Chemistry and Chemical Reactivity Chapter 5 Reactions in Aqueous Solution PDF

Chemistry and Chemical Reactivity 1 6th Edition John C. Kotz Paul M. Treichel...

Chemistry and Chemical Reactivity 1 6th Edition John C. Kotz Paul M. Treichel Gabriela C. Weaver CHAPTER 5 Reactions in Aqueous Solution Lectures written by John Kotz ©2006 © 2006 Brooks/Cole Brooks/Cole Thomson - Thomson  2 Reactions in Aqueous Solution Many reactions involve ionic compounds, especially reactions in water — aqueous solutions. KMnO4 in water K+(aq) + MnO4-(aq) © 2006 Brooks/Cole - Thomson  3 CCR, page 177 An Ionic Compound, CuCl2, in Water © 2006 Brooks/Cole - Thomson  Aqueous Solutions 4 How do we know ions are present in aqueous solutions? The solutions conduct electricity! They are called ELECTROLYTES HCl, CuCl2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions. © 2006 Brooks/Cole - Thomson  Aqueous Solutions 5 HCl, CuCl2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions. © 2006 Brooks/Cole - Thomson  Aqueous Solutions 6 Acetic acid ionizes only to a small extent, so it is a weak electrolyte. CH3CO2H(aq) CH CO 3 2 -(aq) + H+(aq) © 2006 Brooks/Cole - Thomson  Aqueous 7 Solutions Acetic acid ionizes only to a small extent, so it is a weak electrolyte. CH3CO2H(aq) CH CO 3 2 - (aq) + H+(aq) © 2006 Brooks/Cole - Thomson  Aqueous 8 Solutions Some compounds dissolve in water but do not conduct electricity. They are called nonelectrolytes. Examples include: sugar ethanol ethylene glycol © 2006 Brooks/Cole - Thomson  Water Solubility of Ionic Compounds 9 If one ion from the “Soluble Compd.” list is present in a compound, the compound is water soluble. © 2006 Brooks/Cole - Thomson  10 Water Solubility of Ionic Compounds Common minerals are often formed with anions that lead to insolubility: sulfide fluoride carbonate oxide Iron pyrite, a sulfide Orpiment, arsenic sulfide Azurite, a copper carbonate © 2006 Brooks/Cole - Thomson  ACIDS 11 An acid -------> H+ in water Some strong acids are HCl hydrochloric H2SO4 sulfuric HClO4 perchloric HNO3 HNO3 nitric © 2006 Brooks/Cole - Thomson  ACIDS 12 An acid -------> H+ in water HCl(aq) H+(aq) + Cl-(aq) © 2006 Brooks/Cole - Thomson  13 The Nature of Acids HCl Cl- H 2O H 3O+ hydronium ion © 2006 Brooks/Cole - Thomson  14 Weak Acids WEAK ACIDS = weak electrolytes CH3CO2H acetic acid H2CO3 Acetic acid carbonic acid H3PO4 phosphoric acid HF hydrofluoric acid © 2006 Brooks/Cole - Thomson  ACIDS 15 Nonmetal oxides can be acids CO2(aq) + H2O(liq) H2CO3(aq) SO3(aq) + H2O(liq) H2SO4(aq) and can come from burning coal and oil. © 2006 Brooks/Cole - Thomson  BASES 16 see Screen 5.9 and Table 5.2 Base ---> OH- in water NaOH(aq) Na+(aq) + OH-(aq) NaOH is a strong base © 2006 Brooks/Cole - Thomson  17 Ammonia, NH3 An Important Base © 2006 Brooks/Cole - Thomson  18 BASES Metal oxides are bases CaO(s) + H2O(liq) Ca(OH)2(aq) CaO in water. Indicator shows solution is basic. © 2006 Brooks/Cole - Thomson  19 Know the strong acids & bases! © 2006 Brooks/Cole - Thomson  20 Net Ionic Equations Mg(s) + 2 HCl(aq) H2(g) + MgCl2(aq) We really should write Mg(s) + 2 H+(aq) + 2 Cl-(aq) ---> H2(g) + Mg2+(aq) + 2 Cl-(aq) The two Cl- ions are SPECTATOR IONS — they do not participate. Could have used NO3-. © 2006 Brooks/Cole - Thomson  Net Ionic Equations 21 Mg(s) + 2 HCl(aq) H2(g) + MgCl2(aq) Mg(s) + 2 H+(aq) + 2 Cl-(aq) H2(g) + Mg2+(aq) + 2 Cl-(aq) We leave the spectator ions out — Mg(s) + 2 H+(aq) ---> H2(g) + Mg2+(aq) to give the NET IONIC EQUATION © 2006 Brooks/Cole - Thomson  22 Chemical