Precipitation Reactions Solubility Rules

The Ostwald Step Rule, or the rule of stages postulates that the precipitate with the highest solubility, i.e., the least stable solid phase will form first in a consecutive precipitation reaction. This rule is very well documented mineral formation via precursors and intermediates can be explained by the kinetics of the nucleation process. The precipitation sequence results because the nucleation of a more soluble... 

A reaction where a solid forms is called a precipitation reaction. General rules on solubility help predict whether a solid—and what solid—will form when two solutions are mixed. 

The solubility rules in Table 1.1 are used to predict the outcomes of precipitation reactions. 

How would you use the solubility rules in Table 1.1 to separate the following pairs of ions In each case indicate what reagent you would add and write the net ionic equation for the precipitation reaction (a) lead(II) and copper(II) ions ... 

Our goal in this chapter is to help you learn about reactions in aqueous solutions, including titrations. We will present a set of solubility rules you can use to predict whether or not precipitation will take place when two solutions are mixed. You may want to talk to your instructor and/or check your text for other solubility rules. These rules will be useful as you learn to write net ionic equations. If you are unsure about mass/mole relationships, you may want to review Chapter 3. And remember—Practice, Practice, Practice. 

Precipitation reactions involve the formation of an insoluble compound, a precipitate, from the mixing of two aqueous solutions containing soluble compounds. To predict if precipitation will occur upon the mixing of two solutions, you must know and be able to apply the following solubility rules. You should apply these rules to all combination of cations with anions in each of the mixed solutions. 

In this chapter, you learned about solutions and how to use molarity to express the concentration of solutions. You also learned about electrolytes and nonelectrolytes. Using a set of solubility rules allows you to predict whether or not precipitation will occur if two solutions are mixed. You examined the properties of acids and bases and the neutralization reactions that occur between them. You then learned about redox reactions and how to use an activity table to predict redox reactions. You learned about writing net ionic equations. Finally, you learned how to use the technique of titrations to determine the concentration of an acid or base solution. 

To know if a precipitate will form as a product of a reaction in an aqueous solution, it is necessary to know a few simple solubility rules. These are given in all standard general chemistry texts, but they will be given here to aid in writing equations ... 

Write the molecular equation. The molecular equation shows the reactants and products as molecules. The solubility rules are used to determine if a product is insoluble. Precipitation reactions are examples of a double-displacement reaction. Therefore, the cation of... 

Predicting the identity of the solid product in a precipitation reaction requires knowledge of the solubilities of common ionic substances. As an aid in predicting the products of precipitation reactions, some simple solubility rules are given in Table 4.1. You should memorize these rules. 

The Ostwald step rule, or the mle of stages, postulates that the precipitate with the highest solubility (i.e., the least stable solid phase) will form first in a consecutive precipitation reaction. This mle is very well documented mineral formation via precursors and intermediates can be explained by the kinetics of the nucleation process. The precipitation sequence results because the nucleation of a more soluble phase is kinetically favored over that of a less soluble phase because the more soluble phase has the lower solid-solution interfacial tension (7cw) than the less soluble phase (equation 50). In other words, a supersaturated solution will nucleate first the least stable phase (often an amorphous solid phase) because its nucleation rate is larger than that of the more stable phase (Figure 13.26). While the Ostwald step mle can be explained on the basis of nucleation kinetics, there is no thermodynamic contradiction in the initial formation of a finely divided precursor. 

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 the aqueous solutions of two water-soluble compounds are mixed, there may be a reaction between the ions of these solutions. If one of the products is insoluble, crystals of this product fall from the resulting solution. This solid product is called a precipitate. Let us consider the reaction between the solutions of lead(ll) nitrate, Pb(N03)2, and potassium iodide, Kl. According to the solubility table in Appendix A, both are soluble in water. This means that the solution of lead(ll) nitrate contains Pb2+ and N03 ions, and the potassium iodide solution contains K+ and I- ions. The possible products of this reaction are Pbl2 and KN03. According to solubility rules, potassium nitrate is soluble in water, but lead(ll) iodide is not. As soon as the two reactants mix, insoluble lead(ll) iodide crystals settle at the bottom of the container as a yellow precipitate. 

Skill 16.7 Applying solubility rules of inorganic salts to predict the occurrence of precipitation reactions... 

Since the reactants are both ionic species, the expert system will check for aqueous precipitation reactions. In this case, using solubility rules, the expert system predicts logically, but incorrectly, that iron(III) carbonate will precipitate from the solution. In fact, if a precipitate is formed, it is more likely to be iron(III) hydroxide, since iron(III) ion and carbonate ion are both rather susceptible to hydrolysis. Rules have not yet been established to predict when hydrolysis of the components will override precipitate formation. 

