Precipitation of Lead(II) Iodide

When a few crystals of lead nitrate and potassium iodide are added to opposite sides of a Petri dish containing deionized water, after a few minutes, a line of bright yellow lead(II) iodide precipitate forms down the middle of the dish.  Suggestion: have students:  a. predict the products from the Solubility Rules, b. write the molecular equation, c. write the full ionic equation, d. write the net ionic equation, and e. draw particle diagrams.

When combined with computer animations of the dissolving process and the migration of ions to form lead(II) iodide (macroscopic level) this demonstration is an effective teaching tool.

Curriculum Notes 

This demonstration, when presented as a guided-inquiry active student learning approach, combined with particle diagrams can be highly effective because it addresses all three areas of Johnstone's triangle: macroscopic, symbolic, and microscopic (particle level). This demonstration is usually performed when solubility rules, precipitation reactions, and net ionic reactions are being discussed.  It could also be used when the mechanism of dissolution is presented to illustrate that dissolved substance are still present, even though they may become "invisible." 

The solid potassium iodide ane solid lead nitrate dissolve and their constituent ions begin to diffuse through the water. Eventually the lead ions begin to react with the iodide ions and precipitate out where they "meet" which is near the center of the Petri dish. 

The net ionic reaction is Pb2+(aq) + 2 I-(aq) --> PbI2(s)

The Kspof PbI2 is 9.8 X10-91 1http://www.ktf-split.hr/periodni/en/abc/kpt.html

The solubility rules tell us that salts of group 1 (IA) metals are soluble, salts containing the nitrate anion are soluble, and salts containing the iodide anion are insoluble except for those containing the lead(II) cation (among others). This demo illustrates these rules. 

A student activity sheet to accompany this demonstration is available.   A set of lecture presentation slides to accompany this demonstration is available.

This demonstration, with the pedagogy of having students "do the work" of predicting products, writing equations, and drawing particle level diagrams works best if it is presented AFTER the precipitation of silver chromate demonstration - which is done in a teacher directed manner.

Learning Objectives

1.  Reinforce the concept that soluble ionic salts are strong electrolytes and the ions are mobile ions in solution, i.e. the ions disperse in solution.

2.  Application of Solubility Rules.

3.  Write net ionic equations.

Lead Time 
One day of lead time is required for this project.
Discussion 

Presenting the Demonstration   This demonstration takes about 8 minutes to perform if you rush through it and do it is as a teacher presenting information to students. The line of precipitate does not begin to appear until a two or three minutes time have elapsed and it takes a few more minutes for the line to reach across the dish. The instructor should be prepared to lead a discussion of writing the 'molecular equation", "full ionic equation" and the net ionic equation. while the demo proceeds. This demonstration works best if the instructor uses this demonstration as the basis of a presentation and takes about 12-15 minutes to have students do the activities that accompany this demonstration. In a large lecture hall a document camera is necessary to project the image of the reaction. However, if the camera is of low quality students may not see the vivid yellow color of the precipitate.

What is the significance of this demonstration?  The only way the solid lead(II) iodide precipitate can form in the middle of the petri dish is for 1) the Pb(NO3)2 and the KI to dissociate into ions in the water and 2) the Pb2+ ions and I- ions must migrate in the aqueous solution, i.e. the ions are mobile, until they find each other and form a solid.  This demonstration provides evidence to support this claim.

Materials 
  • small bottle of solid potassium iodide
  • small bottle solid lead nitrate
  • Petri dish
  • wash bottle containing DI water
  • 2 tweezers
  • dark background (optional)
Procedure 
  • Place a Petri dish under an document projector camera (preferably on a dark background) or on an overhead projector.
  • Pour enough water into the Petri dish to cover the bottom to a depth of 4-5 mm.
  • Using the tweezers, place a few crystals of each substance on opposite sides of the dish. Use different tweezers for each substance to avoid contamination.
  • After about two or three minutes a bright yellow line of lead iodide precipitate should begin to form down the center of the dish.
Safety Precautions 

Both lead nitrate and potassium iodide are very poisonous. Lead nitrate is also a strong oxidizing agent. Handle them with care. 

Prep. Notes 

Be careful when breaking down this demo. The solution in the Petri dish is very "sloshy." It's probably best to drain it into a waste container of some kind before removing the demo.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group