Reactions in Water Sections 5.2 & 5.4-5.6—CD-ROM Ch. 5 We will look at Pb(NO3) 2(aq) + 2 KI(aq) EXCHANGE ----> PbI2(s) + 2 KNO3 (aq) REACTIONS AX + B Y AY + B X The anions exchange places between cations. © 2006 Brooks/Cole - Thomson  23 Precipitation Reactions The “driving force” is the formation of an insoluble compound — a precipitate. Pb(NO3)2(aq) + 2 KI(aq) 2 KNO3(aq) + PbI2(s) Net ionic equation Pb2+(aq) + 2 I-(aq) PbI2(s) © 2006 Brooks/Cole - Thomson  24 Acid-Base Reactions The “driving force” is the formation of water. NaOH(aq) + HCl(aq) NaCl(aq) + H2O(liq) Net ionic equation OH-(aq) + H+(aq) H2O(liq) This applies to ALL reactions of STRONG acids and bases. © 2006 Brooks/Cole - Thomson  Acid-Base Reactions 25 CCR, page 191 © 2006 Brooks/Cole - Thomson  Acid-Base Reactions 26 A-B reactions are sometimes called NEUTRALIZATIONS because the solution is neither acidic nor basic at the end. The other product of the A-B reaction is a SALT, MX. HX + MOH MX + H2O Mn+ comes from base & Xn- comes from acid This is one way to make compounds! © 2006 Brooks/Cole - Thomson  27 Gas-Forming Reactions This is primarily the chemistry of metal carbonates. CO2 and water H2CO3 H2CO3(aq) + Ca2+ 2 H+(aq) + CaCO3(s) (limestone) Adding acid reverses this reaction. MCO3 + acid CO2 + salt © 2006 Brooks/Cole - Thomson  28 Gas-Forming Reactions CaCO3(s) + H2SO4(aq) 2 CaSO4(s) + H2CO3(aq) Carbonic acid is unstable and forms CO2 & H2O H2CO3(aq) CO2 + water (Antacid tablet has citric acid + NaHCO3) © 2006 Brooks/Cole - Thomson  29 © 2006 Brooks/Cole - Thomson  30 Quantitative Aspects of Reactions in Solution Sections 5.8-5.10 © 2006 Brooks/Cole - Thomson  Terminology 31 In solution we need to define the SOLVENT the component whose physical state is preserved when solution forms SOLUTE the other solution component © 2006 Brooks/Cole - Thomson  32 Concentration of Solute The amount of solute in a solution is given by its concentration. moles solute Molarity(M) = liters of solution Concentration (M) = [ …] © 2006 Brooks/Cole - Thomson  33 1.0 L of water was used to make 1.0 L of solution. Notice the water left over. CCR, page 206 © 2006 Brooks/Cole - Thomson  34 Preparing a Solution Active Figure 5.18 © 2006 Brooks/Cole - Thomson  35 PROBLEM: Dissolve 5.00 g of NiCl2 6 H2O in enough water to make 250 mL of solution. Calculate molarity. © 2006 Brooks/Cole - Thomson  The Nature of a CuCl2 Solution: 36 Ion Concentrations CuCl2(aq) Cu2+(aq) + 2 Cl-(aq) If [CuCl2] = 0.30 M, then [Cu2+] = 0.30 M [Cl-] = 2 x 0.30 M © 2006 Brooks/Cole - Thomson  37 USING MOLARITY What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500 M solution? Because Conc (M) = moles/volume = mol/V this means that moles = M V © 2006 Brooks/Cole - Thomson  USING MOLARITY 38 What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500 M solution? moles = M V © 2006 Brooks/Cole - Thomson  39 Preparing Solutions Weigh out a solid solute and dissolve in a given quantity of solvent. Dilute a concentrated solution to give one that is less concentrated. © 2006 Brooks/Cole - Thomson  40 Preparing a Solution by Dilution © 2006 Brooks/Cole - Thomson  41 PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Add water to the 3.0 M solution to lower its concentration to 0.50 M Dilute the solution! © 2006 Brooks/Cole - Thomson  42 PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? But how much water do we add? © 2006 Brooks/Cole - Thomson  43 PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? How much water is added? The important point is that