Predicting Precipitation Reactions

Kgp values can be used to make predictions as to whether or not a precipitate will form when two solutions are mixed. To do this, we follow an approach very similar to that used in Chapter 12, to determine the direction in which a system will move to reach equilibrium. We work with a quantity Q, which has the same mathematical form as K. The difference is that the concentrations that appear in Q are those that apply at a particular moment. Those that appear in are equilibrium concentrations. Putting it another way, the value of Q is expected to change as a precipitation reaction proceeds, approaching Ksp and eventually becoming equal to it. 

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

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 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. 

To predict whether a precipitation reaction will occur on mixing aqueous solutions of two substances, you must know the solubility of each potential product—that is, how much of each compound will dissolve in a given amount of solvent at a given temperature. If a substance has a low solubility in water, it is likely to precipitate from an aqueous solution. If a substance has a high solubility in water, no precipitate will form. 

Predict whether a precipitation reaction will occur when aqueous solutions of CdCl2 and (NH4)2S are mixed. Write the net ionic equation. 

Of the two possible products, the solubility guidelines predict that CdS, a sulfide, is insoluble and that NH4C1, an ammonium compound, is soluble. Thus, a precipitation reaction will occur ... 

PROBLEM 4.6 Predict whether a precipitation reaction will occur in each of the following situations. Write a net ionic equation for each reaction that does occur. 

There are three important classes of aqueous reactions. Precipitation reactions occur when solutions of two ionic substances are mixed and a precipitate falls from solution. To predict whether a precipitate will form, you must know the solubility of each potential product. Acid-base neutralization reactions occur when an acid is mixed with a base, yielding water and a salt. The neutralization of a strong acid with a strong base can be written as a net ionic equation, in which nonparticipating, spectator ions are not specified ... 

The solubility product constant allows us to predict the degree of completeness of precipitation reactions. Whenever the product of the concentrations (each raised to the appropriate power) exceeds Ksp, the salt will precipitate until the concentration product equals Ksp. 

The solubility product can be used to explain and predict the degree of completeness of precipitation reactions. We can substitute the initial concentrations into the Ksp expression and compare the result, often times referred... 

In spite of its limitations (as outlined in the previous section) the solubility product relation is of great value in qualitative analysis, since with its aid it is possible not only to explain but also to predict precipitation reactions. The solubility product is in reality an ultimate value which is attained by the ionic product when equilibrium has been established between the solid phase of the slightly soluble salt and the solution. If conditions are such that the ionic product is different from the solubility product, the system will seek to adjust itself in such a manner that the ionic product attains the value of the solubility product. Thus, if the ionic product is arbitrarily made greater than the solubility product, for example by the addition of another salt with a common ion, the adjustment of the system results in the precipitation of the solid salt. Conversely, if the ionic product is made smaller than the solubility product, as, for instance, by diminishing the concentration of one of the ions, equilibrium in the system is attained by some of the solid salt passing into solution. 

The distribution of hydrolyzed V02+ as a function of pH at a total vanadium concentration of 10 xM is shown in Fig. 3. The curves in the distribution diagram also depend on the total vanadium concentration because of the dimer formation and the precipitation reactions. While distribution diagrams of this type for V02+ are incomplete, they nevertheless illustrate the interrelationship between some of the species present and are of predictive value below pH 6 and above pH 11, and possibly in the pH 6 to 11 interval provided one starts with a solution below pH 6 and slowly adds base. The unidentified soluble hydroxide species are less likely to form under those conditions. Species distribution diagrams for a number of V02+ complexes with several common ligands are given by Kraglen34. ... 

When two solutions are mixed, an insoluble substance sometimes forms that is, a solid forms and separates from the solution. Such a reaction is called a precipitation reaction and the solid that forms is called a precipitate. For example, a precipitation reaction occurs when an aqueous solution of potassium chromate, K2Cr04(aq), which is yellow, is mixed with a colorless aqueous solution containing barium nitrate, Ba(N03)2(aq). As shown in Fig. 4.12, when these solutions are mixed, a yellow solid forms. What is the equation that describes this chemical change To write the equation we must know the identities of the reactants and products. The reactants have already been described K2Cr04(aq) and Ba(N03)2(tf<7). Is there some way we can predict the identities of the products In particular, what is the yellow solid ... 

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. 

How do we recognize acid-base reactions One of the most difficult tasks for someone inexperienced in chemistry is to predict which reaction might occur when two solutions are mixed. With precipitation reactions we found that... 

We now discuss chemical reactions in further detail. We classify them as oxidation-reduction reactions, combination reactions, decomposition reactions, displacement reactions, and metathesis reactions. The last type can be further described as precipitation reactions, acid-base (neutralization) reactions, and gas-formation reactions. We will see that many reactions, especially oxidation-reduction reactions, fit into more than one category, and that some reactions do not fit neatly into any of them. As we study different kinds of chemical reactions, we will learn to predict the products of other similar reactions. In Chapter 6 we will describe typical reactions of hydrogen, oxygen, and their compounds. These reactions will illustrate periodic relationships with respect to chemical properties. It should be emphasized that our system is not an attempt to transform nature so that it fits into small categories but rather an effort to give some order to our many observations of nature. 

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

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. 

Sample Study Sheet 4.2 Predicting Precipitation Reactions and Writing Precipitation Equations... 

Tip-off You are asked to predict whether a precipitation reaction will take place between two aqueous solutions of ionic compounds, and if the answer is yes, to write the complete equation for the reaction. 

Step 2 Predict whether either of the possible products is water-insoluble. If either possible product is insoluble, a precipitation reaction takes place, and you may continue with step 3. If neither is insoluble, write No reaction. ... 

Predict whether a precipitate will form when water solutions of silver nitrate, AgN03( a ), and sodium phosphate, Na3P04( ), are mixed. If there is a precipitation reaction, write the complete equation that describes the reaction. 

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