1 © 2006 Brooks/Cole - Thomson Chemistry and Chemical Reactivity 6th Edition John C. Kotz Paul M. Treichel Gabriela C. Weaver CHAPTER 5 Reactions in Aqueous Solution © 2006 Brooks/Cole Thomson Lectures written by John Kotz

2 © 2006 Brooks/Cole - Thomson Reactions in Aqueous Solution Many reactions involve ionic compounds, especially reactions in water — aqueous solutions. KMnO 4 in water K + (aq) + MnO 4 - (aq)

3 © 2006 Brooks/Cole - Thomson An Ionic Compound, CuCl 2, in Water CCR, page 177

4 © 2006 Brooks/Cole - Thomson How do we know ions are present in aqueous solutions? The solutions conduct electricity! They are called ELECTROLYTES HCl, CuCl 2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions. Aqueous Solutions

5 © 2006 Brooks/Cole - Thomson HCl, CuCl 2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions. Aqueous Solutions

6 © 2006 Brooks/Cole - Thomson Aqueous Solutions Acetic acid ionizes only to a small extent, so it is a weak electrolyte. CH 3 CO 2 H(aq) ---> CH 3 CO 2 - (aq) + H + (aq)

7 © 2006 Brooks/Cole - Thomson Aqueous Solutions Some compounds dissolve in water but do not conduct electricity. They are called nonelectrolytes. Examples include: sugarethanol ethylene glycol Examples include: sugarethanol ethylene glycol

8 © 2006 Brooks/Cole - Thomson Water Solubility of Ionic Compounds If one ion from the “Soluble Compd.” list is present in a compound, the compound is water soluble.

9 © 2006 Brooks/Cole - Thomson Solubility: Which of the following is the only INSOLUBLE salt in water?

10 © 2006 Brooks/Cole - Thomson An acid > H + in water ACIDSACIDS Some strong acids are HClhydrochloric H 2 SO 4 sulfuric HClO 4 perchloric HNO 3 nitric HNO 3

11 © 2006 Brooks/Cole - Thomson An acid > H + in water ACIDSACIDS HCl(aq) ---> H + (aq) + Cl - (aq)

12 © 2006 Brooks/Cole - Thomson The Nature of Acids HCl H 2 O Cl - H 3 O + hydronium ion

13 © 2006 Brooks/Cole - Thomson Weak Acids WEAK ACIDS = weak electrolytes CH 3 CO 2 H acetic acid H 2 CO 3 carbonic acid H 3 PO 4 phosphoric acid HF hydrofluoric acid Acetic acid

14 © 2006 Brooks/Cole - Thomson ACIDSACIDS Nonmetal oxides can be acids CO 2 (aq) + H 2 O(liq) ---> H 2 CO 3 (aq) SO 3 (aq) + H 2 O(liq) ---> H 2 SO 4 (aq) and can come from burning coal and oil.

15 © 2006 Brooks/Cole - Thomson Base ---> OH - in water BASES see Screen 5.9 and Table 5.2 NaOH(aq) ---> Na + (aq) + OH - (aq) NaOH is a strong base

16 © 2006 Brooks/Cole - Thomson Ammonia, NH 3 Ammonia, NH 3 An Important Base

17 © 2006 Brooks/Cole - Thomson BASESBASES Metal oxides are bases CaO(s) + H 2 O(liq) --> Ca(OH) 2 (aq) --> Ca(OH) 2 (aq) CaO in water. Indicator shows solution is basic.

18 © 2006 Brooks/Cole - Thomson Know the strong acids & bases!

19 © 2006 Brooks/Cole - Thomson Which compound below is an acid in aqueous solution?

20 © 2006 Brooks/Cole - Thomson Which compound below is a base in aqueous solution?

21 © 2006 Brooks/Cole - Thomson Net Ionic Equations Mg(s) + 2 HCl(aq) --> H 2 (g) + MgCl 2 (aq) We really should write Mg(s) + 2 H + (aq) + 2 Cl - (aq) ---> H 2 (g) + Mg 2+ (aq) + 2 Cl - (aq) The two Cl - ions are SPECTATOR IONS — they do not participate. Could have used NO 3 -.

