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Chemistry, The Central Science, 11th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 4 Aqueous Reactions and Solution Stoichiometry Mrs. Debbie Amuso Camden High School Camden, NY Aqueous Reactions © 2009, Prentice-Hall, Inc. Aqueous Solutions (aq) 1. Solutions consist of two or more substances physically combined in a homogeneous mixture. 2. Aqueous solutions consist of solute(s) dissolved in a solvent (water). 3. Solute = substance dissolved 4. Solvent = substance of greater quantity (usually water) Aqueous Reactions © 2009, Prentice-Hall, Inc. Aqueous Solutions (aq) 5. Properties: Clear or transparent but may be colored, Solute doesn’t settle on standing, Can’t be separated by filtering, Components are not in a definite ratio 6. Example: NaCl(aq) Aqueous Reactions Water: The Universal Solvent 1. Due to its polar nature, water will dissolve ionic substances and most polar substances. 2. “Like dissolves like.” 3. Example: NaCl(s) Na 1+ (aq) + Cl 1= (aq) + Aqueous Reactions Dissociation Dissociation is the process that occurs when an ionic substance dissolves in water and the solvent pulls the individual ions from the crystal and solvates them. Recall: Ionic substances contain a metal & nonmetal or polyatomic ion. Examples: NH4Cl or KBr Aqueous Reactions © 2009, Prentice-Hall, Inc. Electrolytes vs Nonelectrolytes • An electrolyte is a substance that dissociates into ions when dissolved in water. • A nonelectrolyte may dissolve in water, but it remains a molecule and does not dissociate into ions. Aqueous Reactions © 2009, Prentice-Hall, Inc. Electrolytes • A strong electrolyte dissociates completely when dissolved in water & forms lots of ions. • A weak electrolyte only dissociates partially when dissolved in water & only forms a few ions. Aqueous Reactions © 2009, Prentice-Hall, Inc. Electrolytes vs Nonelectrolytes 1. 2. 3. 4. Memorize the Chart Note: Ionic compounds contain a metal and nonmetal and/or polyatomic ion. Molecular (also called covalent) compounds contain only nonmetals. There is a separate list of strong acids you need to memorize. The compound must also be soluble in water – Be able to use the Solubility Table. Note: Strong bases are ionic & also strong electrolytes!! Aqueous Reactions © 2009, Prentice-Hall, Inc. Strong Acids & Bases Memorize! Aqueous Reactions © 2009, Prentice-Hall, Inc. Solubility Guidelines Know how to use this Chart. Aqueous Reactions How to Show Strong Electrolyte: KCl (s) K 1+ (aq) + Cl 1- (aq) Weak Electrolyte: HF (g) H 1+ (aq) + F 1- (aq) Nonelectrolyte: C12H22O11 (s) C12H22O11 (aq) Aqueous Reactions © 2009, Prentice-Hall, Inc. Check Yourself: 1. Electrolyte or Nonelectrolyte? a) C6H12O6 b) MgSO4 c) LiBr d) NH3 2. Soluble or Insoluble? a) AgCl b) BaCO3 c) NaC2H3O2 d) Li2S Answers: 1 a) NE b) E c) E d) E 2 a) Ins b) Ins c) Sol d) Sol Aqueous Reactions Solution Chemistry • It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, or an aqueous solution made of ions). • If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction. Aqueous Reactions © 2009, Prentice-Hall, Inc. • • • • Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose.” It appears the ions in the reactant compounds exchange, or transpose, ions. Metathesis is another name for a Double Replacement Reaction. Recall the General form: AB + CD AD + CB AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Aqueous Reactions © 2009, Prentice-Hall, Inc. Precipitation Reactions A Precipitation Reaction is a metathesis reaction that results in the formation of an insoluble solid (precipitate). Aqueous Reactions © 2009, Prentice-Hall, Inc. Precipitates 1. Precipitates are insoluble solids. 2. Practice: Name the precipitate formed when solutions of barium nitrate and sodium sulfate are mixed. Aqueous Reactions Answer: Barium sulfate Writing Net Ionic Equations 1. Write the balanced Molecular Equation. 2. Identify the product that does not ionize. It will be either: Water - H2O A gas - CO2 or H2 or O2 , etc. A precipitate – one of the insoluble ionic solids on the Solubility Table 3. Write the Ionic Equation by ionizing everything else. 4. Cross-out Spectator Ions (Those ions that don’t change.) Aqueous 5. What’s left is the Net Ionic Reaction. Reactions Note 1. It is said that a reaction “takes place” or “is spontaneous” or “goes to completion” only if one of the products is water, a gas, or a precipitate. 2. For metathesis reactions, if the products formed are also aqueous, we say the reaction is “not spontaneous” or doesn’t go to competition. Aqueous Reactions © 2009, Prentice-Hall, Inc. Molecular Equation The molecular equation lists the reactants and products in their molecular form. Be sure to include phases. AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) This rxn is spontaneous because a precipitate forms. Aqueous Reactions © 2009, Prentice-Hall, Inc. Ionic Equation • In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. • This more accurately reflects the species that are found in the reaction mixture. Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) AgCl (s) + K+ (aq) + NO3- (aq) Aqueous Reactions © 2009, Prentice-Hall, Inc. Net Ionic Equation • To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. • The only things left in the equation are those things that change (i.e., react) during the course of the reaction. • Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) Aqueous AgCl (s) + K+(aq) + NO3-(aq) Reactions © 2009, Prentice-Hall, Inc. Practice Completing & Balancing Metathesis Reactions BaCl2 (aq) + K2SO4 (aq) Answer: BaCl2(aq) + K2SO4(aq) BaSO4(s) + 2KCl(aq) Fe2(SO4 )3(aq) + LiOH (aq) Answer: Fe2(SO4 )3(aq) + 6LiOH(aq) 2Fe(OH)3(s) + 3Li2SO4(aq) Aqueous Reactions Write Ionic & Net Ionic Equations too BaCl2 (aq) + K2SO4 (aq) Answer: Ba2+(aq) + 2Cl- (aq) + 2K+(aq) + SO4 2-(aq) BaSO4(s) + 2K+(aq) + 2Cl- (aq) = spectator ions Net Ionic Rx = Ba2+(aq) + SO4 2-(aq) BaSO4(s) Fe2(SO4 )3(aq) + LiOH (aq) Answer: 2Fe3+(aq) + 3SO4 2-(aq) + 6Li+(aq) + 6OH-(aq) 2Fe(OH)3(s) + 6Li+(aq) + SO42= spectator ions Net Ionic Rx = 2Fe3+(aq) + 6OH-(aq) (aq) Aqueous 2Fe(OH)3(s) Reactions Acids • Arrhenius defined acids as substances that increase the concentration of H+ when dissolved in water. • Brønsted and Lowry defined them as proton donors. Aqueous Reactions © 2009, Prentice-Hall, Inc. Acids There are only seven strong acids: • Hydrochloric (HCl) • Hydrobromic (HBr) • Hydroiodic (HI) • Nitric (HNO3) • Sulfuric (H2SO4) • Chloric (HClO3) • Perchloric (HClO4) A common weak acid: – Acetic (HC2H3O2) Aqueous Reactions © 2009, Prentice-Hall, Inc. Bases • Arrhenius defined bases as substances that increase the concentration of OH− when dissolved in water. • Brønsted and Lowry defined them as proton acceptors. Aqueous Reactions © 2009, Prentice-Hall, Inc. Bases The strong bases are the soluble metal salts of hydroxide ion: • Alkali metals • Calcium • Strontium • Barium A common weak base: – Ammonia (NH3) They are: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 Aqueous Reactions © 2009, Prentice-Hall, Inc. Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H+) to the base. Aqueous Reactions © 2009, Prentice-Hall, Inc. Neutralization Reactions Generally, when solutions of an acid and a base are combined, the products are a salt and water. Neutralization Acid + Base Salt + Water Example: CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l) Aqueous Reactions © 2009, Prentice-Hall, Inc. Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) Na+ (aq) + Cl- (aq) + H2O (l) H+ (aq) + OH- (aq) H2O (l) Aqueous Reactions © 2009, Prentice-Hall, Inc. Gas-Forming Reactions Memorize: When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water. CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l) NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l) Aqueous Reactions © 2009, Prentice-Hall, Inc. Gas-Forming Reactions Memorize: Similarly, when a sulfite reacts with an acid, the products are a salt, sulfur dioxide, and water. NO SrSO3 (s) + 2 HI (aq) SrI2 (aq) + H2SO3 (aq) YES SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l) Aqueous Reactions © 2009, Prentice-Hall, Inc. Gas-Forming Reactions • This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! • But just as in the previous examples, a gas is formed as a product of this reaction. Na2S (aq) + H2SO4 (aq) Na2SO4 (aq) + H2S (g) Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation-Reduction Reactions • An oxidation occurs when an atom or ion loses electrons. • A reduction occurs when an atom or ion gains electrons. • One cannot occur without the other. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation-Reduction Reactions • The substance oxidized is called the reducing agent. • The substance reduced is called the oxidizing agent. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation-Reduction Reactions • The substance oxidized experiences an increase in oxidation number. • The substance reduced experiences a decrease in oxidation number. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation Numbers To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity. If the oxidation numbers change, a redox rxn has occurred. Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 1. Elements in their elemental form have an oxidation number of zero. Mg0(s) + 2HCl(aq) MgCl2 (aq) + H20 (g) 2. The oxidation number of a monatomic ion is the same as its charge. Mg0(s) + 2H+1Cl-1(aq) Mg+2Cl-12 (aq) + H20 (g) Note: Mg0 is oxidized (its charge increased) 2H1+ is reduced (its charge decreased) Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 3. The sum of the oxidation numbers in a neutral compound is 0. FeCl3 (Fe = +3 & each Cl = -1) FeCl2 (Fe = +2 & each Cl = -1) 4. