UW Department of Chemistry Lab Lectures Online

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1 Lab 4: Effect of Temperature on Solubility and Fractional Crystallization Part I: Fractional Crystallization of Potassium Nitrate (KNO 3 ) Part II: Determining the Solubility Curve of Potassium Nitrate Introduction Part I. Fractional Crystallization of Potassium Nitrate (KNO 3 ) Mixtures are a fact of life in the chemical laboratory. Consider a synthesis of a desired product, C, by the reaction: 2 A + B C + D The desired product, C, can be expected to be part of a mixture for one or more reasons: (1) Two products are produced; (2) Reactions do not always go to completion and, in such cases, reactants and products could be mixed; and (3) Side reactions produce other products. Because of these reasons, separations are an important part of chemistry. In order to maintain the identity of the separated substances, the majority of separation methods are based on physical methods (e.g. filtration, extraction, distillation, fractional crystallization and chromatography). In this experiment, you will examine the method of crystallization (and re-crystallization) as a purification technique. The techniques of crystallization and re-crystallization are used extensively for the isolation and purification of organic and inorganic compounds. The technique exploits the differences in solubility of the components in a mixture. The desired compound is crystallized while impurities remain in solution. Let's begin by revisiting the definition of solubility. In a former Chemistry 142 experiment, "Household Chemicals", substances were classified as either soluble or not soluble. If a pea sized amount of solid dissolved in ~ 5 ml of solvent (water), the substance was classified as soluble. If any amount of solid still remained undissolved, the substance was classified as insoluble. The same applies to the solubility rules you learned in the section of the Zumdahl text covering precipitation reactions. This simple view of solubility worked well then, but is inadequate for our present purposes. The truth is that as long as we are not dealing with a network covalent compound, a bit of solute always dissolves. For example, although silver chloride is 'insoluble,' the concentration of AgCl(aq) is 1.8 x 10-5 M at 25 o C. This solubility could also be expressed as 0.26 mg AgCl per 100 ml of water. It is important to quote the temperature because, as you already know, solubility changes as a function of temperature. Most substances are more soluble at higher temperatures than at lower temperatures (see the Temperature Effects for Aqueous Solutions section in the Zumdahl text). What is the best way to express solubility? It all depends on what you are doing. For this experiment, the preferred way to express solubility will be the number of grams of solute dissolved in 100 g of solvent (H 2 O). Remember that the solute is the substance being dissolved and the solvent is what the solute is dissolving in. 1 of 6

2 Consider the problem of separating potassium nitrate, KNO 3, from ferrous ammonium sulfate,. 6H2 O. Suppose the mixture consists of 200 grams of KNO 3 and 10 grams of. 6H2 O. Notice the dimension of the problem, i.e. we have a desired compound, KNO 3, that is contaminated with a small amount of impurity,. 6H2 O. To understand how we might isolate KNO 3 from the mixture by crystallization, let us examine how the solubility of KNO 3 and solubility of. 6H2 O vary as a function of temperature (Figure 1). Figure 1. Solubility Curves for KNO 3 and. 6H2 O Consider what happens when the mixture is completely dissolved in a minimum amount of hot water (90 o C < T hot water < 100 o C). If the temperature of our hot water is 90 o C, we will need about 100g of the hot water to dissolve the mixture. Now consider what happens when we cool the mixture. At about 80 o C, KNO 3 will start crystallizing out of the solution. At this temperature, none of the. 6H2 O should crystallize. This is the basis of the separation process. If we continue cooling to 15 o C, about 150 g of the original 200 g KNO 3 will have crystallized leaving about 50 g still dissolved and mixed with the dissolved. 6H2 O. If we continue cooling to 0 o C, we should be able to collect 185 g of nearly pure KNO 3. Figure 1 indicates that all of the 10 g of. 6H2 O we started 2 of 6

3 with should still be dissolved at 0 o C (because at 0 o C the solubility of. 6H2 O is ~25 g/100 g H 2 O). Filtering out and drying the crystallized KNO 3 should complete the task. While we significantly improve the purity of the KNO 3 using this technique, we cannot help but lose some of it to the dissolved state. In the above example, 15 g KNO 3 is lost this is a price we pay for purifying. Although the crystallization process is quite selective, a small amount of. 6H2 O does enter within the KNO 3 crystals, but the trapped amount will be considerably less than the amount that was there originally. One could improve the purity by going through additional re-crystallization procedures, as you will do in this experiment. How many re-crystallizations are necessary depends on the desired purity of the final product. An examination of Figure 1 should convince you that the process would not work if we started with a mixture consisting of 200 grams of KNO 3 and 200 grams of. 6H2 O. Both would crystallize out of solution during cooling. The technique works best if you have a target compound contaminated with a little impurity. Some practical considerations: 1. The selection of the appropriate solvent is critical for a successful purification. The target compound should be soluble in the solvent at or near its boiling point, but relatively insoluble in the cold solvent. Solubility data for many compounds is available from a number of sources: CRC Handbook of Chemistry and Physics, 89 th ed., Lide, D. R., Ed., CRC Press, Cleveland, OH, 2008; Lange's Handbook of Chemistry, 15 th ed., Dean, J. A., Ed., MacGraw Hill, New York, 1999; International Critical Tables of Numerical Data, Physics, Chemistry and Technology, Washburn, E.W., Ed., Vol 1-7, MacGraw-Hill: New York, There are copies of the CRC Handbook of Chemistry and Physics available outside the stockroom (Bagley 271) and also in the Chemistry Study Center (Bagley 330). Also, all three titles are available as electronic resources through the UW Library system. 2. Add a little of the desired solvent to the solute and heat to a few degrees below the expected boiling point. Add additional solvent to dissolve all the mixture while maintaining the temperature near the boiling point. 3. Continue heating the mixture to vaporize some of the solvent. The loss of solvent should eventually lead to the appearance of crystals. (You won t have time to do this in this experiment.) 4. Cool slowly. Slow cooling gives larger and usually purer crystals than does rapid cooling. At this point, there are two important questions that will need to be answered. Is crystallization and re-crystallization really improving the purity of the KNO 3? By how much does the purity 3 of 6

