Stechiometria, wydajność produktu i reagenty ograniczające
View the full transcript and gain access to JoVE Lab Manual videos
Q1: Why does nickel chloride hexahydrate need to be the limiting reactant in this synthesis?
Nickel chloride hexahydrate must be the limiting reactant to ensure formation of the desired tris(ethylenediamine)nickel chloride product. If nickel were in excess, the solution would not reach the purple color indicating complete complex formation. The 3:1 molar ratio of ethylenediamine to nickel is required for the product, making nickel the controlling reactant.
Q2: What color changes indicate the formation of different nickel complexes during the reaction?
The solution progresses through distinct colors as ethylenediamine replaces water molecules. A light blue color appears around a 1:1 molar ratio, indicating tetraaquaethylenediamine nickel. Dark blue emerges at a 2:1 ratio, representing diaquabisethylenediamine nickel. Purple finally appears at a 3:1 ratio, confirming formation of the tris(ethylenediamine)nickel chloride product.
Q3: How does adding acetone help precipitate the nickel complex product?
Tris(ethylenediamine)nickel chloride is soluble in water but insoluble in acetone. Adding acetone to the aqueous product solution shifts the equilibrium to favor precipitation of the solid product. This precipitation is enhanced further by cooling the solution in an ice bath, which lowers the product's solubility even more.
Q4: What is the purpose of washing the precipitate with acetone multiple times?
Multiple acetone washes remove excess water and ethylenediamine trapped in the product. Each 5-minute soak allows these impurities to dissolve into the acetone, which is then removed by vacuum filtration. Thorough washing is critical for obtaining an accurate yield calculation and a pure final product.
Q5: How do you calculate percent yield from the actual and theoretical yields?
Percent yield is calculated as the ratio of actual yield to theoretical yield, multiplied by 100. Theoretical yield is determined using stoichiometric relationships based on the limiting reactant amount. If your percent yield falls below 95% or exceeds 100%, review your experimental technique for issues like excess water, insufficient cooling, or incomplete washing.
Q6: Why is it important to minimize water addition when dissolving nickel chloride hexahydrate?
Excess water makes it harder to precipitate the final product from solution. Since the goal is to recover solid tris(ethylenediamine)nickel chloride, using only the minimum water needed to dissolve the starting nickel chloride hexahydrate ensures better precipitation efficiency and higher product recovery.
Q7: What does the stoichiometric ratio tell you about the reaction between nickel chloride and ethylenediamine?
The stoichiometric ratio of 3:1 (ethylenediamine to nickel chloride hexahydrate) indicates that three ethylenediamine molecules are needed to completely replace the six water molecules coordinated to each nickel ion. This replacement occurs sequentially, producing intermediate complexes with different colors before the final purple product forms.