Chapter 4 Aqueous Reactions and Solution Stoichiometry Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 4 Aqueous Reactions and Solution Stoichiometry

4.1 Properties of Aqueous Solutions

Solutions (abrev. sol’n) Homogeneous mixtures of two or more pure substances can be liquids or gases. Solvent - substance present in the greatest quantity Solute(s) - All other substances present in lesser quantities Solutions in which water is the solvent are called aqueous solutions

Water – a great solvent! H2O molecules can attract, separate, and surround the cations and anions that make up many ionic compounds Solvation - the process of a solvent surrounding and dissolving an ionic compound  the cations & anions become dissociated (or separated) as they are surrounded by the H2O molecules The dissociated ions are free to move and become evenly dispersed throughout the solution. the moving ions can carry a current throughout the solution resulting in electrical conductivity (movement of charged particles through a medium).

1 2 NaCl + H2O 3 4

Ionic Compounds in Water While water is an excellent solvent, not ALL ionic substances will readily dissolve and dissociate in the presence of H2O Those that DO dissociate are known as electrolytes: Electrolyte - a substance which contains ions and conducts electricity in it’s aqueous form There are 3 classifications of electrolytes: strong, weak, and nonelectrolytes

* explained on a later slide Strong Electrolytes dissociates completely into ions when dissolved in H2O sol’n exists as 100% ions (no bonded ionic compounds left undissolved) good conductor of electricity Includes: all soluble * ionic compounds all strong acids & bases* a few covalent compounds * explained on a later slide

Weak Electrolytes dissociate only partially into ions when dissolved in H2O sol’n exists mostly as molecules surrounded by water (very, very few ions present) poorly conducts electricity Includes: weak acids & bases* (ex: acetic acid, HC2H3O2 [only 1% as ions]) slightly soluble ionic compounds* some covalent compounds

Nonelectrolytes do NOT dissociate into ANY ions dissolved substance consists of intact molecules surrounded by H2O does not conduct electricity Includes: most covalent compounds (w/the exception of acids & bases which are either strong or weak)

Electrolytes: a visual

Solubility – ability to be dissolved Soluble (aq) a substance which easily dissolves in water Slightly Soluble (s) a substance that only dissolves a tiny bit in water Insoluble (s) substance that does NOT dissolve in water substance remains separate from the H2O molecules (no interaction) substances are insoluble when the ions attract so strongly that they CANNOT be pulled apart by H2O molecules

Solubility: a visual

Solubility Rules To know if an ionic substance is soluble or not, you must consult the solubility rules lists which ions are typically soluble and insoluble also lists exceptions to the rules Note: Acids are not explicitly listed in the rules because all acids are aqueous solutions (thus soluble)

Solubility Rules for Common Ionic Compounds in Water Soluble Ionic Compounds Exceptions: these are insoluble w/ respective anions Nitrates, NO3- none Acetates, C2H3O2- Chlorides, Cl- Ag+, Hg22+ (2 Hg atoms), Pb2+ Bromides, Br- Ag+, Hg22+, Pb2+ Iodides, I- Sulfates, SO42- Ag+, Hg22+, Pb2+, Sr2+, Ba2+ Alkali metal cations Ammonium, NH4+ Insoluble Ionic Compounds Exceptions: these are soluble w/ respective anions Sulfides, S2- alkali metal cations, NH4+, Ca2+, Sr2+, Ba2+ Carbonates, CO32- alkali metal cations, NH4+ Phosphates, PO43- Hydroxides, OH-

Solubility Rules - Practice Are the following soluble or insoluble? Be(C2H3O2)2 MgS BaSO4 K3PO4 soluble insoluble insoluble soluble

Acids Bases Substances that produce H+ ions when dissolved in water (Arrhenius). Ex) HNO3  H+ + NO3– Substances that produce OH− ions when dissolved in water (Arrhenius). Ex) NaOH  Na+ + OH–

Strong vs. Weak Both strong & weak acids and bases exist Strong acids & bases dissociate completely in solution Exist as 100% ions Strong electrolytes & conductors Weak acids & bases only partially dissociate in solution Few ions exits Weak electrolytes & conductors

