Complex Formation and Precipitation.
Precipitation is the formation of a solid in a solution as the result of either a chemical reaction, or supersaturating a solution with a salt resulting in solid material collecting on the bottom of the beaker (Housecroft & Constable, 2006). When the solution has been supersaturated by a compound and no more material can be supported by the solution, it considered as precipitate. Commonly, the solid will fall out of the solute phase and sinks to the bottom part of the solution. However, the solid may float on top of the solution or suspended within the solution if the precipitate is less than the aqueous solution. On the other hand, complex formation is the soluble compound that can increase the solubility of an insoluble salt form in aqueos solution (Housecroft & Constable, 2006). They are formed from a cation such as water or ammonia that has pair of electrons that become attracted to the positively charged cation. In terms of the solubility, the formation of a complex lowers the concentration of the cation in the solution. In this experiment, there are 3 parts need to be done which is complex formation, weak acids and bases and precipitation reaction. During complex formation experiment, student needs to observe the different reaction occur when adding ammonia or sodium hydroxide in small quantity and in excess. At the second part, student needs to measure the pH of different solution by using universal indicator paper and determine whether each solution is acidic, basic or neutral. Lastly, the precipitation reaction is observed by using phosphates, chlorides and silver halides groups.
The main objective of this experiment was to identify certain inorganic reaction types and the differences in solubility of silver halides. Thus, a series of inorganic reactions was carried out as an introduction to complex formation and precipitation. All the reaction in this experiment involved alteration of equilibrium by changing concentration to make precipitation to form or dissolve. Other than that, weak acid and bases of the solution can be identified and students able to write the balanced equations for inorganic reactions. PRE-LAB QUESTIONS
1. Precipitation reaction is a type of reaction that forms a solid in a solution as a result of chemical reaction or supersaturating a solution. This reaction involves changing the soluble ions into insoluble compound and as a result precipitate is formed when the solution has been saturated by a compound (Heitz, 2005).
2. Inorganic complex consists of a central ion or atom (usually metallic) that is surrounded by anions or molecules bound to the central ion/metal via chemical bonds that range in strength from weak to strong (Heitz, 2005).
3. Trigonal planar, tetrahedral and octahedral.
* PART A: COMPLEX FORMATION (AMMONIA COMPLEXES)
1. 0.5 cm3 of dilute zinc nitrate, cupric nitrate and lead nitrate solutions was poured into 3 different test tubes. 2. 4 M of ammonia solution was added dropwise in each test tube until each solution turn alkaline (test with pH paper). 3. Ammonia solution was added more and observes if any precipitate dissolves. 4. The experiment was repeated by replacing the ammonia solution with 2 M of sodium hydroxide.
* PART A: COMPLEX FORMATION (IODIDE COMPLEXES)
1. Saturated solution of potassium hydroxide was added dropwise to 0.5 cm3 of 0.1 M mercuric nitrate solution until the precipitate of HgI2 just dissolves by forming [HgI4]2-. 2. Saturated solution of potassium hydroxide was added dropwise to 5 drops of 0.1 M lead nitrate solution until the precipitate of PbI2 just dissolves by forming [PbI4]2-. 3. The solution of [PbI4]2- was added to about 10 times its volume of distilled water in a medium test tube. 4. The amounts of I- required to dissolve the precipitates formed was compared between Hg (II) and Pb (II)....
References: Heitz., M(2005) General Chemistry: An Integrated Approach, Chapter 16: More Equilibria in Aqueous Solutions: Slightly Soluble Salts and Complex Ions, 4th Edition, Prentice Hall, Inc., State University of New York. pp. 715-718
Housecroft, C.E. & Constable, E.C. (2006), Introduction to spectroscopy, in Chemistry, 3rd Edition, Pearson Education, England. pp. 951-970
Silberberg, M.S. (2006), Chemistry: The Molecular Nature of Matter and Change, 4th Edition, Mc Graw Hill International Edition, New York, pp. 1013-1019.
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