11/6/20131Fri, Nov 8• Lecture 17 (Change)– Predicting Whether a Reaction Will Occur (7.1)– Reactions in Which a Solid Forms (7.2)– Describing Reactions in the Aqueous Phase (7.3)• Questions we’ll answer:– Many important reactions take place in water environment. How do we describe these reactions? – What are some common types of chemical reactions?Solution FormationWhen ionic compounds dissolve in water, they separate into ions that can move independently of each other.When molecular compounds dissolve in water, the atoms in the molecules do not separate from each other…the molecules remain intact.NaCl( ) Na ( ) Cl ( )saq aq22 22HOCH CH OH( ) HOCH CH OH( )laq• A solution is a mixture of a solute (thing being dissolved) and a solvent (the dissolving medium).• The term “solvent” comes from the Latin solvo: I loosen, untie, undo, free up.11/6/20132Conductivity of SolutionsWhen ionic compounds dissolve in water, the ions separate and move around independently. These charged species can conduct electricity, and are therefore called electrolytes.Covalent compounds (e.g. sugar, ethanol) do not produce charged species in solution.Non‐Electrolyte–+–+–+–+–+Each unit of the substance that dissolves in water produces separated ionsStrong Electrolyte–+–+–+–+–+Only a small fraction of the units produce separated ions; the majority of the units remain undissociatedWeak ElectrolytePrecipitation Reactions• Sometimes when we mix two solutions, an insoluble solid will form: AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)KI(aq) + Pb(NO3)2(aq)  PbI2(s) + KNO3(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 get locked together in large clumps, driving the rxn towards products.• The main challenge in precipitation reaction is predicting what (if anything) will form. 211/6/20133KI(aq)K+K+I‐I‐Pb(NO3)2(aq)Pb2+NO3‐NO3‐KI(aq) + Pb(NO3)2(aq)2 KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2 KNO3(aq)Solid lead(II) iodide precipitates out of solution.Potassium and nitrate ions are left in aqueous solution.Precipitation ReactionsPb2+NO3‐NO3‐K+I‐K+I‐K+K+I‐I‐K+I‐K+I‐Pb2+NO3‐NO3‐Pb2+NO3‐NO3‐Solubility Rules• Notice in this example we know that Pb(NO3)2isn’t the precipitate since in our first example this compound was soluble in water.• Therefore, one way to determine if a precipitate will form is to simply study combinations of reactions where you know that one of the potential precipitates is soluble.• That’s exactly how precipitation or “solubility” rules have been determined.11/6/20134Solubility RulesMemorize these…Soluble CompoundsInsoluble CompoundsPredicting Precipitation ProductsAl(NO3)3(aq) + Ba(OH)2(aq)2FeSO4(aq) + KCl(aq)3CaCl2(aq) + Na2SO4(aq)K2S(aq) + Ni(NO3)2(aq)211/6/201359Total and Net Ionic EquationsConventional equation: a bookkeeping of all species present, and arranged for charge neutrality.Total Ionic equation: all aqueous species are split up into their component ions.Net Ionic equation: indicates exactly the chemical change that occurs, and nothing more.Ba(NO3)2(aq) + Na2SO4(aq) BaSO4(s) + 2NaNO3(aq)Ba2+(aq) + 2NO3-(aq) + 2Na+(aq) + SO42-(aq)BaSO4(s) + 2Na+(aq) + 2NO3-(aq)Ba2+(aq) + SO42-(aq) BaSO4(s)Na+and NO3-are spectator ions in this reaction…they are not involved in the chemical change.Example• Example: Write the balanced chemical equation, complete, and net ionic equation for the reaction of potassium chloride and silver nitrate.• First: Remember nomenclature and write the chemical reaction.• Next: Using solubility rules write the ionic equation.33KCl (aq) AgNO (aq) AgCl (s) KNO (aq)33K (aq) Cl (aq) Ag (aq) + NO (aq) AgCl (s) K (aq) NO (aq)   11/6/20136Example (cont)• Last: Cancel spectator ions for either side of the complete ionic equation to obtained the net ionic equation33K (aq) Cl (aq) Ag (aq) + NO (aq) AgCl (s) K (aq) NO (aq)   Cl (aq) Ag (aq) AgCl