Precipitation reactions predicting solid products

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. 

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. 

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. 

In chemistry, precipitation means something different from when the weather forecaster on the news predicts rain. A precipitate is a solid substance that does not dissolve in a solution, and precipitation describes the process of the precipitate forming. This generally takes place when one or more soluble reactants in a solution undergo a reaction to form one or more insoluble products. The result is that a solid substance will form in a solution, either depositing on the bottom of the container or sometimes floating around in the solution. Depending on the circumstances, precipitation can be a desirable or undesirable result. 

Assume that you have mixed two solutions, and a solid product (a precipitate) forms. How can you find out what the solid is What is its formula There are several possible approaches you can take to answering these questions. For example, we saw in Chapter 7 that we can usually predict the identity of a precipitate formed when two solutions are mixed in a reaction of this type if we know some facts about the solubilities of ionic compounds. 

In the previous section, we used a precipitation reaction to illustrate how to convert a molecular equation to an ionic equation. A precipitation reaction occurs in aqueous solution because one product is insoluble. A precipitate is an insoluble solid compound formed during a chemical reaction in solution. To predict whethCT a precipitate will form when you mix two solutions of ionic compounds, you need to know whether any of the potential products that might form are insoluble or not This is another application of the solubility rules (Section 4.1). 

For the conversion from solid Ca-maleate to solid Ca-D-malate (Scheme 12.2), kinetic models were developed to predict kinetics of dissolution of the substrates, the enzyme-catalyzed reaction, and the precipitation of the reaction product, all of which occur at the same time [44]. 

So far we have considered solids dissolving in aqueous solutions. Now we will consider the reverse process—the formation of precipitates. When solutions are mixed, various reactions can occur. We have already considered acid-base reactions in some detail. In this section we show how to predict whether a precipitate will form when two solutions are mixed. We will use the ion product, which is defined just like the Ksp expression for a given solid except that initial concentrations are used instead of equilibrium concentrations. For solid CaF2 the expression for the ion product (Q) is written... 

Figure 1. The predicted sequence of solid reaction products of reaction between spent nuclear fuel and J-13 well water at 25°C. The width of the bars represents the percentage of the number of moles of all precipitates at any given point of reaction progress. There is a break in the appearance of mesolite. There is no break in the appearance of Pu02(c) (Reproduced with permission from Ref. 19. Copyright 1988 Materials Research Society.)...

Another critical aspect in the effectiveness of mass transfer correlations to predict coefficients in reacting systems is the very troublesome but all-too-common tendency for the surface of a reacting solid, catalyst, or precipitating product to become covered by another solid or second-phase liquid, or by a gas in a three-phase mixture. Such a heterogeneous film would obviously have a profound effect on the expected mass transfer coefficient and in many cases can cause a reaction to stop before the expected conversion. These films are obviously unique to each reacting system, thereby preventing any generahzations as to whether they are susceptible to chemical or physical manipulation. Chemical manipulation could be achieved by addition of a surfactant that would be able to modify surface properties to prevent or modify formation of the film. 

The hydrazones, when solid, may be used as derivatives. The method of testing under B usually leads to a product of higher purity. The time required for the precipitation of thehydrazone is of value in predicting something concerning the nature of the compound. The reaction is not very a( curate as a time test for the reason that supersaturated solutions may be formed. 

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