Precipitates from double-displacement

In a DOUBLE DISPLACEMENT, the two compounds change partners with each other. Think of the time when you precipitated silver chloride from solutions of salt and silver nitrate ... 

A precipitate is a solid that separates from a solution as the result of a chemical reaction. You will learn more about precipitates in Chapter 9. Many double displacement reactions involve the formation of a precipitate. 

When you mix two aqueous ionic compounds together, there are two possible outcomes. Either the compounds will remain in solution without reacting, or one aqueous ionic compound will chemically react with the other. How can you predict which outcome will occur Figure 9.4 shows what happens when an aqueous solution of lead(II) nitrate is added to an aqueous solution of potassium iodide. As you can see, a yellow solid—a precipitate—is forming. This is a double displacement reaction. Recall, from Chapter 4, that a double displacement reaction is a chemical reaction that involves the exchange of ions to form two new compounds. It has the general equation... 

Figure 13 shows the result of the reaction between KI and Pb(N03)2- The products are a yellow precipitate of Pbl2 and a colorless solution of KNO3. From the equation, it appears as though the parts of the compounds just change places. Early chemists called this a double-displacement reaction. It occurs when two compounds in aqueous solution appear to exchange ions and form two new compounds. For this to happen, one of the products must be a solid precipitate, a gas, or a molecular compound, such as water. Water is often written as HOH in these equations. 

Sometimes a double-displacement reaction has one produa that is insoluble in water. As that produa forms, it emerges, or precipitates, from the solution as a solid. This process is called precipitation, such a reaaion is called a precipitation reaction, and the solid is called the precipitate. For example, when water solutions of calcium nitrate and sodium carbonate are mixed, calcium carbonate precipitates from the solution while the other product, sodium nitrate, remains dissolved. 

A double-displacement reaction results from pouring a clear, colorless solution of Pb(N03)2 into a clear, colorless solution of Kl, forming a yellow precipitate of Pblj. 

Many industrial processes are driven forward by separation of at least one of the reaction products. For many of the reactions in aqueous solutit ns one of the products has low solubility in the reaction medium. Its precipitation or evaporation pushes the reaction forward. If, on the other hand, all the products are soluble, an equilibrium is reached and a means of separation has to be applied to recover the product and to recycle the reagents (shift the equilibrium). The economics of such processes is strongly affected by the concentration of the product at equilibrium, which determines the load on the separation and the cost of concentrating the separated product. This could be the case in many processes such as acid production through displacement from its salts production of water-soluble salts, e.g., fertilizers, through metathetic ( double decomposition ) reactions and product-inhibited fermentation,... 

This reaction, which was already cited in this book, can be equilibrated. In strongly acidic medium, it evolves from left to right. It is the inverse in neutral or basic medium. It can be displaced toward the right in neutral medium by the addition of a zinc salt. There is a formation of a poorly soluble double salt, the zinc and potassium ferrocyanide, which precipitates... 

Anti-poly G poly C antibodies have been demonstrated by immunodiffusion and by complement fixation in the sera of rabbits immunized with poly G-poly C — MBSA (Michelson et al., 1971 Nahon-Merlin et al., 1971). The anti-poly G poly C antibodies react not only with poly G poly C but also with a large number of double-helical complexes such as poly A poly U, poly I poly C and poly dG poly dC. It is nevertheless to be noted that the complex poly A poly I in which two purine polynucleotides are involved is not precipitated by these antibodies, which is readily explained by the special stereochemical structure of this complex. The anti-poly G poly C antisera nevertheless precipitate poly iso A poly I, but in this case it can be considered that displacement of the glycosyl-hnkage from N to in polyisoadenylic acid converts this polynucleotide into an analogue of poly C. The complex is thus effectively between a poly purine ribonucleotide and a poly pyrimidine ribonucleotide as in the case of other complexes such as poly A poly U and poly I poly C. Immune sera against poly G poly C also react with the triple-stranded complex 2 poly G poly C. In addition they can precipitate one or the other of the component polynucleotides of the homologous complex. 

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