moles of NaOH in ORIGINAL solution = moles of NaOH in FINAL solution © 2006 Brooks/Cole - Thomson  44 PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? © 2006 Brooks/Cole - Thomson  45 PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Conclusion: add 250 mL of water to 50.0 mL of 3.0 M NaOH to make 300 mL of 0.50 M NaOH. © 2006 Brooks/Cole - Thomson  46 Preparing Solutions by Dilution A shortcut Cinitial Vinitial = Cfinal Vfinal © 2006 Brooks/Cole - Thomson  47 SOLUTION STOICHIOMETRY Section 5.10 Zinc reacts with acids to produce H2 gas. Have 10.0 g of Zn What volume of 2.50 M HCl is needed to convert the Zn completely? © 2006 Brooks/Cole - Thomson  GENERAL PLAN FOR STOICHIOMETRY 48 CALCULATIONS Mass Mass zinc HCl Stoichiometric Moles factor Moles zinc HCl Volume HCl © 2006 Brooks/Cole - Thomson  49 Zinc reacts with acids to produce H2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? © 2006 Brooks/Cole - Thomson  pH, a Concentration Scale 50 pH: a way to express acidity -- the concentration of H+ in solution. Low pH: high [H+] High pH: low [H+] Acidic solution pH < 7 Neutral pH = 7 Basic solution pH > 7 © 2006 Brooks/Cole - Thomson  51 The pH Scale pH = log (1/ [H+]) = - log [H+] In a neutral solution, [H+] = [OH-] = 1.00 x 10-7 M at 25 oC pH = - log [H+] = -log (1.00 x 10-7) = - [0 + (-7)] = 7 See CD Screen 5.17 for a tutorial See book Appendix A.3 for more on logs © 2006 Brooks/Cole - Thomson  52 [H+] and pH If the [H+] of soda is 1.6 x 10-3 M, the pH is ____? Because pH = - log [H+] then pH= - log (1.6 x 10-3) pH = -{log (1.6) + log (10-3)} pH = -{0.20 - 3.00) pH = 2.80 © 2006 Brooks/Cole - Thomson  53 pH and [H+] If the pH of Coke is 3.12, it is ____________. Because pH = - log [H+] then log [H+] = - pH Take antilog and get [H+] = 10-pH [H+] = 10-3.12 = 7.6 x 10-4 M © 2006 Brooks/Cole - Thomson  54 ACID-BASE REACTIONS Titrations H2C2O4(aq) + 2 NaOH(aq) acid base Na2C2O4(aq) + 2 H2O(liq) Carry out this reaction using a TITRATION. Oxalic acid, H2C2O4 © 2006 Brooks/Cole - Thomson  Setup for titrating an acid with a base 55 Active Figure 5.23 © 2006 Brooks/Cole - Thomson  56 1. Add solution from the Titration buret. 2. Reagent (base) reacts with compound (acid) in solution in the flask. 3. Indicator shows when exact stoichiometric reaction has occurred. 4. Net ionic equation H+ + OH- H2O 5. At equivalence point moles H+ = moles OH- © 2006 Brooks/Cole - Thomson  57 LAB PROBLEM #1: Standardize a solution of NaOH — i.e., accurately determine its concentration. 1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? © 2006 Brooks/Cole - Thomson  58 1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? Step 1: Calculate amount of H2C2O4 Step 2: Calculate amount of NaOH req’d Step 3: Calculate concentration of NaOH © 2006 Brooks/Cole - Thomson  59 LAB PROBLEM #2: Use standardized NaOH to determine the amount of an acid in an unknown. Apples contain malic acid, C4H6O5. C4H6O5(aq) + 2 NaOH(aq) Na2C4H4O5(aq) + 2 H2O(liq) 76.80 g of apple requires 34.56 mL of the standardized NaOH for titration. What is weight % of malic acid? © 2006 Brooks/Cole - Thomson  60 76.80 g of apple requires 34.56 mL of 0.663 M NaOH for titration. What is weight % of malic acid? Step 1: Calculate amount of NaOH used. Step 2: Calculate amount of acid titrated. Step 3: Calculate mass of acid titrated. Step 4: Calculate % malic acid. © 2006 Brooks/Cole - Thomson  61 Oxidation-Reduction Reactions Section 5.7 Thermite reaction Fe2O3(s) + 2 Al(s) 2 Fe(s) + Al2O3(s) © 2006 Brooks/Cole - Thomson  62 EXCHANGE: