Ch 4 Reactions in Aqueous Solution Aqueous solution Solution in which water is the dissolving medium 4 1 General Properties of Aqueous Solutions Solution Homogenous mixture of two or more substances Solvent Substance present in greatest quantity Solutes Substances dissolved in solvent Electrolytes and Nonelectrolytes Pure water is poor conductor of electricity as water s conductivity comes from dissolved substances Salt solutions is good conductor of electricity o Electric current flow of electrically charged particles between two electrodes of solution More ions greater conductivity Electrolyte Substances whose aqueous solutions contain ions Nonelectrolyte Substance that does not form ions in solution How Compounds Dissolve in Water Ionic solids dissociate into their component ions as they dissolve Water is effective solvent for ionic cmpds bc of partial neg charge on O and partial pos charge on H As ionic cmpds dissolve ions become surrounded or solvated by H2O molecules o aq denotes solvated ions Solvation Helps stabilize ions in solution and prevents cations and anions from recombining Ions dispersed uniformly in solution Nature of ions predicted from chemical name of substance ex Na 2SO4 Na and SO42 Molecular cmpds like sucrose or methanol usually stay as intact molecules when dissolved in water o Most are nonelectrolytes o Acids can ionize although they are molecular cmpds Strong and Weak Electrolytes Strong electrolytes Solutes that exist in solution completely or nearly completely as ions o All water soluble ionic cmpds and few molecular Weak electrolytes Solutes that exist in solution mostly in form of neutral molecules with small fraction in form of ions o Ex acetic acid Solubility is not related to ionization In equation half arrows pointing in opp directions mean rxn is significant in both directions o Balance of sides determines relative numbers of ions and neutral molecules Chemical equilibrium State in which the relative numbers of each type of ion or molecule in the rxn are constant over time Single reaction arrow used for rxns that largely go forward o Ex HCl ionizes but H and Cl ions do not tend to combine to form HCl Ionic cmpds with ammonium ions are not strong electrolytes 4 2 Precipitation Reactions Precipitation reactions Reactions that result in the formation of a insoluble solid o Occurs when pairs of opp charged ions attract each other so strongly that they form insoluble ionic solid Precipitate Insoluble solid formed by a rxn in a sol n Solubility Guidelines for Ionic Compounds Solubility The amount of a certain substance that can be dissolved in a given quantity of solvent at the given temp Any substance with solubility less than 0 01 mol L considered insoluble in book o Attraction between ions too strong for water to separate ions All ionic compounds with NO3 group 1 elements and ammonium are soluble in water To predict whether precipitate forms when strong electrolyte solutions are mixed o Note ions present in reactants o Consider cation anion combinations o Determine if combinations are insoluble Table 4 1 Solubility Guidelines Exchange Metathesis Reactions Equation can be balanced only after chemical formulas of the products have been determined Exchange reactions metathesis reactions Reactions in which cations and anions appear to exchange partners To complete and balance equation o Use chemical formulas of reactants to determine present ions o Write chemical formulas of products by combining cation from one reactant with anion from other using ionic charges to determine subscripts o Check water solubilities of products precipitation rxns require at least one product to be insoluble in water o Balance equation Ionic Equations and Spectator Ions Indicate whether dissolved substances are present predominantly as ions or as molecules Molecular equation Equation which shows chemical formulas without indicating ionic character Complete ionic equation Equation written with all soluble strong electrolytes as ions Spectator ions Ions that appear in identical forms on both sides of a complete ionic equation Net ionic equation Equation which includes only ions and molecules directly involved in the rxn not repeated on both sides of equation Sum of ionic charges must be same If every ion in complete ionic equation is a spectator no rxn occurs Net ionic equation demonstrates more than one set of reactants can lead to same net rxn Steps for writing net ionic equations o Write balanced molecular equation for rxn o o Rewrite equation to show ions that form in solution when each soluble strong electrolyte dissociates into its ions Only strong electrolytes dissolved in aqueous sol n written in ionic form Identify and cancel spectator ions 4 3 Acids Bases and Neutralization Reactions Acids Acids Substances that ionize in aqueous solution to form hydrogen ions H aq Proton donors Molecules of different acids ionize to form different numbers of H ions Monoprotic Acids that yield one H per molecule of acid ex HCl HNO3 Diprotic Acids that yield two H per molecule of acid ex H2SO4 o Ionization occurs in two steps o Often only first ionization complete so mixture of ions Only one H released from acetic acid bonded to COOH group Bases Bases Substances that accept react w H ions Produce hydroxide ions OH in water Bases do not have to contain OH ions ex NH3 Strong and Weak Acids and Bases Acids and bases that are strong electrolytes are strong acids and strong bases Weak electrolytes are weak acids and weak bases Strong acids more reactive when reactivity depends only on H aq concentration Reactivity of acid can depend on anion in addition to H aq ex HF reactive due to anion Most common strong bases are common soluble metal hydroxides Most common weak base is NH3 Identifying Strong and Weak Electrolytes All soluble ionic compounds are strong electrolytes Molecular compounds o Strong acids and bases are strong electrolytes weak acids and bases are weak electrolytes and all other compounds are nonelectrolytes Neutralization Reactions and Salts Acids have sour taste and bases have bitter taste Acids change colors of dyes differently from bases Neutralization reaction Reaction when solution of an acid and a solution of base are mixed o Products do not have characteristics of acid or base o Between acid and metal hydroxide produces a water and a salt metathesis rxn o Main feature of rxn is combination of H and OH ions to form water Salt Any ionic compound whose cation comes