CHAPTER 4: Types Of Chemical Reactions

CHAPTER 4:Types of Chemical Reactions Dissolution Precipitation Acids and Bases and their reactions Oxidation-Reduction ReactionsCHEM 1310 A/B Fall 2006

Dissolution of Ionic CompoundsCHEM 1310 A/B Fall 2006

Dissolution of MoleculesCHEM 1310 A/B Fall 2006

Electrolytes Substances whichincrease the conductivityof water when theydissolve Usually ionic compounds(e.g., NaCl), but somemolecules can alsodissolve into ions (e.g.,acids like HCl, HNO3).Strong bases can also beelectrolytes (NH3 can splitH2O into ions).CHEM 1310 A/B Fall 2006

Solubility Tells how much of something (the “solute”)will dissolve into a given solvent Example: incomplete dissolution of sugarin iced tea Observation: hot tea dissolves sugar mucheasier than iced tea. This implies Another observation: Coke goes flat fasterat higher temperatures. Why? Solubilityof gases change CHEM 1310 A/B Fall 2006

Rules about solubility Most general rule: “Likedissolves like” Polar solvents (water,methanol, etc) usuallydissolve polar moleculesand ionic substances;nonpolar solutes not verysoluble Nonpolar solvents (CCl4,benzene, etc) usuallydissolve nonpolarmolecules; don’t dissolvepolar molecules wellPolar ethylene glycol dissolves in waterOil and water do not mixCHEM 1310 A/B Fall 2006

More rules about solubility No gases or solids are infinitely soluble (“miscible in allproportions”) with water; many liquids are Common inorganic acids soluble in water; organic acidswhich are small molecules soluble in water Small organic compounds with –OH and –NH2 groupsusually soluble in water Gases are less soluble as temperature increases.Liquids and soluds usually more soluble as temperatureincreases Solubilities of ionic compounds varies in water; seeTable 4-1CHEM 1310 A/B Fall 2006

Precipitation Reactions Opposite of dissolution --- asolid comes out of solutionNa (aq) Cl- (aq) NaCl(s) Can cause precipitation by (a)lowering temperature (e.g.,“rock candy”), (b) evaporatingsolvent, (c ) mixing 2 or moresolutions, e.g,BaCl2 (aq) K2SO4 (aq) BaSO4 (s) 2 KCl (aq)all of these are quite soluble inwater except BaSO4CHEM 1310 A/B Fall 2006Precipitation of lead iodide, PbI2,from KI (aq) Pb(NO3)2 (aq)

Ionic equations andnet ionic equations The previous example,BaCl2 (aq) K2SO4 (aq) BaSO4 (s) 2 KCl (aq),can be written in terms of ionic equations asBa2 (aq) 2 Cl- (aq) 2 K (aq) SO42- (aq) BaSO4 (s) 2 K (aq) 2 Cl- (aq) Notice that 2 Cl- (aq) and 2 K (aq) appear onboth sides of the equation. They “cancel” andwe are left withBa2 (aq) SO42- (aq) BaSO4 (s)the net ionic equation. The Cl- and K ions arespectator ionsCHEM 1310 A/B Fall 2006

Predicting precipitation In the previous example, how did we know that BaSO4would precipitate out, and KCl would not? Need to consider all possible precipitates, and consultsolubility tables to see if they remain dissolved or if theyprecipitate Table 4-1 in book gives rules of thumb Could look up “solubility constants” to do this morequantitatively --- more on this later in the course! Example: what happens when an aqueous solution ofberyllium nitrate is mixes with an aqueous solution ofsodium acetate?CHEM 1310 A/B Fall 2006

ExampleCHEM 1310 A/B Fall 2006

Example, cont’dCHEM 1310 A/B Fall 2006

Acids & Bases Acids––––Citric acid (lemons)Acetic acid (CH3COOH, in vinegar)Hydrochloric (HCl), sulfuric (H2SO4), nitric (HNO3)Taste sour, can be very reactive Bases––––In soaps (KOH, NaOH)Drain openers (NaClO, NaOH)Bleach (NaClO; ClO- is hypochlorite)Taste bitter, are corrosive, degrade organic matterCHEM 1310 A/B Fall 2006

Chemistry of acids & bases All our example acids contained hydrogen Several of our example bases contained hydroxide An “Arrhenius acid” gives H ions when dissolved inwater An “Arrhenius base” gives OH- ions when dissolved inwater Water itself is both an Arrhenius acid and Arrheniusbase. Small amounts undergo this reaction:H2O (l) H (aq) OH- (aq) Note: the H ions stay closely surrounded by at least afew H2O moleculesCHEM 1310 A/B Fall 2006

