Reactions In Aqueous Solutions - Simonedamiano

Reactions in aqueous solutionsPrecipitation Reactions

Aqueous solutionsChemical reactions that occur in water are responsible for creation of cenotes. Whencarbon dioxide, CO2, dissolves in water, the resulting solution is slightly acidic andreacts with CaCO3 in the limestone:CaCO3(s) H2O(l) CO2 (aq) Ca(HCO3)2 (aq)116Reactions in Aqueous SolutionCHAPTER 4A solution in which water is the dissolving medium is called an aqueous solutionCaCO3CO2CHAPTER 4H""1 Atmospheric CO2 dissolvesin groundwater, forming H Reactions in Aqueous Solution–H2O and HCO3!CO2H""Acidic water reacts withCaCO3, dissolving limestoneand forming a cenoteCaCO3HCO3!1H2O!2Atmospheric CO2 dissolvesin groundwater, forming H and HCO3–2Acidic water reacts withCaCO3, dissolving limestoneand forming a cenoteCa2"!HCO3!"2!"!Ca2" " H!"2HCO3!!""HHCO3!!" FIGURE 4.1 Cenote formation.GURE 4.1 Cenote formation. 4.1 GENERAL PROPERTIES OFAQUEOUS cenotesSOLUTIONSin Yucatan - MexicoA solution is a homogeneous mixture of two or more substances. (Section 1.2) The

Aqueous solutionsA solution is a homogeneous mixture of two or more substances. Thesubstance present in the greatest quantity is usually called the solvent, and theother substances are called solutes, they are said to be dissolved in the solvent.Pure water is a very poor conductor of electricity. The conductivity ofbathwater originates from the substances dissolved in the water, not from thewater itself.Some substances (e.g NaCl) that dissolve in water make the resulting solutionconducting.

Electrolytic PropertiesTwo aqueous solutions,1. NaCl in water2. Table sugar (sucrose) in water.Both solutions are colorless, but they have different conductivities: the salt solution is a goodconductor of electricity, whereas the sugar solution is not.Current is a flow of electrically charged particles. In a solution the ions carry electrical charges. Thus,the conductivity of NaCl solutions indicates the presence of ions.The lack of conductivity of sucrose solutions indicates the absence of ions. When NaCl dissolves in water,the solution contains Na and Cl- ions, each surrounded by water molecules. When sucrose (C12H22O11)dissolves in water, the solution contains only neutral sucrose molecules surrounded by water molecules.A substance whose aqueous solutions contain ions is called an electrolyte.SECTION 4.1 General Properties of Aqueous SolutionA substance that does not form ions in solution is called a non electrolyte.Not litNot litLitPure water,H2O(l)does not conduct electricitySucrose solution,C12H22O11(aq)Nonelectrolytedoes not conduct electricitySodium chloride solution,NaCl(aq)Electrolyteconducts electricity! FIGUREElectricalconductiviwater andaqueous s