22 © 2006 Brooks/Cole - Thomson Net Ionic Equations Mg(s) + 2 HCl(aq) --> H 2 (g) + MgCl 2 (aq) Mg(s) + 2 H + (aq) + 2 Cl - (aq) ---> H 2 (g) + Mg 2+ (aq) + 2 Cl - (aq) We leave the spectator ions out — Mg(s) + 2 H + (aq) ---> H 2 (g) + Mg 2+ (aq) NET IONIC EQUATION to give the NET IONIC EQUATION

23 © 2006 Brooks/Cole - Thomson Chemical Reactions in Water Sections 5.2 & —CD-ROM Ch. 5 We will look at EXCHANGE REACTIONS Pb(NO 3 ) 2 (aq) + 2 KI(aq) ----> PbI 2 (s) + 2 KNO 3 (aq) The anions exchange places between cations.

24 © 2006 Brooks/Cole - Thomson Precipitation Reactions The “driving force” is the formation of an insoluble compound — a precipitate. Pb(NO 3 ) 2 (aq) + 2 KI(aq) -----> 2 KNO 3 (aq) + PbI 2 (s) 2 KNO 3 (aq) + PbI 2 (s) Net ionic equation Pb 2+ (aq) + 2 I - (aq) ---> PbI 2 (s)

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26 © 2006 Brooks/Cole - Thomson Acid-Base Reactions The “driving force” is the formation of water.The “driving force” is the formation of water. NaOH(aq) + HCl(aq) ---> NaCl(aq) + H 2 O(liq) Net ionic equationNet ionic equation OH - (aq) + H + (aq) ---> H 2 O(liq) OH - (aq) + H + (aq) ---> H 2 O(liq) This applies to ALL reactions of STRONG acids and bases.This applies to ALL reactions of STRONG acids and bases.

27 © 2006 Brooks/Cole - Thomson Acid-Base Reactions CCR, page 191

28 © 2006 Brooks/Cole - Thomson Acid-Base Reactions A-B reactions are sometimes called NEUTRALIZATIONS because the solution is neither acidic nor basic at the end.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.The other product of the A-B reaction is a SALT, MX. HX + MOH ---> MX + H 2 O HX + MOH ---> MX + H 2 O M n+ comes from base & X n- comes from acid M n+ comes from base & X n- comes from acid This is one way to make compounds! This is one way to make compounds!

29 © 2006 Brooks/Cole - Thomson Gas-Forming Reactions This is primarily the chemistry of metal carbonates. CO 2 and water ---> H 2 CO 3 H 2 CO 3 (aq) + Ca > H 2 CO 3 (aq) + Ca > 2 H + (aq) + CaCO 3 (s) (limestone) 2 H + (aq) + CaCO 3 (s) (limestone) Adding acid reverses this reaction. MCO 3 + acid ---> CO 2 + salt

30 © 2006 Brooks/Cole - Thomson Gas-Forming Reactions CaCO 3 (s) + H 2 SO 4 (aq) ---> 2 CaSO 4 (s) + H 2 CO 3 (aq) 2 CaSO 4 (s) + H 2 CO 3 (aq) Carbonic acid is unstable and forms CO 2 & H 2 O H 2 CO 3 (aq) ---> CO 2 + water (Antacid tablet has citric acid + NaHCO 3 )

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32 © 2006 Brooks/Cole - Thomson Quantitative Aspects of Reactions in Solution Sections

33 © 2006 Brooks/Cole - Thomson TerminologyTerminology In solution we need to define the - SOLVENTSOLVENT the component whose physical state is preserved when solution forms SOLUTESOLUTE the other solution component

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35 © 2006 Brooks/Cole - Thomson I did ____ on the exam. Answer Now 0% 100% 1.great 2.good 3.ok 4.Pretty bad

36 © 2006 Brooks/Cole - Thomson Concentration of Solute The amount of solute in a solution is given by its concentration The amount of solute in a solution is given by its concentration. Concentration (M) = [ …]

37 © 2006 Brooks/Cole - Thomson 1.0 L of water was used to make 1.0 L of solution. Notice the water left over. CCR, page 206

38 © 2006 Brooks/Cole - Thomson Preparing a Solution Active Figure 5.18

39 © 2006 Brooks/Cole - Thomson PROBLEM: Dissolve 5.00 g of NiCl 2 6 H 2 O in enough water to make 250 mL of solution. Calculate molarity. Step 1: Calculate moles of NiCl 2 6H 2 O Step 2: Calculate molarity NiCl 2 6 H 2 O [NiCl 2 6 H 2 O ] = M