Strong vs. Weak Electrolytes Strong Acid: Weak Acid: HCl HNO2

Seven Strong Acids MUST BE MEMORIZED! Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO3) Sulfuric (H2SO4) Chloric (HClO3) Perchloric (HClO4) **When dissolved in water, they dissociate 100% into H+ ions and their respective anion.**

Strong Bases MUST BE MEMORIZED!  Soluble hydroxide compounds Alkali metal hydroxides LiOH, NaOH, KOH, etc… Calcium hydroxide, Ca(OH)2 Strontium hydroxide, Sr(OH)2 Barium hydroxide, Ba(OH)2 When dissolved in water, they dissociate 100% into their respective metal cation and OH–

Predicting Ions in Solution If a substance is soluble, you can predict what ions present are in the solution Ex) Ammonium sulfate (NH4)2SO4  soluble Dissociates into: 2 NH4+ ions 1 SO42- ion Ex) Sulfuric Acid H2SO4  soluble Dissociates into: 2 H+ ions

How to Classify Electrolytes Ask… Is the substance ionic, covalent, or an acid/base? If ionic, is it soluble? If soluble = strong electrolyte If insoluble = nonelectrolyte If covalent If covalent and NOT an acid or base = generally nonelectrolyte (ex: sugar, organic compounds) If acid or a base, is it a strong acid/base? If strong acid or base = strong electrolyte if it is any other acid or base = weak electrolyte

Classify the following as: strong, weak, or nonelectrolyte CaCl2 HNO3 Ag3PO4 HCHO2 KOH C2H5OH – ionic – soluble = Strong – acid = Strong – ionic – insoluble = Nonelectrolyte – acid = Weak – ionic – soluble = Strong (also strong base) – covalent– NOT a base = Nonelectrolyte

Classify the following as: strong, weak, or nonelectrolyte 1) Mg(OH)2 2) HCl 3) H3PO4 4) (NH4)2CO3 5) H2SO4 – ionic – insoluble = nonelectrolyte – strong acid = strong – weak acid = weak – ionic – soluble = strong – strong acid = strong

4.2 Precipitation Rxns

Precipitation Rxns Double replacement reactions that specifically produce a solid product This solid product is called a precipitate (abbreviated ppt.) and is known as insoluble (because it cannot readily dissolve in water) To predict if an insoluble ppt will form, use the solubility rules

Predicting Precipitations Rxns Predict the products of the double replacement reaction (cations switch partners) Use solubility rules to determine the states of matter of the products: insoluble = (s); soluble = (aq) If a solid is formed, a ppt rxn has occurred

Predicting Precipitation Rxns Practice 1. Fe(NO3)3 (aq) + K2S (aq) 2 Fe(NO3)3 (aq) + 3 K2S (aq)  Fe2S3 (s) + 6 KNO3 (aq) 2. Li2CO3 (aq) + KC2H3O2 (aq) Li2CO3 (aq) + KC2H3O2 (aq)  2 LiC2H3O2 (aq) + K2CO3 (aq)  NR (no reaction) = a bunch of ions in a puddle 

Will precipitates form? DEMO!!! Will precipitates form? Solutions Used: 0.1 M KI 0.1 M Pb(NO3)2 0.1 M CaCl2

Balanced Chemical Equations Balanced chemical equations show chemical formulas without considering the ionic character of any substances However… recall that soluble ionic compounds are strong electrolytes strong acids, strong bases, and soluble ionic salts This means they dissociate and are truly present as ions in a sol’n (which leads us to…)

…Ionic Equations Complete Ionic (CIE): explicitly indicates all the IONS that are in sol’n throughout the rxn Spectator Ions = ions present on BOTH sides of the chemical equation Ions are present but play NO DIRECT ROLE in chemical rxn When ALL reactants and products are AQUEOUS (no reaction), every ion is a spectator ion (so CIE is not usually written)

