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Honors Chemistry Lecture 5: Qualitative Analysis Using Precipitation

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Carnegie Mellon University

September 6, 2013 Honors Chemistry Lecture 5 Today: Qualitative analysis using precipitation in water For our purposes, when dealing with precipitates we will assume the following:  Our solvent is water  We are dealing with a neutral pH of 7 2- 2-  We will not really discuss sulfides (S ) or oxides (O ) Ionic bonding in solids: Coulomb’s law: E = ~ (q q 1 /2r (where E = energy, q = charge of ion, ~ = constants we aren’t worried about, r = distance between two ions) 2- 2-  Therefore O and S will form very stable solids  Therefore SO 2-is soluble because it’s so big; it’s size causes Coulomb’s law to 4 counteract the charge General Solubility Rules:  Group 1 alkali metals are always soluble  Compounds containing NO are always soluble 3  Compounds containing NH are sol4ble - 2+ 2+ 2+  Compounds containing OH are insoluble (except with Group I alkali metals and Ba , Ca , Sr )  Compounds containing S are insoluble (except with Group I alkali and Group II alkaline metals) - - - 3- +  Compounds containing SO , CO ,3CrO ,3and PO 4 4 are insoluble (except for those with NH or 4 Group I alkali metals) Besides the general solubility rules, you can also remember solubility rules for certain groups of ions (i.e., the silver group, the copper-arsenic group, the aluminum-nickel group, and the barium-magnesium group) The Silver Group: + + 2+  The silver group is made up of silver (I) [Ag ], mercury (I) [Hg ], and lead (II) [Pb ]  All of these form insoluble chlorides  In general, chloride salts are soluble  Tend to form white precipitates (ppt)  Remember that Hg is a paramagnetic species so it cannot stay in this form. Instead, it becomes 2+ + 2+ Hg 2 [Hg  Hg ] 2  Side note: Pb is always insoluble Some Compounds with Ag and their Solubility Soluble (all colorless) Insoluble (form colored ppt) AgNO , AgF, Ag SO (slightly soluble), AgCl (white), AgBr (yellow), AgI (yellow), Ag O 3 2 4 2 Ag(CN) (2asic solution) (brown), Ag C2 (ye3low), Ag CrO 2dark 4ed) 2+ Some Compounds with Hg 2 and their Solubility Soluble (all colorless) Insoluble (form colored ppt
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