4 improve? In this experiment, you will find out the answers to these questions by determining the percent of. 6H2 O remaining in the re-crystallized product using absorbance measurements and Beer's law. Recall the iron-phenanthroline experiments from Chemistry 142 and 152: when Fe 2+ is present in solution with 1,10-phenanthroline, it reacts to form the ferroin complex which has a strong absorption at 508 nm. Part II. Determining the Solubility Curve of Potassium Nitrate Figure 1 does not have enough data points to accurately depict the solubility of. 6H2 O or KNO 3. In Part II, you will construct a more accurate solubility curve for KNO 3 by collecting 6 data points and plotting a more accurate curve of solubility versus temperature. You will do so by measuring the crystallization temperature of 6 solutions that differ in KNO 3 concentration. From the data in this part of the experiment, you will also be able to calculate the enthalpy of solution (ΔH o soln) and the entropy of solution (ΔS o soln). At the moment of crystallization, a solution is saturated with solute, such that no more solute can remain in solution and the solid starts to precipitate in the form of crystals. The K sp at that temperature can be calculated by multiplying the molality of K + times the molality of NO - 3. Recall from the experiment in Chemistry 152, in which the temperature dependence of K sp was investigated, that the temperature and K sp data can be plotted, according to the following equation: ln K sp = ΔH R T so ln 1 + ΔS R so ln By plotting ln(k sp ) as a function of 1/T(Kelvin), the ΔH o soln can be determined from the slope of the plot and ΔS o soln from the y-intercept. The prelab assignment on WebAssign addresses the following: Semi-quantitative evaluation of solubility curves. After studying the solubility curves for four compounds, o Determine if you can use fractional crystallization to separate two of the compounds by targeting one of the compounds for crystallization. Hint: Keep a close eye on the mass of each solute in the questions as well as what mass of solute can remain in solution at each temperature. o Predict the sign of ΔH o soln for the specific solutes. Given three choices, identify the plot that shows how you expect the purity of KNO 3 to change as a function of the number of crystallizations performed. 4 of 6

5 Helpful information Time management is critical in this experiment, so be sure to follow the lab manual and your TA s instructions regarding multi-tasking so that you can finish the experiment. Solute: a substance that dissolves in a liquid to form a solution Solvent: the liquid into which a solute dissolves Solubility: the amount of solute that dissolves in a given volume of solvent at a specific temperature There are several ways to describe the composition of a solution. In an earlier chapter of Zumdahl (covered in Chem 142) you learned about molarity (M): Molarity = moles of solute L of solution In the chapter covering properties of solutions (covered in ), you are introduced to other terms used to describe solution composition, one of which is molality (m): molality = moles of solute kg of solvent It is worth noting that, since the volume of a solution changes some with temperature, molarity is temperature dependent, but because molality is based solely on mass, it is independent of temperature. In the post-lab report, you will plot the % impurity versus the # of crystallizations (0, 1, and 2). The % impurity is the % iron compound in the crystals you recover from the solution. Crystallization 0 is the mixture of solids you weighed out at the start of the lab, so the % impurity is the % iron compound in your starting mixture. When determining the amount of iron compound in the crystals, you will measure the absorbance of the ferroin complex produced by the reaction between iron(ii) and 1,10-phenanthroline. In the report, you will be given the value of the molar absorptivity (ε), which, along with the absorbance (A) and pathlength (b), will allow you to calculate the concentration of the ferroin complex solution according to Beer s law: A = ε b c. In the report, you will also plot the amount of KNO 3 crystallized as the % of original amount versus the number of crystallizations. This is the amount of KNO 3 you have recovered compared to the original amount in your starting mixture. Crystallization 0 refers to how much you weighed out, so the % of original amount is 100%. 5 of 6

6 Safety Considerations With multiple beakers on the hot plate at the same time, take care not to knock them over. 6 of 6