Chapter 1: Matter and Measurement

Chapter 1: Matter and Measurement

General Chemistry Principles and Modern Applications Petrucci Harwood Herring 8th Edition Chapter 5: Introduction to Reactions in Aqueous Solutions Philip Dutton University of Windsor, Canada Prentice-Hall 2002 Slide 1 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

Contents 5-1 5-2 5-3 5-4 5-5 5-6 5-7 The Nature of Aqueous Solutions Precipitation Reactions Acid-Base Reactions Oxidation-Reduction: Some General Principles Balancing Oxidation-Reduction Equations Oxidizing and Reducing Agents Stoichiometry of Reactions in Aqueous Solutions: Titrations

Focus on Water Treatment Slide 2 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 5.1 The Nature of Aqueous Solutions Slide 3 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Electrolytes

Some solutes can dissociate into ions. Electric charge can be carried. Slide 4 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Types of Electrolytes Strong electrolyte dissociates completely. Good electrical conduction.

Weak electrolyte partially dissociates. Fair conductor of electricity. Non-electrolyte does not dissociate. Poor conductor of electricity. Slide 5 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Representation of Electrolytes using Chemical Equations A strong electrolyte: MgCl2(s) Mg2+(aq) + 2 Cl-(aq) A weak electrolyte:

CH3CO2H(aq) CH3CO2-(aq) + H+(aq) A non-electrolyte: CH3OH(aq) Slide 6 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Notation for Concentration MgCl2(s) Mg2+(aq) + 2 Cl-(aq) In 0.0050 M MgCl2: Stoichiometry is important.

[Mg2+] = 0.0050 M [Cl-] = 0.0100 M Slide 7 of 43 General Chemistry: Chapter 5 [MgCl2] = 0 M Prentice-Hall 2002 Example 5-1 Calculating Ion concentrations in a Solution of a Strong Electolyte. What are the aluminum and sulfate ion concentrations in 0.0165 M Al2(SO4)3?. Balanced Chemical Equation:

Al2(SO4)3 (s) 2 Al3+(aq) + Slide 8 of 43 General Chemistry: Chapter 5 3 SO42-(aq) Prentice-Hall 2002 Example 5-1 Aluminum Concentration: 0.0165 mol Al2(SO4)3 2 mol Al3+ [Al] = 1L

1 mol Al2(SO4)3 = 0.0330 M Al3+ Sulfate Concentration: 0.0165 mol Al2(SO4)3 3 mol SO42[SO4 ] = 1L 1 mol Al2(SO4)3 2- Slide 9 of 43 General Chemistry: Chapter 5 = 0.0495 M SO42-

Prentice-Hall 2002 5-2 Precipitation Reactions Soluble ions can combine to form an insoluble compound. Precipitation occurs. Ag+(aq) + Cl-(aq) AgCl(s) Slide 10 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

Net Ionic Equation Overall Precipitation Reaction: AgNO3(aq) +NaI (aq) AgI(s) + NaNO3(aq) Complete ionic equation: Spectator ions Ag+(aq) + NO3-(aq) + Na+(aq) + I-(aq) AgI(s) + Na+(aq) + NO3-(aq) Net ionic equation: Ag+(aq) + I-(aq) AgI(s) Slide 11 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

Solubility Rules Compounds that are soluble: Alkali metal ion and ammonium ion salts Li+, Na+, K+, Rb+, Cs+ NH4+ Nitrates, perchlorates and acetates NO3- Slide 12 of 43 ClO4- CH3CO2-

General Chemistry: Chapter 5 Prentice-Hall 2002 Solubility Rules Compounds that are mostly soluble: Chlorides, bromides and iodides Cl-, Br-, I- Except those of Pb2+, Ag+, and Hg22+. Sulfates SO42- Except those of Sr2+, Ba2+, Pb2+ and Hg22+.

