0.8 Metathesis: to exchange or not?  (Page 3/4)

In the second part of this experiment, you will study the effect of temperature on solubility. The effect that temperature has on solubility varies from salt to salt. We conclude that mixing solutions of KCl and ${\text{NaNO}}_3$ resulted in no reaction (see Equations 6 and 7). What would happen if we cooled such a mixture? The solution would eventually become saturated with respect to one of the salts, and crystals of that salt would begin to appear as its solubility was exceeded. Examination of Equation 6 reveals that crystals of any of the following salts could appear initially: ${\text{KNO}}_3$ , KCl, ${\text{NaNO}}_3$ , or NaCl.Consequently, if a solution containing ${\text{Na}}^{+}$ , $K^{+}$ , ${\text{Cl}}^{-}$ , and ${\text{NO}}_{3^{-}}$ ions is evaporated at a given temperature, the solution becomes more and more concentrated and will eventually become saturated with respect to one of the four compounds. If a evaporation is continued, that compound will crystallize out, removing its' ions from solution. The other ions will remain in solution and increase in concentration. Before beginning this laboratory exercise you are to plot a graph of the solubilities of the four salts given in Table 3 on your report sheet.

Experimental procedure

Part 1: metathesis reactions

CAUTION WEAR EYE PROTECTION

• The report sheet lists 16 pairs of chemicals that are to be mixed. Use about 1 mL of the reagents to be combined as indicated on the report sheet.
• Mix the solutions in small test tubes and record your observations on the report sheet. If there is no reaction, write N.R. (The reactions need not be carried out in the order listed. In order to reduce congestion at the reagent shelf, half the class will start in reverse order). Dispose of the contents of your test tubes in the designated receptacles.

Part 2: solubility, temperature and crystallization

• Place 8.5 g of sodium nitrate and 7.5 g of potassium chloride in a 100-mL beaker and add 25 mL of water. Warm the mixture on an hotplate, stirring, until the solids completely dissolve.
• Assuming a volume of 25mL for the solution, calculate the molarity of the solution with respect to ${\text{NaNO}}_3$ , KCl, NaCl, and ${\text{KNO}}_3$ , and record these molarities on your report form.
• Cool the solution to about 10°C by placing the beaker in ice water in a 600-mL beaker and stir the solution carefully with a thermometer, being careful not to break it.
• When no more crystals form, at approximately 10°C, filter the cold solution quickly and allow the filtrate to drain thoroughly into an evaporating dish. Dry the crystals between two dry pieces of filter paper or paper towels.
• Examine the crystals with a magnifying glass (or fill a Florence flask with water and look at the crystals through it). Describe the shape of the crystals—that is, needles, cubes, plates, rhombs, and so forth on your report form.
• Based upon your solubility graph, which compound crystallized out of solution and write that in the appropriate place on your report form
• Evaporate the filtrate to about half of its volume using a Bunsen burner and ring stand. A second crop of crystals should form. Record the temperature and rapidly filter the hot solution, collecting the filtrate in a clean 100-mL beaker.
• Dry the second batch of crystals between two pieces of filter paper and examine their shape. Compare their shape with the first batch of crystals.
• Based upon your solubility graph, what is this substance?
• Finally, cool the filtrate to 10°C while stirring carefully with a thermometer to obtain a third crop of crystals. Carefully observe their shapes and compare them with those of the first and second batches.
• What compound is the third batch of crystals? Dispose of the chemicals in the designated receptacles.