40 © 2006 Brooks/Cole - Thomson The Nature of a CuCl 2 Solution: Ion Concentrations CuCl 2 (aq) --> Cu 2+ (aq) + 2 Cl - (aq) If [CuCl 2 ] = 0.30 M, then [Cu 2+ ] = 0.30 M [Cl - ] = 2 x 0.30 M

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42 © 2006 Brooks/Cole - Thomson USING MOLARITY What mass of oxalic acid, H 2 C 2 O 4, is required to make 250. mL of a M solution? Because Conc (M) = moles/volume = mol/V this means that moles = MV

43 © 2006 Brooks/Cole - Thomson Step 1: Calculate moles of acid required. ( mol/L)(0.250 L) = mol Step 2: Calculate mass of acid required. ( mol )(90.00 g/mol) = 1.13 g USING MOLARITY moles = MV What mass of oxalic acid, H 2 C 2 O 4, is required to make 250. mL of a M solution?

44 © 2006 Brooks/Cole - Thomson You have 60.0 mL of 0.25 M HCl. What amount of HCl (moles) is present? mol mol mol mol mol

45 © 2006 Brooks/Cole - Thomson M M M M

46 © 2006 Brooks/Cole - Thomson Preparing Solutions Weigh out a solid solute and dissolve in a given quantity of solvent.Weigh out a solid solute and dissolve in a given quantity of solvent. Dilute a concentrated solution to give one that is less concentrated.Dilute a concentrated solution to give one that is less concentrated.

47 © 2006 Brooks/Cole - Thomson Preparing a Solution by Dilution

48 © 2006 Brooks/Cole - Thomson 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!

49 © 2006 Brooks/Cole - Thomson 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?

50 © 2006 Brooks/Cole - Thomson 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 moles of NaOH in FINAL solution

51 © 2006 Brooks/Cole - Thomson PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Amount of NaOH in original solution = M V = M V = (3.0 mol/L)(0.050 L) = 0.15 mol NaOH Amount of NaOH in final solution must also = 0.15 mol NaOH Volume of final solution = (0.15 mol NaOH)(1 L/0.50 mol) = 0.30 L or 300 mL

52 © 2006 Brooks/Cole - Thomson You add 60.0 mL of 0.25 M HCl to a 500 mL volumetric flask and then add water to the mark on the flask. What is the concentration of HCl in the diluted solution? M M M M M

53 © 2006 Brooks/Cole - Thomson A shortcut A shortcut C initial V initial = C final V final Preparing Solutions by Dilution

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55 © 2006 Brooks/Cole - Thomson Zinc reacts with acids to produce H 2 gas.Zinc reacts with acids to produce H 2 gas. Have 10.0 g of ZnHave 10.0 g of Zn What volume of 2.50 M HCl is needed to convert the Zn completely? What volume of 2.50 M HCl is needed to convert the Zn completely? SOLUTION STOICHIOMETRY Section 5.10

56 © 2006 Brooks/Cole - Thomson GENERAL PLAN FOR STOICHIOMETRY CALCULATIONS Mass zinc Stoichiometric factor Moles zinc Moles HCl Mass HCl Volume HCl

57 © 2006 Brooks/Cole - Thomson Step 1: Write the balanced equation Zn(s) + 2 HCl(aq) --> ZnCl 2 (aq) + H 2 (g) Step 2: Calculate amount of Zn Zinc reacts with acids to produce H 2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? Step 3: Use the stoichiometric factor

58 © 2006 Brooks/Cole - Thomson Step 3: Use the stoichiometric factor Zinc reacts with acids to produce H 2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? Step 4: Calculate volume of HCl req’d

59 © 2006 Brooks/Cole - Thomson ACID-BASE REACTIONS Titrations H 2 C 2 O 4 (aq) + 2 NaOH(aq) ---> acid base acid base Na 2 C 2 O 4 (aq) + 2 H 2 O(liq) Carry out this reaction using a TITRATION. Oxalic acid, H 2 C 2 O 4

60 © 2006 Brooks/Cole - Thomson Setup for titrating an acid with a base Active Figure 5.23

61 © 2006 Brooks/Cole - Thomson TitrationTitration 1. Add solution from the 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 - --> H 2 O H + + OH - --> H 2 O 5. At equivalence point moles H + = moles OH - moles H + = moles OH -

62 © 2006 Brooks/Cole - Thomson g of H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentra-tion of the NaOH? g of H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentra-tion of the NaOH? LAB PROBLEM #1: Standardize a solution of NaOH — i.e., accurately determine its concentration.

