Zinc Complex Ion Formation

1M NaOH is added to a solution of ZnSO4. A precipitate is formed and the mixture is noticeably white and cloudy. 50% NaOH is then added. The mixture becomes clear due to the formation of a complex ion that is soluble in water. When more ZnSO4 is added to alter the relative concentrations in the mixture, allowing the precipitation reaction to predominate, the mixture to be cloudy again. 

Curriculum Notes 

This demonstration is a great way to apply Le Chatelier's principle to the formation of complex ions.

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

A double replacement reaction occurs when Zn2+ is in the presence of a stoichiometric amount of OH-. The product, Zn(OH)2, is insoluble in water. However, when a large excess of OH- is introduced to the system by adding a highly concentrated solution of the base, complexation occurs. The complex product, Zn(OH)42-, is soluble in water. The formation of complex ion can be reversed by adding more of the Zn2+; Precipitate Reaction: Zn2+ + 2OH- <==> Zn(OH)2Complex Ion Reaction: Zn2+ + 4OH- <==> Zn(OH)42-

Materials 

Stir Plate Magnetic Stir Bar 400 mL Beaker 220 mL .1M ZnSO4Solution 30 mL 1M NaOH Solution 5 mL 50% NaOH Solution 

Procedure 

1. To a 400mL beaker on a stir plate add 120mL ZnSO4 solution (.1M) and a magnetic stir bar 2. Continue stirring throughout. 3. Add 30mL NaOH solution (1M) and observe white cloudy precipitate formation. 4. Add 5mL concentrated NaOH solution (50%) and observe clearing due to complex ion formation 5. Add 100mL ZnSO4solution (.1M) and observe white cloudy precipitate formation. 

Safety Precautions 

Gloves and goggles. Take caution with concentrated NaOH solution. It is corossive, may be fatal if swallowed, harmful if inhaled, and causes burns to any area of contact. Avoid contact with 1M NaOH, it is an irritant.

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