disperses throughout the solution [" FIGURE 4.3(a)]. The ionic solid dissociates into itscomponent ions as it dissolves.Ionic Compounds in Water118CHAPTER 4Reactions in Aqueous SolutionWhendissolves in water, each ion separates from the solid structure and disperses in theG O NaClFIGUREWhich solution,CH3OH(aq),conducts intoelectricity?Water is a very effsolution.TheNaCl(aq)ionic orsoliddissociatesits component ions.cally neutral molecule,denoted by d- . Each Hd"attracted by the negatiAs an ionic compod!shown in Figure 4.3(a)note solvated ions by wfor “aqueous.”Water is polar and is an effective solvent forionic (Sevents cations and aniocompounds. The O atom is rich in l negative charge, denoted by δ-. EachpersedH atomhas throuaWe can usually prpartial positive Methanolcharge (δ ).from the chemical namciates into sodium ioformulas and charges owhich ionic compoundd!1H2O moleculesseparate Na and Cl–ions from solid NaClIonic compounddissolves in water3Cl"2Na and Cl– ionsdispersethroughout thesolutionNa!Cations are attracted by the negative end ofG I Vend.E ITH2O, anions are attracted by the positiveSOWhat dissolved spa. KCN,b. NaClO4?H2O molecules surroundNa and Cl– ions(b) Molecular substances like methanol,(a) Ionic compounds like sodium chloride,As an ioniccompounddissolves,surroundedby H2ONaCl,form ions whenthey dissolvethe ions becomeCH3OH, dissolve without forming ionsmolecules (solvation). In chemical equations, solvated ions are noted as# FIGURE 4.3 Dissolution in water. (a) When an ionic compound, such as sodium chloride, NaCl, dissolves in water, H O molecules (aq)-(aq),separate,surround,and Cluniformlydispersethe ions intoaqthe liquid.substances thatdissolvein water, such as methanol, CH OH,Naandwhereasis an(b) Molecularabbreviationfor“aqueous”.usually do so without forming ions. We can think of this as a simple mixing of two molecular species. In both (a) and (b) the water molecules have23been moved apart so that the solute particles can be seen clearly.Solvation stabilizes the ions in solution and prevents cations and anionsfrom recombining.Molecular ComWhen a molecular commolecules dispersed tpounds are nonelectroanother example, a solumolecules dispersed inA few molecular smost important of theshydrochloric acid, HCl

structure andociates into itsMolecular Compounds in WaterWhen a molecular compound dissolves in water, the molecules remain intact and dispersedthroughout the solution. Consequently, most molecular compounds are non-electrolytes.Molecular substances like methanol, CH3OH, dissolvewithout forming ions.MethanolA few molecular substances have aqueous solutions thatcontain ions. Acids are the most important of thesesolutions. For example, when HCl(g) dissolves in water toform hydrochloric acid, HCl(aq), it ionizes; that is, itdissociates into H (aq) and Cl- (aq) ions.(b) Molecular substances like methanol,CH3OH, dissolve without forming ionsdium chloride, NaCl, dissolves in water, H2O moleculesbstances that dissolve in water, such as methanol, CH3OH,molecular species. In both (a) and (b) the water molecules have

Strong and Weak ElectrolytesStrong electrolytes are those solutes that exist in solution completely or nearly completely asions.Essentially all water-soluble ionic compounds (such as NaCl) and a few molecular compounds(such as HCl) are strong electrolytes.Weak electrolytes are those solutes that exist in solution mostly in the form of neutralmolecules.For example, in a solution of acetic acid (CH3COOH) most of the solute is present asCH3COOH (aq) molecules. Only a small fraction of the CH3COOH is dissociated intoH ions (aq) and CH3COO- (aq) ions.When a weak electrolyte, such as acetic acid, ionizes in solution, we write the reaction asCH3COOH (aq) CH3COO- (aq) H (aq)The arrows pointing in opposite directions mean that the reaction is at thechemical equilibrium.At any given moment some CH3COOH molecules are ionizing to form H and CH3COO- ionsbut those are recombining to form CH3COOH.

PRECIPITATION REACTIONS120CHAPTER 4Reactions in Aqueous SolutionReactions that result in the formation of an insoluble solid are called precipitation reactions.GO FIGUREA precipitate is formedbyremaina reactionWhich ionsin solution afterinPbI solution.precipitation is complete?2Pb2!NO3"I"K!Reactants2 KI(aq) ! Pb(NO3)2(aq)Pb2!(aq) and I–(aq) combineto form precipitateProductsPbI2(s) ! 2 KI(aq)FIGURE 4.4 A precipitation reaction.The figure shows two!clearsolutions being mixed.One solution contains potassium iodide, KI, dissolved in water and the other contains lead nitrate,Precipitation reactions occur when pairs of oppositely charged ions attract eachPb(NO3)2, dissolved in water.other so strongly that they form an insoluble ionic solid. To predict whether certaincombinations ofform insoluble compounds,yellowwe must considersome guidelinesThe reaction between these two solutes producesa ionswater-insolublesolid.concerning the solubilities of common ionic compounds.The precipitate is lead iodide (PbI2), a compoundthat has a very low solubility. The other product of thisSolubility Guidelines for Ionic Compoundsreaction, potassium nitrate (KNO3), remainsTheinsolubilitysolution.of a substance at a given temperature is the amount of the substance thatcan be dissolved in a given quantity of solvent at the given temperature. In our discussions, any substance with a solubility less than 0.01 mol/L will be referred to as insoluble.In those cases the attraction2 between the oppositely charged3ions in the solid is too greatfor the water molecules to separate the ions to any significant extent; the substance remains largely undissolved.Pb(NO3)2 (aq) KI (aq) PbI (s) 2 KNO (aq)

