EXP.1 RESOURCE - Chemistry 111 Laboratory Experiment 6...

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1 Chemistry 111 Laboratory Experiment 6: Synthesis of a Coordination Compound Introduction This experiment serves as an introduction to the area of synthetic chemistry – and more specifically, to inorganic synthesis. You are going to make some new chemical bonds as you synthesize a large molecule from smaller ones! You will be synthesizing a coordination compound . The bonds in coordination compounds are two-electron bonds (like covalent bonds), but both electrons come from the ligand (a Lewis base with an unpaired electron pair). The metal ion serves as a Lewis acid (an electron-deficient species which can use its empty orbitals for the bond). The pictures below show the ligands we will be using and a representation of the bonding in a coordination complex. You will have the opportunity to synthesize one of two possible coordination complexes in this experiment. One molecule is named hexaamminecobalt(III) chloride. Its formula is written [Co(NH 3 ) 6 ]Cl 3 . The other molecule is named pentaamminechlorocobalt(III) chloride. Its formula is written [Co(NH 3 ) 5 Cl]Cl 2 . The ammonia and chloride ligands of these complexes use lone pairs of electrons to form bonds with the cobalt cation. The coordination number for the cobalt in both products is six and these cations are octahedral, as predicted by VSEPR theory. The cobalt ion ends up having a charge N H H H Cl These entities can function as ligands since they possess lone (nonbonding) pairs of electrons. C o 2 + N H H H N H H H C l Lewis bases can approach and subsquently "coordinate" to transition metal cations via the participation of nonbonding electron pairs. Electron deficient transition metal cations are strong Lewis acids.
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2 of 3+; each chloride ion is 1-, and the ammonia ligands are neutral, so [Co(NH 3 ) 6 ] has a net 3+ charge and [Co(NH 3 ) 5 Cl] has a net 2+ charge. These complex cations crystallize from solution as chloride salts. The overall reactions for the syntheses of these salts are very similar: 2CoCl 2 6H 2 O (s) + 2 NH 4 Cl (s) + 10 NH 3(aq) + H 2 O 2(aq) + 3 H 2 O ( l ) → charcoal 2 [Co(NH 3 ) 6 ]Cl 3(s) + ½ O 2(g) 2CoCl 2 6H 2 O (s) + 2 NH 4 Cl (s) + 8 NH 3(aq) + H 2 O 2(aq) + 3 H 2 O ( l ) 2 [Co(NH 3 ) 5 Cl]Cl 2(s) + ½ O 2(g) Notice that cobalt(II) chloride is hygroscopic and is available as the hydrate: CoCl 2 6 H 2 O. The dot in this formula implies a strong association between the cobalt(II) chloride and the water molecules. This is not a chemical bond, but a result of water being incorporated into the crystal structure of CoCl 2 (in a very specific way!) as the solid forms. The 6 does indicate the stoichiometric relationship between the components of this material: there are 6 water molecules in the crystal for every Co atom and every two Cl atoms. As you follow the procedure, keep complete records of your observations in your laboratory notebook. Record what your eyes, ears, and nose detect! (But don’t inhale the fumes above or stick your ear into the reaction vessel – that’s dangerous!) Often in syntheses small things can go slightly wrong which alter the reaction in some way.
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This note was uploaded on 11/13/2011 for the course CHEM 1b taught by Professor Cabrera during the Spring '10 term at San Jose City College.

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EXP.1 RESOURCE - Chemistry 111 Laboratory Experiment 6...

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