Ca(SO4) is slightly soluble. Slide 13 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Solubility Rules Compounds that are insoluble: Hydroxides and sulfides HO-, S2- Except alkali metal and ammonium salts Sulfides of alkaline earths are soluble Hydroxides of Sr2+ and Ca2+ are slightly soluble.

Carbonates and phosphates CO32-, PO43- Except alkali metal and ammonium salts Slide 14 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 5-3 Acid-Base Reactions Latin acidus (sour) Sour taste

Arabic al-qali (ashes of certain plants) Bitter taste Svante Arrhenius 1884 Acid-Base theory. Slide 15 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Acids Acids provide H+ in aqueous solution. Strong acids: HCl(aq)

H+(aq) + Cl-(aq) H+(aq) + CH3CO2-(aq) Weak acids: CH3CO2H(aq) Slide 16 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

Bases Bases provide OH- in aqueous solution. Strong bases: NaOH(aq) Na+(aq) + OH-(aq) H2O Weak bases: NH3(aq) + H2O(l) Slide 17 of 43 OH-(aq) + NH4+(aq)

General Chemistry: Chapter 5 Prentice-Hall 2002 Recognizing Acids and Bases. Acids have ionizable hydrogen ions. CH3CO2H or HC2H3O2 Bases have OH- combined with a metal ion. KOH or are identified by chemical equations Na2CO3(s) + H2O(l) HCO3-(aq) + 2 Na+(aq) + OH-(aq) Slide 18 of 43

General Chemistry: Chapter 5 Prentice-Hall 2002 More Acid-Base Reactions Milk of magnesia Mg(OH)2 Mg(OH)2(s) + 2 H+(aq) Mg2+(aq) + 2 H2O(l) Mg(OH)2(s) + 2 CH3CO2H(aq) Mg2+(aq) + 2 CH3CO2-(aq) + 2 H2O(l) Slide 19 of 43 General Chemistry: Chapter 5

Prentice-Hall 2002 More Acid-Base Reactions Limestone and marble. CaCO3(s) + 2 H+(aq) Ca2+(aq) + H2CO3(aq) But: H2CO3(aq) H2O(l) + CO2(g) CaCO3(s) + 2 H+(aq) Ca2+(aq) + H2O(l) + CO2(g) Slide 20 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Limestone and Marble Slide 21 of 43

General Chemistry: Chapter 5 Prentice-Hall 2002 Gas Forming Reactions Slide 22 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 5-4 Oxidation-Reduction: Some General Principles Hematite is converted to iron in a blast furnace.

Fe2O3(s) + 3 CO(g) 2 Fe(l) + 3 CO2(g) Oxidation and reduction always occur together. Fe3+ is reduced to metallic iron. CO(g) is oxidized to carbon dioxide. Slide 23 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Oxidation State Changes Assign oxidation states: 3+ 2-

0 2+ 2- 4+ 2- Fe2O3(s) + 3 CO(g) 2 Fe(l) + 3 CO2(g) Fe3+ is reduced to metallic iron. CO(g) is oxidized to carbon dioxide. Slide 24 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

Oxidation and Reduction Oxidation O.S. of some element increases in the reaction. Electrons are on the right of the equation Reduction O.S. of some element decreases in the reaction. Electrons are on the left of the equation. Slide 25 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Zinc in Copper Sulfate

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) Slide 26 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Half-Reactions Represent a reaction by two half-reactions. Oxidation: Zn(s) Zn2+(aq) + 2 e- Reduction: Cu2+(aq) + 2 e- Cu(s)

Overall: Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq) Slide 27 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Balancing Oxidation-Reduction Equations Few can be balanced by inspection. Systematic approach required. The Half-Reaction (Ion-Electron) Method Slide 28 of 43

General Chemistry: Chapter 5 Prentice-Hall 2002 Example 5-6 Balancing the Equation for a Redox Reaction in Acidic Solution. The reaction described below is used to determine the sulfite ion concentration present in wastewater from a papermaking plant. Write the balanced equation for this reaction in acidic solution. . SO32-(aq) + MnO4-(aq) SO42-(aq) + Mn2+(aq) Slide 29 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

