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
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
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.
A. Qualitative Analysis
B. Quantitative Examination of Lead Nitrate-Potassium Iodide
- 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.
- 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.
- Qualitative Identification of the Precipitate Formed.
- 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.
- 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.
- Dissolve the precipitate by adding 5 drops of dilute
nitric acid, stirring with a clean glass rod, and heating
in a boiling water bath.
- Perform the test for the presence of iodide ions
- 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.
- Stock solutions of known concentrations of lead nitrate
and of potassium iodide will be provided.
- Using a buret, add 20 mL of potassium iodide solution to a
150 mL beaker. Add 25 mL of deionized water to the same
- 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.
|volume of Pb(NO3)2
- Slowly, and with constant stirring, combine the solutions
into a 250 mL beaker. Continue to stir for 5 minutes.
- Weigh a piece of filter paper to the nearest milligram.
Fold the paper and place it in the supported filter funnel.
See Figure 8.
Figure 8. Use of filter funnel
- 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
- 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 100°C.
- 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
- 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
- From the qualitative and quantitative information collected,
determine the complete and balanced equation for the reaction
between potassium iodide and lead nitrate.
- Explain the meaning of the terms qualitative and
quantitative, as they are used in this exercise.
- What would be the effect upon the quantitative results if
the student did not dry the final precipitate thoroughly?
- What would be the shape of a plot of mass of precipitate
versus the ratio of the masses of KI to