63 © 2006 Brooks/Cole - Thomson g of H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? Step 1: Calculate amount of H 2 C 2 O 4 Step 2: Calculate amount of NaOH req’d

64 © 2006 Brooks/Cole - Thomson g of H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? Step 1: Calculate amount of H 2 C 2 O 4 = mol acid = mol acid Step 2: Calculate amount of NaOH req’d = mol NaOH = mol NaOH Step 3: Calculate concentration of NaOH [NaOH] = M

65 © 2006 Brooks/Cole - Thomson LAB PROBLEM #2: Use standardized NaOH to determine the amount of an acid in an unknown. Apples contain malic acid, C 4 H 6 O 5. C 4 H 6 O 5 (aq) + 2 NaOH(aq) ---> Na 2 C 4 H 4 O 5 (aq) + 2 H 2 O(liq) Na 2 C 4 H 4 O 5 (aq) + 2 H 2 O(liq) g of apple requires mL of M NaOH for titration. What is weight % of malic acid?

66 © 2006 Brooks/Cole - Thomson g of apple requires mL of M NaOH for titration. What is weight % of malic acid? Step 1: Calculate amount of NaOH used. C V = (0.663 M)( L) = mol NaOH = mol NaOH Step 2: Calculate amount of acid titrated. = mol acid

67 © 2006 Brooks/Cole - Thomson g of apple requires mL of M NaOH for titration. What is weight % of malic acid? Step 3: Calculate mass of acid titrated. Step 1: Calculate amount of NaOH used. = mol NaOH = mol NaOH Step 2: Calculate amount of acid titrated = mol acid = mol acid

68 © 2006 Brooks/Cole - Thomson g of apple requires mL of M NaOH for titration. What is weight % of malic acid? Step 1: Calculate amount of NaOH used. = mol NaOH = mol NaOH Step 2: Calculate amount of acid titrated = mol acid = mol acid Step 3: Calculate mass of acid titrated. = 1.54 g acid = 1.54 g acid Step 4: Calculate % malic acid.

69 © 2006 Brooks/Cole - Thomson Oxidation-Reduction Reactions Section 5.7 Thermite reaction Fe 2 O 3 (s) + 2 Al(s) ----> ----> 2 Fe(s) + Al 2 O 3 (s)

70 © 2006 Brooks/Cole - Thomson REDOX REACTIONS EXCHANGEAcid-BaseReactionsEXCHANGEGas-FormingReactions EXCHANGE: Precipitation Reactions REACTIONS

71 © 2006 Brooks/Cole - Thomson REDOX REACTIONS REDOX = reduction & oxidation 2 H 2 (g) + O 2 (g) ---> 2 H 2 O(liq)

72 © 2006 Brooks/Cole - Thomson REDOX REACTIONS REDOX = reduction & oxidation Corrosion of aluminum 2 Al(s) + 3 Cu 2+ (aq) ---> 2 Al 3+ (aq) + 3 Cu(s)

73 © 2006 Brooks/Cole - Thomson In all reactions if something has been oxidized then something has also been reduced REDOX REACTIONS Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)

74 © 2006 Brooks/Cole - Thomson Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s) REDOX REACTIONS

75 © 2006 Brooks/Cole - Thomson Why Study Redox Reactions Manufacturing metals FuelsFuels CorrosionCorrosion BatteriesBatteries

76 © 2006 Brooks/Cole - Thomson 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 REDOX REACTIONS

77 © 2006 Brooks/Cole - Thomson 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 O 2 I 2 S 8 Zn O 2 I 2 S 8 2.In simple ions, ox. no. = charge on ion. -1 for Cl - +2 for Mg for Cl - +2 for Mg 2+

78 © 2006 Brooks/Cole - Thomson Group 1 = +1, group 2 = +2 F is –1 H is +1 O is –2 The remaining atoms are determined by algebra. (Neutral species have oxidation numbers that add to zero; charged species add to the expressed charge.)

