, sometimes only the cation or anion of a dissolved compound reacts. The other ion merely watches the whole affair, twiddling its charged thumbs in electrostatic boredom. These uninvolved ions cire called spectator ions. 

J.12 Identify the following as a strong acid, a weak add, a base, a soluble ionic compound, or an insoluble ionic compound in water (a) HN03 (b) KOH ... 

Strong electrolytes are usu-v / ally soluble ionic compounds or strong acids. Weak electrolytes are typically weak acids or weak bases. Common nonelectrolytes are water, alcohols, and sugars. 

Many biological and environmental processes involve the dissolution or precipitation of a sparingly soluble ionic compound. Tooth decay, for example, begins when tooth enamel, composed of the mineral hydroxyapatite, Cas PO OH,... 

In general, the solubility of a slightly soluble ionic compound is decreased by the presence of a common ion in the solution, as illustrated in Figure 16.11. The quantitative aspects of the common-ion effect are explored in Worked Example 16.11. 

P The solubility of a sparingly 1 soluble ionic compound is increased on addition of H30 + if the anion of the compound is the conjugate base of a weak acid. 

When solutions of soluble ionic compounds are mixed, an insoluble compound will precipitate if the ion product (IP) for the insoluble compound exceeds its fCsp. The IP is defined in the same way as /equilibrium concentrations. Certain metal cations can be separated by selective precipitation of metal sulfides. Selective precipitation is important in qualitative analysis, a procedure for identifying the ions present in an unknown solution. 

Bases accept (or take) hydrogen ions. In water, the strongest possible base is the hydroxide ion, OH-. A strong base ionizes essentially 100% to produce the OH- ion, so a strong base is a soluble ionic hydroxide. Like all soluble ionic compounds, ionic hydroxides are strong electrolytes and dissociate completely... 

What happens if both products are soluble ionic compounds Both ionic compounds will be ions dissolved in the water. If neither product precipitates out, no reaction occurs. Try the following problem to practise writing the products of double displacement reactions and predicting their states. 

This equation does not show the change that occurs, however. It shows the reactants and products as intact compounds. In reality, soluble ionic compounds dissociate into their respective ions in solution. So chemists often use a total ionic equation to show the dissociated ions of the soluble ionic compounds. 

Write the soluble ionic compounds as ions. For example, write NH4+(aq) and ClTaq), instead of NH4Cl(aq). 

Group I ions, hydrogen ions, and ammonium ions always form soluble ionic compounds.)... 

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