E770: Acid/Base – Common Ion Effect
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-.
To Conduct Demonstration:
- Add water to each of two beakers. Add 2 drops of phenolphthalein to each.
- Add a few drops of 3M NH3 to each beaker. The solution will turn pink.
- 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.
Variant (courtesy Doug Gin):
- Add water to one of the beakers. Add 2 drops of phenolphthalein and a few drops of 3 M aq. NH3. This reference solution will turn pink (basic, pH > 7).
- Add water to the other beaker, and a scoop of solid NH4Cl. Add a few drops of 3 M aq. NH3, stir, and add 2 drops of phenolphthalein. This solution will not shift as much to the right and will not turn pink or as pink (less basic).
- Use the first solution without added solid NH4Cl as a reference.
Safety
Goggles should be worn.