Predicting Whether a Precipitate Will Form

If you mix aqueous solutions of two ionic compounds, can you predict if a precipitate will form Consider this example. When solid sodium iodide and potassium nitrate are each dissolved in water, each solution consists of separated ions dispersed throughout the solution  

Let s follow three steps to predict whether a precipitate will form  

Note the ions present in the reactants. The reactant ions are 

Consider the possible cation-anion combinations. In addition to the two original ones, Nal and KNO3, which you know are soluble, the other possible cation-anion combinations are NaN03 and KI. 

Decide whether any of the combinations is insoluble. A reaction does not occur when you mix these starting solutions because all the combinations—Nal, KNO3, NaN03, and KI—are soluble. All the ions remain in solution. (You ll see shortly a set of rules for deciding if a product is soluble or not.) 

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EXAMPLE 11.10 Predicting whether a precipitate will form when two solutions are mixed... 

Knowing the value of the solubility product constant can also allow us to predict whether a precipitate will form if two solutions, each containing an ion component of a slightly soluble salt, are mixed. The ion-product, sometimes represented as Q (same form as the solubility product constant), is calculated taking into consideration the mixing of the volumes of the two solutions, and this ion-product is compared to IQp. If it is greater than IQp, precipitation will occur until the ion concentrations have been reduced to the solubility level. 

In the next chapter, you will extend your knowledge of equilibria involving aqueous ions. You will learn how to calculate the pH at an equivalence point, so you can select an appropriate indicator for any acid-hase titration. You will also learn why equilihrium is important to the solubility of compounds that are slightly soluble, and how to predict whether a precipitate will form as the result of a reaction between ions in solution. 

You can use the relationship between the ion product expression and the solubility product expression to predict whether a precipitate will form in a given system. One common system involves mixing solutions of two soluble ionic compounds, which react to form an ionic compound with a very low solubility. If Qsp > Kp. based on the initial concentrations of the ions in solution, the sparingly soluble compound will form a precipitate. 

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 ... 

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... 

Use the reaction quotient to predict whether a precipitate will form when two solutions are mixed, and then calculate the equilibrium concentrations that result (Section 16.3, Problems 17-24). 

In each of the following, both compounds are water-soluble. Predict whether a precipitate will form when solutions of the two are mixed, and, if so, identify the compound that precipitates, (a) Pb(N03)2, Nal (b) Ba(N03)2, KCl (c) (NH4)2S, AgNO,... 

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. 

How can we predict whether a precipitate will form when a compound is added to a solntion or when two solutions are mixed It depends on the solubility of the solute, which is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. Chemists refer to substances as soluble, slightly soluble, or insoluble in a qualitative sense. A substance is said to be soluble if a fair amount of it visibly dissolves when added to water. If not, the substance is described as slightly soluble or insoluble. All ionic compounds are strong electrolytes, but they are not equally soluble. 

For equilibrium reactions involving an ionic soM in aqueous solution, any one of the following conditions may exist (1) the solution is unsaturated, (2) the solution is saturated, or (3) the solution is supersaturated. For concentrations of ions that do not correspond to equilibrium conditions we use the reaction quotient (see Section 14.4), which in this case is called the ion product (Q), to predict whether a precipitate will form. Note that Q has the same form as K p except that the concentrations of ions are not equilibrium concentrations. For example, if we mix a solution containing Ag" ions with one containing CP ions, then the ion product is given by... 

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. 

The Key Event Formation of a Solid Predicting Whether a Precipitate Will Form... 

When the FeCl3 and Fe4(Fe(CN)6)3(s) solutions are mixed, if the concentrations of the ions Fe + and Fe(CN)6 are greater than those that can exist in a saturated solution of Fe4(Fe(CN)6)3, the equilibrium will shift to the left and Fe4(Fe(CN)6)3(s) will precipitate. To predict whether a precipitate will form when the two solutions are mixed, you must first calculate the concentrations of the ions. 

One of the greatest tools available to chemists is experimentation. If you perform an experiment in which two solutions are mixed, you can make observations that help you understand what is happening. For example, using Table 4.1 to predict whether a precipitate will form is not nearly as exciting as seeing the precipitate form, as in Figure 4.4. Careful observations in the laboratory portion of the course will make your lecture material easier to master. 

For concentrations of ions that do not correspond to equilibrium conditions, we use the reaction quotient (see Section 10.2), which in this case is called the ion product (Q), to predict whether a precipitate will form. Note that Q has the same form... 

From knowledge of the solubility rules (see Section 12.5) and the solubility products listed in Table 12.3, 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. 

SAMPLE PROBLEM 19.9 Predicting Whether a Precipitate Will Form Problem A common laboratory method for preparing a precipitate is to mix solutions containing the component ions. Does a precipitate form when 0.100 L of 0.30 M Ca(N03)2 is mixed with 0.200 L of 0.060 M NaF ... 

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