Inorganic Qualitative Analysis (QUAL) 11 11. Zn2+(aq)   +  OH– (aq)   ®   N.R. NH3 N.R. Zn2+(aq)  +  OH– (aq)   ®   N.R. H+ N.R. 12. Ca2+(aq)  +  2OH– (aq)   ®   N.R. or Ca(OH)2(s)   H2O   Ca2+(aq)+2OH–(aq) Ca2+(aq)  +  2OH– (aq)   ®   N.R. or Ca(OH)2(s)H + Ca2+(aq) 13. Cu2+(aq)  +  2OH– (aq)   ®   Cu(OH)2(s) NH3 Cu(NH3)42+(aq)+2OH–(aq) Cu2+(aq)  +  2OH– (aq)   ®   Cu(OH)2(s) H+ Cu2+(aq)  +  2H2O(l) 14. Al3+(aq)  +  OH– (aq)   ®   N.R. NH3 N.R. Al3+(aq)  +  OH– (aq)   ®   N.R. H+ N.R. 15. Co2+(aq)  +  2OH– (aq)   ®   Co(OH)2(s) NH3 N.R. Co2+(aq)  +  2OH– (aq)   ®   Co(OH)2(s) H+ Co2+(aq)  +  2H2O Notes on the Tests and Equations 1.    Students may point out that most solubility rules would indicate that Zn2+ and Al3+ form insoluble hydroxides. Some may have noticed precipitates appearing when OH– is added. This precipitate disappears when additional OH– isadded.BothZn(OH)2andAl(OH)3areamphoteric(i.e.,theyaresoluble in both strong base and strong acid). They form soluble ions when an excess of OH– is present. For example: Zn2+(aq)  +  2OH–(aq)   ® Zn(OH)2(s) Zn(OH)2(s)  +  2OH–(aq)   ® Zn(OH)42–(aq) In a similar manner Al(OH)4 – is formed. Students will probably not be able to write an equation for the complex ion formation and will merely indicate no reaction since that is consistent with their observations. The point of the test is that it allows the separation of Zn2+ and Al3+ from the other ions. Whether or not you discuss complex formation by these ions depends upon the background of students. 2.    The Ca(OH)2 precipitate dissolves in the washings because although it is “insoluble,” it is relatively more soluble than most “insoluble” substances. (Ksp  =  5.5 x 10–6) 3.    The Cu(OH)2 precipitate is difficult to see until the solution is centrifuged. The green solution formed when the Cu(OH)2 is acidified is very pale. Most students will probably observe no color. The dark blue solution formed when the ammonia is added to the Cu(OH)2 is due to the formation of the complex ion Cu(NH3)42+. Students are unlikely to write the correct formula for the tetraammine copper complex, and might just write CuNH3 2+. It is probably not important that they write the correct formula, although it does not do any harm just to tell them the correct formula. The equations for the competing equilibria follow: Cu(OH)2(s) Cu2+(aq)  +  2OH–(aq) Ksp   =  1.6 x 10–19 Cu2+(aq)  +  4NH3(aq) Cu(NH3)4 2+(aq) Kf   =  2 x 1014 Concept/Skills Development ® ® ® ® ® ® ® ® ®