Lecture 5

Lecture 5 - CHEM 7L Experiment 5: Qualitative Analysis...

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CHEM 7L Experiment 5: Qualitative Analysis Page 1 Analytical Chemistry Fundamental branch of Chemistry that involves separating, identifying, and determining the relative amounts of the components in a sample of matte of matter Qualitative Analysis : Identifying the chemical species that make-up a sample Quantitative Analysis : Determining the relative amount of one or more of these species, or analytes , in numerical terms 1 Experiment #5: Qualitative Analysis Relevant Concepts/Topics: Aqueous Equilibria & Solubility Le Chatelier’s Principle (H S 2H + +S 2 2 Le Chatelier’s Principle (H 2 S 2 H + S 2- ) Redox Reactions (in acid or base ) More Inorganic Complex Chemistry Goal: identify metal ions in an unknown ion solution (detective work) Qualitative Analysis Similar to gravimetric analysis (Expt. #1) in that precipitation is involved Bt i ti l i i tit ti 3 But gravimetric analysis is quantitative Expt. #5: You will use your knowledge of precipitation and K sp values to identify various metal ions qualitatively
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CHEM 7L Experiment 5: Qualitative Analysis Page 2 Qualitative Analysis: ID'ing Ions You will investigate a mixture of nine common metal ions in this experiment: 4 Ag + , Hg 2 2+ Cu 2+ , Bi 3+ , {Sn 4+ } Mn 2+ , Zn 2+ , Ni 2+ , Fe 3+ , Cr 3+ Solubility Rules 5 Experiment #5: Groups A Experiment #5: Groups A-C 9 metal ions separated into 3 major groups: “Group A”: Ag + , Hg I 2 2+ Form insoluble chlorides 6 “Group B”: Cu 2+ , Bi 3+ , {Sn 4+ } Form insoluble acidic sulfides “Group C”: Mn 2+ , Zn 2+ , Ni 2+ , Fe 3+ , Cr 3+ Form insoluble hydroxides (and basic sulfides)
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CHEM 7L Experiment 5: Qualitative Analysis Page 3 Equilibrium Constant K Describes the equilibrium composition of a reaction mixture a A + b B p P + q Q  q p Q P 7 Large K: products favored Small K: reactants favored Solubility Product (K sp ) When reactants = solid salt in equilibrium with its dissolved ions, [reactants] term drops out b a c B A K c & K sp Examples K = SO 2 = 0 041 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) 8 Ag + (aq) + Cl - (aq) AgCl(s) c [SO 3 ] 0.041 [SO 2 ] 2 [O 2 ] K sp = [Ag + ][Cl ] = 1.6 10 -10 (favors reactants) Equilibrium Composition 9 Favors Reactants Favors Products
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CHEM 7L Experiment 5: Qualitative Analysis Page 4 Reaction Product Q Same form as K, but arbitrary stage of reaction Plug concentration values into Q c Compare Q c to K c to determine whether existing 10 c c mixture of reactants and products will tend to favor the formation of more reactant(s) or product(s) Q < K : reaction tends to proceed toward products Q = K: no tendency to change in either direction Q > K : reaction tends to proceed toward reactants The Direction of Reaction 11 Q c & Q sp Examples Q c = [SO 3 ] 2 plug in values [SO 2 [O 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) 12 Ag + (aq) + Cl - (aq) AgCl(s) 2 ] [O 2 ] Q c > K c too much SO 3 : product forms reactants Q sp =[Ag + ][Cl ] plug in values Q sp > K sp Precipitation occurs
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CHEM 7L Experiment 5: Qualitative Analysis Page 5 Predicting Precipitation 13 A salt precipitates if Q sp > K sp By controlling the concentrations of ions (e.g., pH, complexation), one can effect precipitation Q sp vs. K
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Lecture 5 - CHEM 7L Experiment 5: Qualitative Analysis...

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