[WW HOME] [CHEMISTRY] [CONSUMER] [SEARCH] [DISCLAIMER]

REACTIONS BETWEEN IONS IN SOLUTION USING CONSUMER MATERIALS

* PURPOSE

This activity is designed to allow students to observe the formation of precipitates in aqueous reactions. It also demonstrates the fact that certain combinations of ions do not form precipitates but remain in solution as spectator ions. Practice in the writing of ionic and net ionic equations is also provided.

* DESCRIPTION

The experiment is appropriate for general and first year college-prep courses. The content relates well to a unit on solution chemistry or salts. If Ksp's are added, this experiment could also be appropriate for second-year or Advanced Placement courses. Various ionic solutions will be mixed, two at a time, to determine which combinations form precipitates. Knowledge of which ions are present makes it possible to deduce which of the possible ion combinations are responsible for the formation of the precipitates.

* TIME REQUIRED

One to two lab periods.

* MATERIALS

Chemicals:
iron(II) sulfate (All salt solutions can be made by dissolving one tablespoon of solute in water and diluting to 100 mL)*
sodium hydroxide
sodium chloride
magnesium sulfate
sodium phosphate
hydrochloric acid (dilute 500 mL concentrated HCl solution to 1.00 L with distilled or deionized water)*
iron *
zinc*
distilled or deionized water
Equipment:
dropper bottles, small test tubes or small bottles
eye droppers or disposable pipets*
plastic wrap
funnel and support*
filter paper*
250-mL beakers
100-mL graduated cylinder*
*See Modifications/Substitutions

* HAZARDS

Hydrochloric acid is a corrosive liquid. Keep away from skin and eyes. Avoid breathing vapors. When reacting the penny with this acid, perform the procedure under a hood. If spilled, use baking soda to neutralize acid before cleaning up. Sodium hydroxide and trisodium phosphate are very caustic; avoid contact with skin. If spilled, immediately flush the area with water and clean up. Goggles must be worn throughout this experiment.

* MODIFICATIONS/SUBSTITUTIONS

  1. Consumer sources of all chemicals used in this experiment are available. Iron(II) sulfate is available at garden supply centers. Sodium hydroxide is available as lye, sodium chloride as table salt, and sodium carbonate as washing soda at grocery stores. Magnesium sulfate is available as Epsom salts at grocery or drug stores. Sodium phosphate is available as TSP at grocery or hardware stores.
  2. Hydrochloric acid is available at hardware stores as muriatic acid, 28% HCl.
  3. To obtain a stock solution of ZnCl2, two pennies (post-1983), having a zinc core are scratched to expose the core in several places, and are immersed in 10 mL of muriatic acid and left to react overnight. The resulting solution is diluted to 200 mL. At this dilution, production of carbon dioxide (due to excess HCl) with carbonates is minimized and the precipitates, normally soluble in acid, will not dissolve.
  4. The Fe+2 ion can be obtained from an FeCl2 solution. This solution is made by adding 5 #2d common nails to 50 mL of muriatic acid. After being allowed to react overnight, the solution is filtered, diluted to 100 mL and stored with a clean iron nail. The solution should be neutralized with Na2CO3 until almost neutral.
  5. For filtration purposes, Whatman #1 paper is recommended. However, several thicknesses of paper towel in a strainer clear the solution adequately for this experiment.
  6. Plastic straws may be used in place of eye droppers or disposable pipets.
  7. Glass plates, spot plates, or transparencies may be substituted for the plastic wrap.
  8. Styrofoam cups may be used in place of beakers.
  9. A measuring cup may be substituted for the graduated cylinder.

* PROCEDURE

  1. Test each possible pair of solutions by combining 1 to 2 drops of each member of the pair on a sheet of plastic wrap.
  2. Record observations, noting formation and color of any precipitate. A data table similar to the one below provides an efficient means of recording and comparing results.
  3. Use the data obtained to write complete and/or net ionic equations for those combinations that result in a precipitate.
Solution: NaOH MgSO4 FeSO4 ZnCl2 Na3PO4 Na2CO3 NaCl
NaOH






MgSO4






FeSO4






ZnCl2






Na3PO4






Na2CO3






NaCl







* DISPOSAL

All solutions may be flushed down the sink with plenty of water. Since students use only a very small quantity of each solution, they can dispose of their samples by folding up the plastic wrap and placing it in with the solid waste.

* DISCUSSION

Before performing the experiment, students should be aware of the nature of ionic compounds, or, at least, know that many substances form ions in solution. The fact that many ionic compounds have limited solubilities can be discussed if care is taken not to reveal the results of this investigation. The following equations describe the reactions that form precipitates:

  1. Fe2+(aq) + 2 OH-(aq) ----> Fe(OH)2 (s)

  2. Zn2+(aq) + 2 OH-(aq) ----> Zn(OH)2 (s)

  3. Mg2+(aq) + 2 OH-(aq) ----> Mg(OH)2 (s)

  4. Fe2+(aq) + CO32-(aq) ----> FeCO3 (s)

  5. Mg2+(aq) + CO32-(aq) ----> MgCO3 (s)

  6. 3 Zn2+(aq) + 2 PO43-(aq) ----> Zn3(PO4)2 (s)

  7. 3 Fe2+(aq) + 2 PO43-(aq) ----> Fe3(PO4)2 (s)

  8. 3 Mg2+(aq) + 2 PO43-(aq) ----> Mg3(PO4)2 (s)

The students should be asked to write the overall and the net ionic equations. The distinction between reacting species and spectator ions should be stressed. The experiment can be used as an introduction to solubility rules.

* TIPS

  1. One of the benefits of using consumer products is the familiarity students may have with them. It is also possible that they may be able to supply some of the products. However, it is necessary to caution the students to read the labels that are on these products. The original containers should be brought into the lab so that the labels can be read by the students for additional emphasis on the need for caution.
  2. Stock solutions can be prepared in advance. The use of consumer products to prepare these solutions would be an excellent project for "advanced" students.
  3. During the formation of precipitates, some irregularities may be observed.
    1. Fe2+, usually green, may rapidly oxidize to orange Fe3+ therefore forming precipitates which obviously are different.
    2. The Zn+2 ion solution is fairly acidic since it was prepared with HCl. A very dilute solution is used to insure formation of the Zn(OH)2 precipitate.

* REFERENCES

Merrill, P., Parry, R.W., Tellefsen, R.L., and Bassow, H., Chemistry: Experimental Foundations, Laboratory Manual, Prentice Hall, Inc, Engelwood Cliffs, NJ, 1982, p. 50. This work contains an activity similar to that described here, but uses different chemicals and equipment.

Submitted by Judy Bazler, Larry Kresse, Joe MacQuade, Glenda Murshman, and Jackie Simms


[WW HOME] [CONSUMER] [SEARCH] [FEEDBACK] [DISCLAIMER]

Woodrow Wilson Leadership Program in Chemistry * lpt@www.woodrow.org
The Woodrow Wilson National Fellowship Foundation * webmaster@woodrow.org
CN 5281, Princeton NJ 08543-5281 * Tel:(609)452-7007 * Fax:(609)452-0066