PRECIPITATION REACTIONSSolubility Guidelines for Ionic CompoundsPrecipitation reactions occur when pairs of oppositely charged ions attract eachother so strongly that they form an insoluble ionic solid.The solubility of a substance is the amount of the substance that can be dissolved in a givenquantity of solvent at the given temperature.In the case of an insoluble substance the attraction between the ions in the ionic solid is toogreat for the water molecules to separate the ions.All common ionic compounds of the alkali metal ions (group 1A of the PTE)and of the ammonium ion (NH4 ) are soluble in water.

PRECIPITATION REACTIONSSolubility Guidelines for Ionic CompoundsExperimental observations, however, have led to guidelines for predicting solubility for ioniccompounds. E.g. all common ionic compounds that contain the nitrate anion, NO3-, are solublein water.Solubility guidelines for common ionic compoundsSoluble Ionic CompoundsCompounds containingImportant ExceptionsNO3CH3COOClBrISO42-Insoluble Ionic CompoundsCompounds containingNoneNoneCompounds of Ag , Hg22 , and Pb2 Compounds of Ag , Hg22 , and Pb2 Compounds of Ag , Hg22 , and Pb2 Compounds of Sr2 , Ba2 , Hg22 , and Pb2 Important ExceptionsS2CO32PO43OH-Compounds of NH4 , the alkali metal cations, Ca2 , Sr2 ,and Ba2 Compoundsof NH4 and the alkali metal cationsCompounds of NH4 and the alkali metal cationsCompounds of NH4 , the alkali metal cations, Ca2 , Sr2 ,and Ba2

PRECIPITATION REACTIONSSolubility Guidelines for Ionic CompoundsTo predict whether a precipitate, we must1.note the ions present in the reactants2.consider the possible cation-anion combinations,3.use the “Solubility guidelines for common ionic compounds” table to determine ifany of these combinations is insoluble.E.g. Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed?Both substances are soluble ionic compounds and strong electrolytes (see the “Solubility guidelines forcommon ionic compounds” table).Mixing the solutions first produces a solution containing Mg2 , NO3-, Na , and OH- ions.Will either cation interact with either anion to form an insoluble compound?Knowing that Mg(NO3)2 and NaOH are both soluble in water, our only possibilities are Mg2 with OH- andNa with NO3-.From the table we see that hydroxides are generally insoluble.Because Mg2 is not an exception, Mg(OH)2 is insoluble and thus forms a precipitate. NaNO3, however, issoluble, so Na and NO3- remain in solution.The balanced equation for the precipitation reaction isMg(NO3)2 (aq) NaOH (aq) Mg(OH)2 (s) 2 NaNO3 (aq)

PRECIPITATION REACTIONSPRACTICE EXERCISE Classify these ionic compounds as soluble or insoluble in water:(a) sodium carbonate, Na2CO3(b) lead sulfate, PbSO4SOLUTIONWe are given the names and formulas of two ionic compounds and asked to predict whether they aresoluble or insoluble in water.We can use the “Solubility guidelines for common ionic compounds” table to answer the question.Thus, we need to focus on the anion in each compound because the table is organized by anions.(a) According to the table, most carbonates are insoluble. But carbonates of the alkali metal cations(such as sodium ion) are an exception to this rule and are soluble. Thus, Na2CO3 is soluble in water.(b) Although most sulfates are water soluble, the sulfate of Pb2 is an exception. Thus, PbSO4 isinsoluble in water. Classify the following compounds as soluble or insoluble in water:(a) cobalt(II) hydroxide(b) barium nitrate(c) ammonium phosphate.