Lab_5-S08-CoSynth_amended

Lab_5-S08-CoSynth_amended - Lab 5 SYNTHESIS OF Co(NH3)6Cl3...

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Lab 5 SYNTHESIS OF Co(NH 3 ) 6 Cl 3 Laboratory Goals In this lab you will: Synthesize a cobalt compound Recrystallize the impure compound to form pure crystals Safety Notes You will be working with concentrated acids and bases and a very concentrated hydrogen peroxide. Keep in mind that acids and bases of this strength can cause permanent damage (especially to eyes.) WEAR SAFETY GOGGLES AT ALL TIMES! Introduction One activity that occupies chemists is the synthesis or preparation of chemical compounds. A goal of synthetic chemistry is to make substances that have never been previously prepared. Some chemists work in the laboratory to prepare compounds that are known to occur in nature, others strive to make new, unknown compounds. The purpose of this experiment is to learn some laboratory techniques involved in synthetic chemistry. The compound to be prepared is Co(NH 3 ) 6 Cl 3 . This is an example of a coordination compound: the cation has six molecules of ammonia covalently bonded to Co(III) in an octahedral arrangement (see figure 1). The ammonia molecules are bound to the cobalt through the lone pair on the nitrogen. Figure 1: Geometric configuration of Co(NH 3 ) 6 Cl 3 Co 3+ H 3 N H 3 N NH 3 NH 3 NH 3 NH 3 There are also three chloride ions in the crystal to balance the charge on this cation. They are not covalently bonded to it. The compounds will be prepared by the reaction of cobalt chloride, ammonia and hydrogen peroxide as in the following overall equation. 2Co(H 2 O) 6 Cl 2 + 2NH 4 Cl + 10NH 3 + H 2 O 2 2Co(NH 3 ) 6 Cl 3 + 14H 2 O However, if the reactants are simply mixed together, the desired product will not form. To get the desired product, the procedure must be done stepwise. The sequence of reactions is: 5-1
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Co(H 2 O) 6 2+ (aq) + 4Cl - (aq) CoCl 4 2- (aq) + 6H 2 O (l) CoCl 4 2- (aq) + 6NH 3 (aq) Co(NH 3 ) 6 2+ (aq) + 4Cl - (aq) 2Co(NH 3 ) 6 2+ (aq) + 2H 3 O + (aq) + H 2 O 2 (aq) 2Co(NH 3 ) 6 3+ (aq) + 4H 2 O(l) Co(NH 3 ) 6 3+ (aq) + 3Cl - (aq) Co(NH 3 ) 6 Cl 3 (s) Charcoal (Pink) (Pink) (Pink) (Blue) (Blue) (Yellow) (Yellow) (Yellow) Heat Heat Cold Many synthetic reactions are carried out in solution by dissolving the reactants and allowing them to form the desired product. After a compound forms in solution, it must be isolated and purified in order to rid it of the unused reactants and any side products. Since the solubility of most compounds decreases with temperature, a common technique
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This note was uploaded on 04/15/2009 for the course LB 171L taught by Professor All during the Spring '08 term at Michigan State University.

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Lab_5-S08-CoSynth_amended - Lab 5 SYNTHESIS OF Co(NH3)6Cl3...

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