Precipitation Reactions EXCHANGE EXCHANGE Gas-Forming REACTIONS Acid-Base Reactions Reactions REDOX REACTIONS © 2006 Brooks/Cole - Thomson  63 REDOX REACTIONS REDOX = reduction & oxidation 2 H2(g) + O2(g) 2 H2O(liq) © 2006 Brooks/Cole - Thomson  64 REDOX REACTIONS REDOX = reduction & oxidation Corrosion of aluminum 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) © 2006 Brooks/Cole - Thomson  65 REDOX REACTIONS Cu(s) + 2 Ag+(aq) Cu2+(aq) + 2 Ag(s) In all reactions if something has been oxidized then something has also been reduced © 2006 Brooks/Cole - Thomson  66 REDOX REACTIONS Cu(s) + 2 Ag+(aq) Cu2+(aq) + 2 Ag(s) © 2006 Brooks/Cole - Thomson  67 Why Study Redox Reactions Batteries Corrosion Manufacturing metals Fuels © 2006 Brooks/Cole - Thomson  68 REDOX REACTIONS Redox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor. Transfer leads to— 1. increase in oxidation number of some element = OXIDATION 2. decrease in oxidation number of some element = REDUCTION © 2006 Brooks/Cole - Thomson  69 OXIDATION NUMBERS The electric charge an element APPEARS to have when electrons are counted by some arbitrary rules: 1. Each atom in free element has ox. no. = 0. Zn O2 I2 S8 2. In simple ions, ox. no. = charge on ion. -1 for Cl- +2 for Mg2+ © 2006 Brooks/Cole - Thomson  70 OXIDATION NUMBERS 3. O has ox. no. = -2 (except in peroxides: in H2O2, O = -1) 4. Ox. no. of H = +1 (except when H is associated with a metal as in NaH where it is -1) 5. Algebraic sum of oxidation numbers = 0 for a compound = overall charge for an ion © 2006 Brooks/Cole - Thomson  71 OXIDATION NUMBERS NH3 N = Oxidation ClO- Cl = number of F in HF? H3PO4 P = MnO4- Mn = Cr2O72- Cr = C3H8 C = © 2006 Brooks/Cole - Thomson  72 Recognizing a Redox Reaction Corrosion of aluminum 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) Al(s) Al3+(aq) + 3 e- Ox. no. of Al increases as e- are donated by the metal. Therefore, Al is OXIDIZED Al is the REDUCING AGENT in this balanced half- reaction. © 2006 Brooks/Cole - Thomson  73 Recognizing a Redox Reaction Corrosion of aluminum 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) Cu2+(aq) + 2 e- Cu(s) Ox. no. of Cu decreases as e- are accepted by the ion. Therefore, Cu is REDUCED Cu is the OXIDIZING AGENT in this balanced half- reaction. © 2006 Brooks/Cole - Thomson  Recognizing a Redox Reaction 74 Notice that the 2 half-reactions add up to give the overall reaction —if we use 2 mol of Al and 3 mol of Cu2+. 2 Al(s) 2 Al3+(aq) + 6 e- 3 Cu2+(aq) + 6 e- 3 Cu(s) ----------------------------------------------------------- 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) Final eqn. is balanced for mass and charge. © 2006 Brooks/Cole - Thomson  75 Examples of Redox Reactions Metal + halogen 2 Al + 3 Br2 ---> Al2Br6 © 2006 Brooks/Cole - Thomson  76 Examples of Redox Reactions Nonmetal (P) + Oxygen P4O10 Metal (Mg) + Oxygen MgO © 2006 Brooks/Cole - Thomson  77 Recognizing a Redox Reaction See Table 5.4 Reaction Type Oxidation Reduction In terms of oxygen gain loss In terms of halogen gain loss In terms of electrons loss gain © 2006 Brooks/Cole - Thomson  Common Oxidizing and 78 Reducing Agents See Table 5.4 Metals (Cu) are Metals reducing (Na, K, agents Mg, Fe) HNO3 is an are oxidizing reducing agent agents Cu + HNO3 --> 2 K + 2 H2O --> Cu2+ + NO2 2 KOH + H2 © 2006 Brooks/Cole - Thomson  79 Examples of Redox Reactions Metal + acid Mg + HCl Mg = reducing agent H+ = oxidizing agent Metal + acid Cu + HNO3 Cu = reducing agent HNO3 = oxidizing agent © 2006 Brooks/Cole - Thomson