Ch 4_Part 2_101811 - 10/18/2011 Reaction Classes...

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10/18/2011 1 Reaction Classes Precipitation : synthesis of an ionic solid – a solid precipitate forms when aqueous solutions of certain ions are mixed Acid-Base : proton transfer reactions – acid donates a proton to a base, forming a molecule (water or another weak acid) and an aqueous salt Acid : proton-donor; Base : proton-acceptor Oxidation-Reduction : electron transfer reactions – electron transfer from one species to another, causing a change in the oxidation state of the two species OIL RIG : O xidation I s L oss (of e - ), R eduction I s G ain (of e - ) – includes combustion, the reaction of a substance with oxygen 1 Precipitation Reactions Sometimes when we mix two solutions together, an insoluble solid will form: AgNO 3 (aq) + NaCl(aq) K 2 CrO 4 (aq) + Ba(NO 3 ) 2 (aq) The solid, called a precipitate (or insoluble salt ) is insoluble in water. It is so insoluble that when its component ions find each other in solution, they very rapidly get locked together in large clumps, driving the reaction towards the products. 2
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10/18/2011 2 The Solubility of Ionic Compounds in Water The solubility of ionic compounds in water depends upon the relative strengths of the electrostatic forces between ions in the ionic compound and the attractive forces between the ions and solvent molecules (often water). There is a tremendous range in the solubility of ionic compounds in water. The solubility of so called “insoluble” compounds may be several orders of magnitude less than ones that are called “soluble” in water. For example, consider the solubility (in g/L) of the following compounds in water at 20 o C : Solubility of NaCl = 365 Solubility of MgCl 2 = 542.5 Solubility of AlCl 3 = 699 Solubility of PbCl 2 = 9.9 Solubility of AgCl = 0.009 Solubility of CuCl = 0.0062 3 Precipitation Reactions The main challenge with precipitation reactions is predicting what solid (if any) will form. Example: Addition of potassium chromate to barium nitrate. K 2 CrO 4 ( aq ) + Ba(NO 3 ) 3 ( aq ) What is the precipitate? 4
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10/18/2011 3 One approach: consider the ions present in the solutions: K 2 CrO 4 ( aq ) Ba(NO 3 ) 2 ( aq ) Precipitation Reactions 5 Clues: Potassium forms soluble salts. Chromate is yellow. BaCrO 4 precipitates! Precipitation Reactions When combining ions, consider which precipitates could form Precipitates must have a net charge of zero Possible choices are BaCrO 4 and KNO 3 6
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10/18/2011 4 BaCl 2 (aq) Ba 2+ Ba 2+ Cl - Cl - Cl - Cl - AgNO 3 (aq) Ag + Ag + NO 3 - NO 3 - BaCl 2 (aq) + AgNO 3 (aq) Ag + Ag + NO 3 - NO 3 - Ba 2+ Ba 2+ Cl - Cl - Cl - Cl - 2AgNO 3 (aq) + BaCl 2 (aq) 2AgCl(s) + Ba(NO 3 ) 2 (aq) Solid silver chloride precipitates out of solution. Barium and nitrate ions are left in aqueous solution. Precipitation Reactions 7 Precipitation Reactions In the previous example, we know that Ba(NO 3 ) 2 is not the precipitate since in our first example this compound was soluble in water.
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This note was uploaded on 01/18/2012 for the course CHEM 142B taught by Professor John during the Fall '11 term at University of Washington.

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Ch 4_Part 2_101811 - 10/18/2011 Reaction Classes...

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