From a knowledge of the solubility rules (see Section 4.2) and the solubility products listed in Table 16.2, we can predict whether a precipitate will form when we mix two solutions or add a soluble compound to a solution. This ability often has practical value. In industrial and laboratory preparations, we can adjust the concentrations of ions until the ion product exceeds K p in order to obtain a given compound (in the form of a precipitate). The ability to predict precipitation reactions is also useful in medicine. For example, kidney stones, which can be extremely painful, consist largely of calcium oxalate, CaC204 (K p = 2.3 X 10 ). The normal physiological concentration of calcium ions in blood plasma is about 5 mM (1 mM = 1 X 10 M). Oxalate ions ( 204 ), derived from oxalic acid present in many vegetables such as rhubarb and spinach, react with the calcium ions to form insoluble calcium oxalate, which can gradually build up in the kidneys. Proper adjustment of a patient s diet can help to reduce precipitate formation. Example 16.10 illustrates the steps involved in precipitation reactions. 

As we said, there is no simple way to decide whether any given ion combination is soluble or not, so Table 4.1 provides a short list of solubility rules to memorize. They allow you to predict the outcome of many precipitation reactions. 

Precipitation reactions are those in which the reactants exchange ions to form an insoluble salt—one which does not dissolve in water. Reaction oc-ctirs when two ions combine to form an insoluble solid or precipitate. We predict whether such a compound can be formed by consulting solubility rules (see Table 1). If a possible product is insoluble, a precipitation reaction should occur. 

On the basis of the general solubility rules given in Table 8.1, write a balanced molecular equation for the precipitation reactions that take place when the following aqueous solutions are mixed. Underline the formula of the precipitate (solid) that forms. If no precipitation reaction is likely for the solutes given, so indicate. 

Using the solubility rules provided in the table above, complete the following chemical equations. Indicate whether a precipitate forms or not. Identify the precipitate. If no reaction occurs, write NR. 

For most analyses, standard solutions are aqueous solutions containing several percent of a mineral acid such as HCl or nitric acid. When mixing different elements in acids, it is important to remember basic chemistry and solubility rules for inorganic compounds. The elements must be compatible with each other and soluble in the acid used so that no precipitation reactions occur. Such reactions would change the solution concentration of the elements involved in the reaction and make the standard useless. Combinations to be avoided are silver and HCl, barium and sulfuric acid, and similar... 

The products of many of these reactions can be predicted by using the solubility rules. Ion-combination reactions that form precipitates (water-insoluble compounds) are known as precipitation reactions. In the reaction above, only AgQ is insolnble (solnbility rule 3) and precipitates as soon as the reactants are mixed... 

Barium sulfate precipitates (solubility rule 6) when a solution of barium chloride reacts with a solution of sodium sulfate. The other product of the reaction, NaCl, is soluble and does not precipitate ... 

Malachite and aurichalcite cannot be distinguished by using only precipitation reactions and Bronsted-Lowry add-base reactions. Another type of acid-base reaction, the Lewis reaction (reaction category 3), is necessary. Zinc ion will react with hydroxide ion to form insoluble zinc hydroxide (solubility rule 4), which will then react with excess hydroxide ion as a Lewis add to form the soluble Lewis adduct Zn(OH)4 Although the copper ion will react with hydroxide ion to form insoluble Cu(OH)2, this hydroxide does not ad as a Lewis acid and will not dissolve in excess hydroxide ion. 

The combined solution (before any reaction occurs) contains the ions K, OH , Fe ", and NO,. The salts that might precipitate are KNO, and Fe(OH),. The solubility rules in Table 4.1 indicate that both and NO, salts are soluble. However, Fe(OH), is only slightly soluble (Rule 5) and hence will precipitate. The balanced equation is... 

Suppose you are trying to help your friend understand the general solubility rules for ionic substances in water. Explain in general terms to your friend what the solubility rules mean, and give an example of how the rules could be applied in determining the identity of the precipitate in a reaction between solutions of two ionic compounds. 

Summarize the simple solubility rules for ionic compounds. How do we use these rules in determining the identity of the solid formed in a precipitation reaction Give examples including balanced complete and net ionic equations. 

In our earlier discussion of precipitation reactions, we considered general rules for predicting the solubility of common salts in water. (Section 4.2) These rules give us a qualitative sense of whether a compound has a low or high solubility in water. By considering solubility equilibria, however, we can make quantitative predictions about solubility. 

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