E770: Prep Notes

Introduction

The addition of NH4Cl to a solution of ammonia shows the effect of adding a common ion to a system at equilibrium.

Equations

 NH3 + H2O ↔ NH4+ + OH-

 Addition of NH4Cl adds NH4+, driving the reaction to the left and consuming OH-.

Prior to Lecture  (prep time ~ 5 min.)

1. Prepare the following chemicals:

1.   3M NH3 (ammonia) in a dropper bottle
2.  a few grams of NH4Cl (ammonium chloride)
3.  phenolphthalein indicator in dropper bottle
4.  distilled water in wash bottle
5.  two small beakers (50 or 100mL)
6.  scoopula

To Conduct Demonstration:

1. Add water to each of two beakers.  Add 2 drops of phenolphthalein to each.
2.  Add a few drops of 3M NH3 to each beaker.  The solution will turn pink.
3.  Add a scoop of solid NH4Cl to one of the beakers.  The pink color will become very faint or disappear.  Use the other beaker as a reference.

Safety and Disposal

Goggles should be worn. 

Chemicals may be sink disposed.