KI + Pb(NO₃)₂

Introduction

This experiment will demonstrate a typical double replacement reaction. It also shows precipitation of a product from a solution (and thus looks very cool).

WARNING AND DISCLAIMER

Lead (II) nitrate is a strong oxidizer and must be stored away from any reducing agents. It is recommended that, if at all possible, a dilute solution be obtained ready-mixed, so as to avoid the need to make a solution. Potassium iodide in any form is relatively stable and non-toxic, but, as with all chemicals, should be handled with care. This said, the usual disclaimer applies:

The International Order of Nitrogen, its officers, and/or its members are not responsible for your actions.  Carry this out at your own risk.

Read the entire procedure TWICE before starting the experiment!

Reaction In Summary

Pb(NO₃)₂ (aq) + 2KI (aq) --> 2KNO₃ (aq) + PbI₂ (s)

Facilities

To do this experiment safely, you will need to have ready access to the following:

• Water supply
• Eye wash station

Supplies and Equipment

The following are listed in the order they will be used.

• 10 mL 1 M lead (II) nitrate solution
• One 100 mL laboratory beaker
• 10 mL 1 M potassium iodide solution
• One glass stirring rod

Procedure

Before going any further, be sure to have read the warning and disclaimer.

Lead (II) nitrate is a strong oxidizing agent. Avoid contact with skin. If contact occurs, flush with water. If eye contact occurs, use the eye wash station and seek prompt medical attention.

Promptly clean up any spills.

1. Add the potassium iodide to the beaker.
2. Record observations.
3. Pour the lead (II) nitrate into the beaker.
4. Record observations.
5. Without splashing, stir the contents of the beaker with the stirring rod for three minutes.
6. Record observations.
7. Flush the contents down the sink with water (unless otherwise directed by the lab director).

Photos and Movies

The following are thumbnails of digital photos of this reaction being performed. Click on them to open a full size photo. Thanks to John Thompson of Crescent Valley High School for the use of his lab.

Potassium iodide and lead nitrate, with a beaker	Adding potassium iodide to the beaker
Potassium iodide in the beaker	Lead iodide, after the reaction

•  To view a slow-motion video of the reaction (about 834 KB), click here. The video is an animated GIF image.

What's Happening

The potassium iodide reacts with the lead (II) nitrate to yield lead (II) iodide and potassium nitrate. However, lead (II) iodide is not very soluble in water, and thus precipitates out as a yellow solid. It is important to note, though, that it is not completely insoluble. Some (and occasionally all, depending on the solute to solvent ratio) can be forced back into solution by stirring.

Questions

If you fully understand what's happening in this reaction, see if you can answer the following questions for yourself:

1. Why is 25 mL of Pb(NO₃)₂ used while 50 mL of KI is used?
2. What does the "(II)" mean in lead (II) nitrate?
3. If this reaction were done on a hot-plate (at 100 ^oC), what would happen?

Author: C. Shultz