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion. Sulfate Ion = -2, so for SO4 2(S = +6 and each O = -2 which adds up to -2) Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 5. Nonmetals tend to have negative oxidation numbers, although there are some exceptions. Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1. O2 (Here oxygen is 0) MgO (Here Mg is +2 and O is -2) Aqueous Na2O2 (In peroxides, Na is +1 and O is -1)Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 5. Nonmetals tend to have negative oxidation numbers, although there are some exceptions. Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal. HCl NH3 NaH (Here H is +1 and Cl is -1) (Here N is -3 and H is still +1) (Here Na is +1 and H is -1) Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 5. Nonmetals tend to have negative oxidation numbers, although there are some exceptions. Fluorine always has an oxidation number of −1. NaF (Na is +1 and F is -1) The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions. KClO3 (K is +1, Cl is +5 and each O is -2) Aqueous Reactions © 2009, Prentice-Hall, Inc. Redox Reactions 1. Be able to identify which substance is oxidized and which substance is reduced. Example: 0 0 H2 + O2 +1 -2 H2O (H2 is oxidized because its charge increases) (O2 is reduced because its charge decreases) 2. Metathesis Rxn are not redox because the charges don’t change. Aqueous Reactions © 2009, Prentice-Hall, Inc. Displacement Reactions • In displacement reactions, ions oxidize an element. • The ions, then, are reduced. Aqueous Reactions © 2009, Prentice-Hall, Inc. Displacement Reactions In this reaction, silver ions oxidize copper metal. Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s) Net Ionic Rx: Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s) NO31- is the spectator ion Aqueous Reactions © 2009, Prentice-Hall, Inc. Displacement Reactions The reverse reaction, however, does not occur. x Cu (s) + 2 Ag+ (aq) Cu2+ (aq) + 2 Ag (s) Aqueous Reactions © 2009, Prentice-Hall, Inc. Activity Series Rule: A metal will replace (reduce) any metal below it on the table. Given: A + BC AC + B The reaction will take place only if A is more reactive (above) B on the table! If so, we say the reaction isAqueous Reactions spontaneous. © 2009, Prentice-Hall, Inc. Molarity • Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. • Molarity is one way to measure the concentration of a solution. Molarity (M) = moles of solute volume of solution in liters M = mol/L (Also helpful: mol = mass/gfm) Aqueous Reactions © 2009, Prentice-Hall, Inc. Mixing a Solution • To create a solution of a known molarity, one weighs out a known mass (and, therefore, number of moles) of the solute. • The solute is added to a volumetric flask, and solvent is added to the line on the neck of the flask. mass = (Molarity) (gfm) (Vliters ) mass = (M) (gfm) (VL) Aqueous Reactions © 2009, Prentice-Hall, Inc. Solution Dilution • One can also dilute a more concentrated solution by – Using a pipet to deliver a volume of the solution to a new volumetric flask, and – Adding solvent to the line on the neck of the new flask. Aqueous Reactions © 2009, Prentice-Hall, Inc. Solution Dilution The molarity of the new solution can be determined from the equation M1 V1 = M2 V2, where M1 and M2 are the molarity of the concentrated and dilute solutions, respectively, and V2 and V2 are the volumes of the two solutions. Aqueous Reactions © 2009, Prentice-Hall, Inc. Titration Titration is an analytical technique in which one can calculate the concentration of a solute in a solution. Aqueous Reactions © 2009, Prentice-Hall, Inc. Acid-Base Titration Use: (Ma)(Va) = (Mb)(Vb) Shortcut: Diprotic Acid Like H2SO4 : 2 (Ma)(Va) = (Mb)(Vb) Dihydroxyl Base Like Ca(OH)2 : (Ma)(Va) = 2 (Mb)(Vb) Aqueous Reactions © 2009, Prentice-Hall, Inc. Using Molarities in Stoichiometric Calculations Aqueous Reactions © 2009, Prentice-Hall, Inc. Stoichiometric Calculation Steps Step 1) Complete & balance equation using coefficients. Step 2) Identify Given & Unknown. Step 3) Convert Given into moles using either #moles = mass / GFM -or- #moles = (Molarity)(VL) Step 4) Use mole ratio to find moles of Unknown. #molesgiven Coeffgiven = #molesunk Coeffunk Step 5) Convert moles of ‘unknown’ to answer using either mass = (#moles) (GFM) –or- Molarity = #moles / VL Aqueous Reactions © 2009, Prentice-Hall, Inc. Example: How many grams of NaOH(s) are needed to neutralize exactly 50.0 mL of 2.00 M H2SO4 ? Step 1) 2 NaOH(s) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) Step 2) Given = 50.0mL of 2.00M H2SO4 Unknown = mass of NaOH Step 3) Convert Given into moles using #moles = (Molarity)(VL) #moles H2SO4 = (2.00M)(0.0500L) = 0.100 moles H2SO4 Step 4) Use mole ratio to find moles of Unknown. #molesgiven = #molesunk 0.100 Coeffgiven Coeffunk 1 = x 2 x = 0.200 mol Step 5) Convert moles of ‘unk’ to answer using mass = (#moles) (GFM) Aqueous mass = (0.200moles)(40.0g/mol) = 8.00 grams NaOHReactions © 2009, Prentice-Hall, Inc.