79 © 2006 Brooks/Cole - Thomson OXIDATION NUMBERS NH 3 N = ClO - Cl = H 3 PO 4 P = MnO 4 - Mn = Cr 2 O 7 2- Cr = C 3 H 8 C = Oxidation number of F in HF?

80 © 2006 Brooks/Cole - Thomson In the demonstration of the reaction of Zn with VO 2 +, the oxidation number of V in VO 2 + is:

81 © 2006 Brooks/Cole - Thomson Recognizing a Redox Reaction Corrosion of aluminum 2 Al(s) + 3 Cu 2+ (aq) --> 2 Al 3+ (aq) + 3 Cu(s) Al(s) --> Al 3+ (aq) + 3 e - Ox. no. of Al increases as e - are donated by the metal.Ox. no. of Al increases as e - are donated by the metal. Therefore, Al is OXIDIZEDTherefore, Al is OXIDIZED Al is the REDUCING AGENT in this balanced half- reaction.Al is the REDUCING AGENT in this balanced half- reaction.

82 © 2006 Brooks/Cole - Thomson Recognizing a Redox Reaction Corrosion of aluminum 2 Al(s) + 3 Cu 2+ (aq) --> 2 Al 3+ (aq) + 3 Cu(s) Cu 2+ (aq) + 2 e - --> Cu(s) Ox. no. of Cu decreases as e - are accepted by the ion.Ox. no. of Cu decreases as e - are accepted by the ion. Therefore, Cu is REDUCEDTherefore, Cu is REDUCED Cu is the OXIDIZING AGENT in this balanced half- reaction.Cu is the OXIDIZING AGENT in this balanced half- reaction.

83 © 2006 Brooks/Cole - Thomson Recognizing a Redox Reaction Notice that the 2 half-reactions add up to give the overall reaction —if we use 2 mol of Al and 3 mol of Cu Al(s) --> 2 Al 3+ (aq) + 6 e - 2 Al(s) --> 2 Al 3+ (aq) + 6 e - 3 Cu 2+ (aq) + 6 e - --> 3 Cu(s) 3 Cu 2+ (aq) + 6 e - --> 3 Cu(s) Al(s) + 3 Cu 2+ (aq) ---> 2 Al 3+ (aq) + 3 Cu(s) Final eqn. is balanced for mass and charge.

84 © 2006 Brooks/Cole - Thomson Examples of Redox Reactions Metal + halogen 2 Al + 3 Br 2 ---> Al 2 Br 6

85 © 2006 Brooks/Cole - Thomson Examples of Redox Reactions Metal (Mg) + Oxygen ---> MgO Nonmetal (P) + Oxygen ---> P 4 O 10

86 © 2006 Brooks/Cole - Thomson Recognizing a Redox Reaction See Table 5.4 In terms of oxygengainloss In terms of halogengainloss In terms of electronslossgain Reaction Type OxidationReduction

87 © 2006 Brooks/Cole - Thomson Common Oxidizing and Reducing Agents See Table 5.4 HNO 3 is an oxidizing agent 2 K + 2 H 2 O --> 2 KOH + H 2 Metals (Na, K, Mg, Fe) are reducing agents Metals (Cu) are reducing agents Cu + HNO 3 --> Cu 2+ + NO 2

88 © 2006 Brooks/Cole - Thomson Examples of Redox Reactions Metal + acid Mg + HCl Mg = reducing agent H + = oxidizing agent Metal + acid Cu + HNO 3 Cu = reducing agent HNO 3 = oxidizing agent

89 © 2006 Brooks/Cole - Thomson Copper metal reacts with nitric acid (HNO 3 ) to give Cu 2+ ions and NO 2 gas.

90 © 2006 Brooks/Cole - Thomson 1.an oxidation-reduction reaction. 2.a precipitation reaction. 3.an acid-base reaction.

91 © 2006 Brooks/Cole - Thomson 1.an oxidation-reduction reaction. 2.a precipitation reaction. 3.an acid-base reaction.

92 © 2006 Brooks/Cole - Thomson 1.an oxidation-reduction reaction. 2.a precipitation reaction. 3.an acid-base reaction.

93 © 2006 Brooks/Cole - Thomson Please make your selection... 1.Choice One 2.Choice Two 3.Choice Three 4.Choice Four Answer Now 0% 100%