Ionic compounds predicting solubility

Predict whether ionic compounds are soluble or insoluble in water. See Section 4.2. 

Given the formula for an ionic compound, predict whether it is soluble in water or not. 

Using the solubility rules Given the formula of an ionic compound, predict its solnbihty in water. (EXAMPLE 4.1)... 

If we knew in advance which combinations of ions yield insoluble compounds, we could predict precipitation reactions. These compounds have been identified in the laboratory. Table 9.3 shows the results of such experiments for a large number of ionic compounds. Their solubilities have been summarized in a set of solubility guidelines that your instructor may ask you to memorize. These guidelines are in Active Figure 9.13. 

Predict whether each of the following ionic compounds is soluble in water ... 

EXAMPLE 4.9 Predicting whether an Ionic Compound Is Soluble... 

Prediction of solubility for simple ionic compounds is difficult since we need to know not only values of hydration and lattice enthalpies but also entropy changes on solution before any informed prediction can be given. Even then kinetic factors must be considered. 

Just as with replacement reactions, double-replacement reactions may or may not proceed. They need a driving force. The driving force in replacement reactions is reactivity here it is insolubility or covalence. In order for you to be able to predict if a double-replacement reaction will proceed, you must know some solubilities of ionic compounds. A short list of solubilities is given in Table 7-2. 

Hard water is the name given to water supplies that contain significant concentrations of Mg2+ and Ca2+ ions. Check on the solubility of ionic compounds formed with these ions and predict what problems they may cause. 

In this chapter, you will continue your study of acid-base reactions. You will find out how ions in aqueous solution can act as acids or bases. Then, by applying equilibrium concepts to ions in solution, you will be able to predict the solubility of ionic compounds in water and the formation of a precipitate. 

In this section, you learned why solutions of different salts have different pH values. You learned how to analyze the composition of a salt to predict whether the salt forms an acidic, basic, or neutral solution. Finally, you learned how to apply your understanding of the properties of salts to calculate the pH at the equivalence point of a titration. You used the pH to determine a suitable indicator for the titration. In section 9.2, you will further investigate the equilibria of solutions and learn how to predict the solubility of ionic compounds in solution. 

The next Sample Problem shows how to predict the solubility of an ionic compound when a common ion is present in solution. 

How do you predict whether a given concentration of ions will result in the precipitation of an ionic compound How can you tell if a solution is saturated You substitute the concentrations of the ions into an expression that is identical to the solubility product expression. Because these concentrations may not be the same as the concentrations that the equilibrium system would have, however, the expression has a different name the ion product. 

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. 

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. 

The solubility of most ionic compounds increases with temperature, despite the fact that the standard heat of solution (AH°) is negative for about half of them. Discussions of this seeming contradiction can be found in G. M. Bodner, On the Misuse of Le Chatelier s Principle for the Prediction of the Temperature Dependence of the Solubility of Salts, J. Chem. Ed. 1980,57, 117, and R. S. Treptow, Le Chatelier s Principle Applied to the Temperature Dependence of Solubility, J. Chem. Ed. 1984,61, 499. 

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. 

Suppose that you did not have any information about the solubility of various compounds, but you did have access to a large variety of ionic compounds. What would you need to do before predicting the products of the displacement reactions above Outline a brief procedure. 

Write the chemical formulas for calcium chloride and ammonium sulfate. Predict what kind of reaction will occur between them. Write a balanced chemical equation to show the reaction. Ionic compounds containing the ammonium ion are soluble. Ammonium sulfate is soluble, but barium chloride is not. 

Many interrelated factors affect the solubility of substances in water. This makes it challenging to predict which ionic substances will dissolve in water. By performing experiments, chemists have developed guidelines to help them make predictions about solubility. In Investigation 9-A, you will perform your own experiments to develop quidelines about the solubility of ionic compounds in water. 

The driving force behind double substitution reactions is the formation of a covalent compound (including water or a gaseous compound) or an insoluble ionic compound from ions in solution. A solid formed from ions in solution is called a precipitate. We can thus predict that a reaction will occur if soluble ionic compounds yield at least one insoluble ionic compound or one covalent compound. We need to be familiar with the solubilities of some common ionic compounds in water. Some types of ionic compounds that are soluble or insoluble in water are listed in Table 8.3. A more comprehensive tabulation of solubilities is presented in Table 8.4 for reference, not necessarily to be memorized. 

Predict the solubility of an ionic compound by using a solubility table. Describe solutions in terms of their degree of saturation. 

Notice that the title of the rules is general solubility rules. There are exceptions to the rules listed there. For example, although ionic compounds that contain halogens tend to be soluble, when the cation is lead (II) (as in lead (II) chloride or lead (II) iodide) the ionic compound will be insoluble in water. The rules listed are really meant to be general trends, which you may want to memorize. They will give you the ability to make fast predictions about the identity of a precipitate that is found after many ionic reactions, just not the one in our first example. 

When predicting solubility, scientists often use the phrase like dissolves like. Explain how water, a covalent compound, can be like an ionic compound. 

In order to predict whether a precipitation reaction will take place when two aqueous ionic compounds are mixed, you need to be able to predict whether the possible products of the double-displacement reaction are soluble or insoluble in water. 

It is more difficult to predict the solubility of polar molecular substances than to predict the solubility of ionic compounds and nonpolar molecular substances. Many polar molecular substances are soluble in both water and hexane. For example, ethanol is miscible with both water and hexane. The following generalization is helpfiil ... 

The factors that determine whether or not an ionic compound will be soluble in water are complex, making predictions difficult. As a result, a series of statements or rules have come into being that guide predictions. These rules are called the solubility rules. As with most general rules, exceptions exist, but they are correct most of the time. The following rules are organized in a hierarchal structure—that is, the first rule takes precedence over the second, the second over the third, and so forth. 

Let s apply these rules to predict the solubility of a few ionic compounds. 

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