Strong acids and bases Dissociate completely in water (and aretherefore also strong electrolytes) HCl (g) H (aq) Cl- (aq) [demo] NaOH (s) Na (aq) OH- (aq) Strong acids: HCl, HNO3, H2SO4, HClO3, HClO4,HBr, HI Strong bases: NaOH, KOH Neutralization reaction: H from acid combineswith OH- from base, e.g.,HCl (aq) NaOH (aq) H2O (l) NaCl (aq)What’s the “net ionic equation” for this reaction?CHEM 1310 A/B Fall 2006

Weak acids Do not completely dissociate in water; weak electrolytes E.g., HF, acetic acid, CH3COOH, formic acid HCOOH,phosphoric acid H3PO4, [demo] Although they don’t normally completely dissociate inwater, a strong base can still “force” them to fullydissociate, e.g.,CH3COOH (aq) NaOH (aq) H2O (l) NaCH3COO (aq)most of the acetic acid remains undissociated before thereaction, so the net ionic equation isCH3COOH (aq) OH- (aq) H2O (l) CH3COO- (aq) See book about naming acidsCHEM 1310 A/B Fall 2006

Weak bases The only strong bases are alkali metalhydroxides (e.g., NaOH, KOH) andBa(OH)2 Everything else is a weak base(incomplete dissociation in water unlessforced by an acid)CHEM 1310 A/B Fall 2006

Generalization of Arrhenius Model Ammonia (NH3) tastes bitter, feels soapy,neutralizes acids, just like a base --- but it doesnot contain OH- ions! Revised definition: An Arrhenius base, whendissolved in water, increases the amount ofhydroxide ion over that present in pure solvent Definition of Arrhenius acid expanded similarlyfor H ion NH3 (aq) H2O (l) NH4 (aq) OH- (aq)[demo]CHEM 1310 A/B Fall 2006

Acid & base anhydrides Substances which become H or OH- donatorswhen or after they react with water– Acid anhydride: SO3 (g) H2O (l) H2SO4 (aq)– Base anhydride: Na2O (s) H2O (l) 2 NaOH (aq) Usually oxides of nonmetals are acid anhydrides Usually oxides of metals are base anhydrides (More electronegative more acidic,more electropositive more basic)CHEM 1310 A/B Fall 2006

Other reactions of acids Besides neutralization reactions, acids React with carbonates and hydrogen carbonatesto liberate gaseous CO2, producing a salt andwater, e.g., CaCO3 (s) HCl (aq) CaCl2 (aq) H2O (l) CO2 (g) [demo] act with oxides of metals to form salts and water,e.g.,CuO (s) H2SO4 (aq) ? React with various metals to liberate H2 andform a salt, e.g.,Mg (s) 2 HCl (aq) ?[demo]CHEM 1310 A/B Fall 2006

Other reactions of bases Bases (except NH3) act on ammonium salts togenerate gaseous ammonia, a salt, and water.E.g.,NH4Cl (s) NaOH (aq) NaCl (aq) NH3 (g) H2O (l)Note: gaseous ammonia can be dangerous! Bases react with the oxides of non-metals toproduce salts and water, e.g.,SO3 (g) 2 NaOH (aq) Na2SO4 (aq) H2O (l)CHEM 1310 A/B Fall 2006

Oxidation-Reduction (“Redox”)Reactions Example: Cu wire in AgNO3 (aq) [demo].Why does this Rx happen? Why does thesolution turn blue? Goals: to understand the twin prcesses ofoxidation and reduction; assign “oxidationnumbers”; become familiar with the basictypes of redox reactionsCHEM 1310 A/B Fall 2006

A simple redox reaction Recall periodic table, electronegativity,octets 2 Na (s) Cl2 (g) 2 Na Cl- (s)each Na atomloses an e“oxidation”(“LEO”)each Cl atomgains an e“reduction”(“GERtrude”)Na and Cl- bothhave filled shellslike a noble gas happy Transfer of electrons. To do bookkeepingon electrons transferred, use “oxidationnumbers”CHEM 1310 A/B Fall 2006

Oxidation numbersA bookkeeping device, like formal charges (but not the same)Book has a set of 5 rules 1. Sum of oxidation numbers must equal total charge ( 0 if neutralmolecule)2.Group I has oxidation number of 1 alwaysGroup II has 2 alwaysGroup III has 3 always3. Halogens have o.n. -1 usually (always for F) except in compoundswith O or other halogens4. H has 1 except for metal hydrides like LiH, where it has -15. O has -2 except (a) in compounds with F, use rule 3; (b) in O-Obonds, use rules 2, 4CHEM 1310 A/B Fall 2006

Examples What are the oxidation numbers for H2O2,NaO2, ClO- ?CHEM 1310 A/B Fall 2006

