Experiment_13__-__relative_stability_of_Cu+2_compounds

Experiment_13__-__relative_stability_of_Cu+2_compounds - 13...

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13 Relative Stability of Precipitates and Complex Ions of Cu 2+ Introduction In this experiment, you will prepare solid compounds and aqueous complex ions that contain Cu 2+ ions combined with NH 3 , Cl , OH , CO 3 2– , C 2 O 4 2– , S 2– , NO 2 , or PO 4 3– ions. The goal is to rank these copper species in order of stability. Complex ions. In aqueous solution, metal cations do not exist as free ions. Instead, they form complex ions with surrounding water molecules. In most hydrated cations, either two, four, or six water molecules are loosely bonded to a central metal cation. Copper ion has a coordination number of 4, so it exists in solution as Cu(H 2 O) 4 2+ , with four water molecules arranged in a square around the copper ion at the center. If a hydrated cation such as Cu(H 2 O) 4 2+ is mixed with other species that can bond to Cu 2+ , those other species (ligands) may displace one or more H 2 O molecules. For instance, NH 3 may replace H 2 O from the hydrated copper ion, Cu(H 2 O) 4 2+ , to form Cu(H 2 O) 3 NH 3 2+ , Cu(H 2 O) 2 (NH 3 ) 2 2+ , Cu(H 2 O)(NH 3 ) 3 2+ , or Cu(NH 3 ) 4 2+ . At moderate concentrations of NH 3 , essentially all of the H 2 O molecules are displaced by NH 3 molecules. Cu(H 2 O) 4 2+ ( aq ) + 4 NH 3 ( aq ) → Cu(NH 3 ) 4 2+ ( aq ) + 4 H 2 O ( l ) All metal cations are Lewis acids. All ligands are Lewis bases, with lone pairs of electrons to donate; some ligands are molecules including H 2 O and NH 3 , others are anions such as Cl and CN . Cu(H 2 O) 4 2+ ( aq ) + 4 CN ( aq ) → Cu(CN) 4 2– ( aq ) + 4 H 2 O ( l ) A ligand that displaces two water molecules and connects to copper at two corners of the square is called bidentate; an example is ethylenediamine, NH 2 CH 2 CH 2 NH 2 (en). Chemistry 1B Experiment 13 57
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Cu(H 2 O) 4 2+ ( aq ) + 2 en ( aq ) → Cu(en) 2 2+ ( aq ) + 4 H 2 O ( l ) All complex ions are species that are dissolved in solution. While solutions containing complex ions may be colored, they are always transparent. Because complex ions are charged, they repel each other in solution and cannot precipitate by themselves. Precipitates. Metal cations also form precipitates with various anions. This can occur when the combined negative charge of the coordinated anions balances the positive charge of the cations, to form a neutral compound. For example, OH ions may displace water from the hydrated copper ion, Cu(H 2 O) 4 2+ , to form Cu(H 2 O) 3 OH + (a complex ion), Cu(H 2 O) 2 (OH) 2 (a precipitate), Cu(H 2 O)(OH) 3 (a complex ion), and Cu(OH) 4 2– (a complex ion). At most concentrations of OH , the neutral precipitate is most stable. Cu(H 2 O) 4 2+ ( aq ) + 2 OH ( aq ) → Cu(H 2 O) 2 (OH) 2 ( s ) + 2 H 2 O ( l ) Another, familiar way of writing Cu(H 2 O) 2 (OH) 2 ( s ), a hydrated solid, is Cu(OH) 2 •2H 2 O. When the waters of hydration are ignored, it is most simply written as Cu(OH) 2 . Other ligands form both stable precipitates and complex ions.
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This note was uploaded on 09/04/2011 for the course CHEM 1B taught by Professor Fossum during the Spring '10 term at Laney College.

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Experiment_13__-__relative_stability_of_Cu+2_compounds - 13...

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