4. ESTABLISHMENT OF A CHEMICAL EQUATION

A chemical equation is essentially a description of the reactive relationship between two or more substances. The chemical equation describes qualitatively the nature of the reaction by stating the identity (chemical formula) of the reactant and product substances and also describes the quantitative relationship between the relative amounts of the substances involved. The quantitative information is stated in terms of "reactive units", that is either atoms or molecules, or the "collective unit", moles. In order to write a chemical equation the chemist must know the identity of the reacting substances and of the products formed by the reaction and must also know the quantitative relationship between the substances. In this exercise, the student will develop the equation for the reaction between potassium iodide and lead nitrate.

This exercise must then have both a qualitative stage and a quantitative stage. In order to simplify the process the formulas for the two reactants are given - KI and Pb(NO3)2. Both of these substances are known to be water-soluble ionic salts; thus in solution they will be present as ions. In other words, a solution of lead nitrate contains lead ions and nitrate ions, and a solution of potassium iodide contains potassium ions and iodide ions. As you will see when solutions of these two substances are mixed, a yellow precipitate is formed. It may be presumed that the yellow solid is formed by the combination of the positive ions from one compound with the negative ions of the other. The two possible combinations are lead iodide and potassium nitrate. If the solid is lead iodide, then the potassium ions and nitrate ions must be left in the solution; if the insoluble product is potassium nitrate, the lead ions and iodide ions must be left in solution. Identification of the precipitate might presumably be done either by examining the solid itself or by examining the supernatant solution. In the latter case, care must be taken to not confuse an excess of one of the ions (that reacted to form the precipitate and some of which remains in solution) with the ions which did not react.

In order to carry out the qualitative identification, tests are needed for two of the four ions involved, one from each reacting substance. Tests for only two are needed since the presence or absence of the other two can be inferred.

PROCEDURE

A. Qualitative Analysis
  1. Test for Lead Ions. Place 5 drops of the known lead nitrate solution in a test tube. Add 2 drops of dilute nitric acid and 20 drops of thioacetamide solution. Heat the test tube and contents in a boiling water bath for 5 minutes. The presence of a black precipitate, which is in fact lead sulfide, confirms the presence of lead cation in the test solution. You should note that several of the transition metal cations also form a precipitate with sulfide.
  2. Test for Iodide Ions. Place 5 drops of the known potassium iodide solution in a test tube. Add 5 drops of dichloromethane, 5 drops of dilute nitric acid, and 5 drops of the hydrogen peroxide solution. Shake the test tube vigorously. The presence of a violet color in the lower dichloromethane layer, which is in fact a solution of elemental iodine, confirms the presence of iodide anion in the test solution.
  3. Qualitative Identification of the Precipitate Formed.
    1. Place 5 mL of potassium iodide solution and 5 drops of lead nitrate solution into a test tube. The bright yellow precipitate formed must now be identified.
    2. Centrifuge the solution for one minute. Remove the supernatant with a Pasteur pipet. Add a pipetful of water to the solid. Stir the mixture with a clean stirring rod and centrifuge again. Repeat this procedure one more time, finally leaving the residual precipitate which has been washed free of any supernatant.
    3. Dissolve the precipitate by adding 5 drops of dilute nitric acid, stirring with a clean glass rod, and heating in a boiling water bath.
    4. Perform the test for the presence of iodide ions described above.
    5. Since the reagents used to perform the iodide test interfere with the test for lead, prepare a new sample of the yellow precipitate, wash it free of any supernatant solution, dissolve the precipitate, and perform the test for the presence of lead ions.
B. Quantitative Examination of Lead Nitrate-Potassium Iodide Interactions
  1. Stock solutions of known concentrations of lead nitrate and of potassium iodide will be provided.
  2. Using a buret, add 20 mL of potassium iodide solution to a 150 mL beaker. Add 25 mL of deionized water to the same beaker.
  3. Each student will be assigned one volume of lead nitrate solution from the table below. Add the assigned volume of lead nitrate from the delivery buret to a 150 mL beaker. Add 25 mL of deionized water to the beaker.
  4. assignment a b c d e f
    volume of Pb(NO3)2 2mL 5mL 9mL 12mL 15mL 18mL
  5. Slowly, and with constant stirring, combine the solutions into a 250 mL beaker. Continue to stir for 5 minutes.
  6. Weigh a piece of filter paper to the nearest milligram. Fold the paper and place it in the supported filter funnel. See Figure 8.

  7. Figure 8. Use of filter funnel

  8. Decant most of the supernatant solution through the filter. Complete the filtration by transferring all of the solid to the filter paper. Wash the precipitate with 10 mL of cold distilled water. Rinse with small portions of methanol and ether.
  9. Carefully remove the filter paper and contents from the funnel and place them upon a folded paper towel. When all odor of ether is gone, the last traces of moisture may be removed by placing the sample in the oven for a short period at 100C.
  10. When the precipitate is dry, reweigh the solid and filter paper. Calculate the net mass of precipitate formed. Report this value and your assignment number to the laboratory instructor.
  11. All student results will be pooled. From the pooled data, prepare a plot of the mass of precipitate versus the mass of lead nitrate added to the constant mass of potassium iodide. From the shape of that curve, determine the composition of the precipitate.
  12. From the qualitative and quantitative information collected, determine the complete and balanced equation for the reaction between potassium iodide and lead nitrate.

QUESTIONS

  1. Explain the meaning of the terms qualitative and quantitative, as they are used in this exercise.
  2. What would be the effect upon the quantitative results if the student did not dry the final precipitate thoroughly?
  3. What would be the shape of a plot of mass of precipitate versus the ratio of the masses of KI to Pb(NO3)2 used?