Oxidation and Reduction Use oxidation numbers to tell if an atom isoxidized or reduced by a reaction “Oxidizing agents” promote the oxidation ofother compounds. The oxidizing agent itselfmust be reduced (e.g., MnO4-) Redox reactions can change a lot withconditions (acidity, basicity, etc) Some strong oxidizing/reducing agents canreact slowly, some weak ones can react rapidlyCHEM 1310 A/B Fall 2006

Example of reaction conditionsIn acidic solution 5 Fe2 (aq) MnO4- (aq) 8 H (aq) 5 Fe3 (aq) Mn2 (aq) 4 H2O (l)Which is oxidized and which is reduced?In neutral solution 3 Fe3 (aq) MnO4- (aq) 4 H (aq) 3 Fe2 (aq) MnO2 (s) 2 H2O (l)CHEM 1310 A/B Fall 2006

Types of redox reactions Combination reactions: 2 elements combine toform a compound. Metals usually lose electrons(are oxidized)2 Fe (s) 3 Cl2 (g) 2 FeCl3 (s) [demo] Decomposition reactions: Compound breaks upinto elements or simpler compounds2 Ag2O 4 Ag (s) O2 (g) Oxygenation: Reaction of element or compoundwith oxygen (either O2 or O3, both oxidants)4 Li (s) O2 (g) 2 Li2O (s)CHEM 1310 A/B Fall 2006

Redox reaction types, cont’d Hydrogenation: H2 is a good reducing agent2 Na (l) H2 (g) 2 NaH (s)In organic chem, H2 often adds to double or triple bonds,e.g.,H2C O H2 H3C-OH Displacement:2 AgNO3 (aq) Cu(s) Cu(NO3)2 2 Ag (s) [from the demo!]Can predict ability to displace using “activity series”(Table 4-2). Ag is a weaker reducing agent than Cu (notas good at giving up electrons)CHEM 1310 A/B Fall 2006

More on displacement reactions The famous sodium in water Rx is also a“displacement” Rx.2 Na (s) 2 H2O (l) 2 NaOH (aq) H2 (g)Na “displaces” hydrogen Halogens displace “backwards”; they areoxidizing agents. Increasing oxidizing power:I2 Br2 Cl2 F2Cl2 (g) 2 KI (aq) I2 (g) 2 KCl (aq)CHEM 1310 A/B Fall 2006

Redox reaction types, cont’d 2 Disproportionation: A single ion undergoesboth oxidation and reduction2 H2O2 (l) 2 H2O (l) O2 (g)O reducedO oxidizedCHEM 1310 A/B Fall 2006

Stoichiometry of Reactions inSolution Quantitative problems like in chapter 2, but nowwith the solution phase Usually easiest to measure volumes of solutions.Need to know how much of a compound for agiven volume. “Concentration”Csolute nsolute / Volumemolarity moles of solute / liters of solution1.0 M 1.0 mol L-1 “1 molar”CHEM 1310 A/B Fall 2006

Example of molarity What molarity is a solution made bydissolving 10.0 g of Al(NO3)3 in enoughwater to make 250. mL of solution?CHEM 1310 A/B Fall 2006

Solution stoichiometry, cont’d Note the wording of the last example.Don’t dissolve solute in exactly 250 mL ofsolvent. Dissolve in less solvent, then addsolvent until 250 mL of solution. Dilution: See book. Basically, to getsolution 1/3 as concentrated, need toincrease volume by a factor of 3.CHEM 1310 A/B Fall 2006

Stoichiometry example When treated with acid, lead (IV) oxide isreduced to a lead (II) salt, liberatingoxygen2 PbO2 (s) 4 HNO3 (aq) 2 Pb(NO3)2 (aq) [reduced Pb!] 2 H2O (l) O2 (g) What volume of a 7.91 M solution of HNO3is needed to react with 15.9 g of PbO2?CHEM 1310 A/B Fall 2006

Example, cont’d 2 PbO2 (s) 4 HNO3 (aq) 2 Pb(NO3)2 (aq) 2 H2O (l) O2 (g)CHEM 1310 A/B Fall 2006

Titrations A quantitative way toperform a reaction withsolutions by measuringvolume of one solutionadded to another using aburet Frequently add base fromburet to an acidic solutionuntil the indicator in theflask just barely turnscolor when it turns basic “End point” – when basehas just neutralized theacidCHEM 1310 A/B Fall 2006

Titrations, cont’d Can know the amount of base added (fromaccurate concentration plus the knownvolume added) and get acid concentration(from volume of acid in solution, plus theknowledge that at endpoint, moles of acid moles of base) Or, sometimes “standardize” baseconcentration by titrating against knownamount of acidCHEM 1310 A/B Fall 2006