Precipitation reactions in aqueous solutions

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

Exercises 69 and 70 describe precipitation reactions in aqueous solutions. For each, write balanced (i) formula unit, (ii) total ionic, and (hi) net ionic equations. Refer to the solubility guidelines as necessary. 

Before examining the specific behavior of a few elements, this chapter will identify the main characteristics of condensation and precipitation reactions in aqueous solution. 

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. 

It undergoes double decomposition reactions in aqueous solution forming precipitates of insoluble products ... 

Cobalt(II) chloride undergoes many double decomposition reactions in aqueous solution to produce precipitates of insoluble cobalt salts. For example, heating its solution with sodium carbonate yields cobalt(lI) carbonate ... 

Magnesium sulfate undergoes three important types of reactions in aqueous solutions double decomposition, double salt formation, and formation of oxysulfate cements. Many insoluble magnesium salts may be precipitated out by double decomposition reactions ... 

This series of salts contains a hydroxyl group in the nucleus, and shows a distinctly alkaline reaction in aqueous solution, although they do not precipitate silver oxide from silver nitrate. With concentrated solutions of ammonium salts ammonia is evolved and the corresponding aquo-pentammino-salt is formed.4... 

Give balanced ionic equations for the following reactions in aqueous solution. If a reaction does not occur, write NR. Indicate precipitates by J. and gases by f. 

Dissolution-precipitation reactions often exhibit characteristic time scales that are much larger than those for complexation reactions in aqueous solution. When this is true, the aqueous species in a soil solution will come to mutual equilibrium long before they equilibrate with the solid phase via the reaction in l-q. 3.1. It is possible under these circumstances to define two useful criteria for dissolution precipiialionequilibrium, the ion activity product (IAP) ... 

Predict whether reactions in aqueous solutions will produce a precipitate, water, or a gas. 

Three types of products produced during reactions in aqueous solutions are precipitates, water, and gases. 

Unfortunately, this route is expensive and forms a large amount of silver halide as by-product. Complete precipitation of silver halides from organic solvents can also be quite slow, leading to silver-contaminated products. The nature of the precipitate can also be troublesome in some cases the silver halide forms as submicron particles which are difficult to filter. For these reasons, the preferred and most common metathesis approach is still to carry out the reaction in aqueous solution with either the free acid of the appropriate anion, or its ammonium or alkali metal salt as described in Sect. 2.1.1. 

The same factors that determine the reactions in aqueous solutions self-evidently also occur at the particle surfaces in equilibrium with the solution. The benefit of recognizing this fact is that the same tabulations of critical properties may be used to predict the surface reactions. The sol particles may be considered to represent a rigidized macrocluster of ion species. It should, however, be kept in mind that corrections have to be made for the adsorption against an ion cloud and against a surface potential. Both the surface charge and the site distribution at the surface (the surface morphology) influence the gel formation and the surface precipitation as well. The specific adsorption may, of course, be used for surface modification of the particle surfaces. 

Would tbe collection of LiC104 precipitate from tbe reaction in aqueous solution of Li2C03 and NaC104 be a convenient way of preparing and isolating LiC104 ... 

We will now deal with the stoichiometry of acid-base reactions in aqueous solutions. The procedure is fundamentally the same as that used previously for precipitation reactions. 

In writing equations for reactions in aqueous solution, it is often useful to indicate whether the dissolved substances are present predominantly as ions or as molecules. Let s reconsider the precipitation reaction between Pb(N03)2 and 2 KI ... 

This simplification (assumption (1) in Section 4.2.1) assumes that the concentration of at least one speeies is mueh larger than that of others. For example, in models of many reactions in aqueous solutions, the water concentration is considered to be constant. Similarly, in reactions in which precipitation occurs, the concentration of the solid phase is taken as the constant. These constant values are incorporated into the kinetic coefficients. In typical heterogeneous gas-solid reactions, the amount of reacting gas molecules is assumed to be much larger than the total amount of active catalyst sites. In this case, the concentration of the abundant gaseous species is included in the reaction rate coefficient as a constant (apparent kinetic coefficient). This simplifies the reaction model and often results in a linear model. Thus, for a surface catalytic process at steady state, the rate of adsorption for the reaction A+Z—> AZ can be expressed as... 

Evidence of a Chemical Reaction 206 7.6 Precipitation Reactions Reactions in Aqueous Solution Reactions 226... 

On an industrial scale, the reaction in aqueous solution is a disadvantage because concentrated water-free products cannot be obtained without additional working-up steps. Therefore, a water-free process was developed, which is outlined in Fig. 44. The alkyl polyglycoside is initially introduced into the reactor with an excess of butyl chloride and heated to 80 °C. The reaction is initiated by addition of potassium hydroxide as the catalyst. On completion of the reaction, the reaction mixture is neutralized, the potassium chloride precipitate is filtered off, and the excess butyl chloride is distilled off. The product is composed of various alkyl polyglycosides and alkyl polyglyco-... 

Reactions that form water or a gas Some double-replacement reactions in aqueous solution produce water or a gas (or both) rather than a precipitate. In such cases, the water or gas is shown as a product in the net ionic equation, as are the ions that produced it. The remaining ions are eUminated as spectator ions. The following example problem illustrates this concept. 

Precipitation Reactions—Some reactions in aqueous solution involve the combination of ions to yield a water-insoluble solid—a precipitate. Precipitation reactions are generally represented by net ionic equations, a form in which only the reacting ions and solid precipitates are shown, and spectator ions are deleted. Precipitation reactions usually can be predicted by using a few simple solubility guidelines (Table 5.1). 

Stoichiometry of Reactions in Aqueous Solutions Titrations— A common laboratory technique applicable to precipitation, acid-base, and redox reactions is titration. The key point in a titration is the equivalence point, which can be observed with the aid of an indicator. Titration data can be used to establish a solution s molarity, called standardization of a solution, or to provide other information about the compositions of samples being analyzed. 

This is an acid-base reaction, in which the base is the oxide ion (p. 89) the acidic oxide SiOj displaces the weaker acidic oxide CO2 in the fused mixture. But in aqueous solution, where the 0 ion cannot function as a strong basefp. 89),carbon dioxide displaces silica, which, therefore, precipitates when the gas is passed through the aqueous silicate solution. In a fused mixture of silica and a nitrate or phosphate, the silica again displaces the weaker acidic oxides N2O5 and P4OJ0 ... 

This reaction proceeds slowly in aqueous solution, so that the basic salt. Sn(OH)Cl, is slowly precipitated. Addition of excess hydrochloric acid gives the acids of formulae HSnCl3 and H2SnCl4. Salts of these acids containing the ions SnCl J and SnCl (chloro-stannates(II)) are known. 

Copper(II) ions in aqueous solution are readily obtained from any copper-containing material. The reactions with (a) alkali (p. 430), (b) concentrated ammonia (p 413) and (c) hydrogen sulphide (p. 413) provide satisfactory tests for aqueous copper(II) ions. A further test is to add a hexacyanoferrate(II) (usually as the potassium salt) when a chocolate-brown precipitate of copper(II) hexacyanoferrate(II) is obtained ... 

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