…Ionic Equations Net Ionic (NIE): includes only ions and compounds directly involved in the chemical rxn This means NO spectators are included

CIE & NIE Practice Mg(NO3)2 (aq) + Li2SO4 (aq) Balanced Equation: Mg(NO3)2 (aq) + Li2SO4 (aq)  MgSO4 (aq) + 2 LiNO3 (aq) = NR Complete Ionic Equation: Mg2+ (aq) + 2 NO3- (aq) + 2 Li+ (aq) + SO42- (aq)  Mg2+ (aq) + SO42- (aq) + 2 Li+ (aq) + 2 NO3- (aq) Net Ionic Equation: NONE! – all spectators

CIE & NIE Practice (NH4)3PO4 (aq) + BaS (aq) Balanced Equation:  3 (NH4)2S (aq) + Ba3(PO4)2 (s) Complete Ionic Equation: Net Ionic Equation: 2 PO43- (aq) + 3 Ba2+ (aq)  Ba3(PO4)2 (s) … NO spectators! 6 NH4+ (aq) + 2 PO43- (aq) + 3 Ba2+ (aq) + 3 S2- (aq)  6 NH4+ (aq) + 3 S2- (aq) + Ba3(PO4)2 (s)

4.4 Oxidation-Reduction Reactions

Single Replacement Rxns General Rxn Formula: A + BC  AC + B The reactants: One reactant is a single element by itself The other reactant is an ionic compound The products: If the single element is a metal, it will replace the cation in the ionic compound If the single element is a halogen gas, it will replace the anion in the ionic compound

Oxidation-Reduction Rxns (aka: Redox) All single replacement rxns can be classified as Redox rxns (but not all redox rxns are single replacement) Redox rxns occur when electrons are transferred between reactants Oxidation occurs when an atom, ion, or molecule loses electrons. (LEO) Reduction occurs when an atom, ion, or molecule gains electrons. (GER) * Both processes happen simultaneously in a rxn

Activity Series of Metals - Ease of Oxidation When a single replacement rxn occurs, the metal that is alone will be oxidized. However, some metals are easier to oxidize than others (they lose electrons more easily than others, thus forming cations) Active Metals: metals at the top of the activity series are more reactive and more easily oxidized Noble Metals: metals at the bottom of the activity series which have very low reactivity

Activity Series of Metals In a single replacement rxn: ---------------------------------------------------------------------- a solid metal element can be OXIDIZED by any elements listed below it in the activity series Look to see if the metal solid is ABOVE the metal cation (in the 2nd reactant) If yes, the rxn does happen  predict products If not, NR (b/c the solid metal cannot be oxidized) Activity Series of Metals

Using the Activity Series Examples 1) magnesium sulfate+ zinc metal 2) iron metal + copper (II) nitrate  NR Fe (s) + Cu(NO3)2 (aq)  Cu (s) + Fe(NO3)2 We can also write CIE & NIE!!! (only aqueous dissociate) CIE: Fe (s) + Cu2+ (aq) + 2 NO3– (aq)  Cu (s) + Fe2+ (aq) + 2 NO3– (aq) NIE: Fe (s) + Cu2+ (aq)  Cu (s) + Fe2+ (aq)

Metals Reacting w/Acids Only metals listed ABOVE hydrogen in the series are able to react with acids If the reaction does occur, H2 gas is produced. Example #1: ___Ni (s) + ___HCl (aq)  Ni is above H2, so the rxn occurs! Example #2: ___Cu (s) + ___HCl (aq)  Cu is below H2, so no rxn occurs! NiCl2(aq) + H2 (g) 2 NR

Activity Series of Halogens: The idea is the same here… Fluorine is the most reactive Iodine is the least reactive Only a MORE reactive halogen can replace a LESS reactive one In these single replacement rxns, the single element (halogen gas) must be listed ABOVE the halogen anion in the 2nd reactant for a rxn to occur ___Cl2 + ___NaBr (aq)    Activity Series of Halogens: Fluorine Chlorine Bromine Iodine ___NaCl (aq) + ___Br2 (g) 2 2