Example 5-6 Determine the oxidation states: 4+ 6+ 7+ 2+ SO32-(aq) + MnO4-(aq) SO42-(aq) + Mn2+(aq) Write the half-reactions: SO32-(aq) SO42-(aq) + 2 e-(aq) 5 e-(aq) +MnO4-(aq) Mn2+(aq) Balance atoms other than H and O: Already balanced for elements. Slide 30 of 43 General Chemistry: Chapter 5

Prentice-Hall 2002 Example 5-6 Balance O by adding H2O: H2O(l) + SO32-(aq) SO42-(aq) + 2 e-(aq) 5 e-(aq) +MnO4-(aq) Mn2+(aq) + 4 H2O(l) Balance hydrogen by adding H+: H2O(l) + SO32-(aq) SO42-(aq) + 2 e-(aq) + 2 H+(aq) 8 H+(aq) + 5 e-(aq) +MnO4-(aq) Mn2+(aq) + 4 H2O(l) Check that the charges are balanced: Slide 31 of 43 General Chemistry: Chapter 5 Add e- if necessary. Prentice-Hall 2002

Example 5-6 Multiply the half-reactions to balance all e-: 5 H2O(l) + 5 SO32-(aq) 5 SO42-(aq) + 10 e-(aq) + 10 H+(aq) 16 H+(aq) + 10 e-(aq) + 2 MnO4-(aq) 2 Mn2+(aq) + 8 H2O(l) Add both equations and simplify: 5 SO32-(aq) + 2 MnO4-(aq) + 6H+(aq) 5 SO42-(aq) + 2 Mn2+(aq) + 3 H2O(l) Check the balance! Slide 32 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Balancing in Acid

Write the equations for the half-reactions. Balance all atoms except H and O. Balance oxygen using H2O. Balance hydrogen using H+. Balance charge using e-. Equalize the number of electrons. Add the half reactions. Check the balance. Slide 33 of 43

General Chemistry: Chapter 5 Prentice-Hall 2002 Balancing in Basic Solution OH- appears instead of H+. Treat the equation as if it were in acid. Then add OH- to each side to neutralize H+. Remove H2O appearing on both sides of equation. Check the balance. Slide 34 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

5-6 Oxidizing and Reducing Agents. An oxidizing agent (oxidant ): Contains an element whose oxidation state decreases in a redox reaction A reducing agent (reductant): Contains an element whose oxidation state increases in a redox reaction. Slide 35 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Redox

Slide 36 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Example 5-8 Identifying Oxidizing and Reducing Agents. Hydrogen peroxide, H2O2, is a versatile chemical. Its uses include bleaching wood pulp and fabrics and substituting for chlorine in water purification. One reason for its versatility is that it can be either an oxidizing or a reducing agent. For the following reactions, identify whether hydrogen peroxide is an oxidizing or reducing agent. Slide 37 of 43

General Chemistry: Chapter 5 Prentice-Hall 2002 Example 5-8 H2O2(aq) + 2 Fe2+(aq) + 2 H+ 2 H2O(l) + 2 Fe3+(aq) Iron is oxidized and peroxide is reduced. 5 H2O2(aq) + 2 MnO4-(aq) + 6 H+ 8 H2O(l) + 2 Mn2+(aq) + 5 O2(g) Manganese is reduced and peroxide is oxidized. Slide 38 of 43 General Chemistry: Chapter 5

Prentice-Hall 2002 5-7 Stoichiometry of Reactions in Aqueous Solutions: Titrations. Titration Carefully controlled addition of one solution to another. Equivalence Point Both reactants have reacted completely. Indicators Substances which change colour near an equivalence point. Slide 39 of 43

General Chemistry: Chapter 5 Prentice-Hall 2002 Indicators Slide 40 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Example 5-10 Standardizing a Solution for Use in Redox Titrations. A piece of iron wire weighing 0.1568 g is converted to Fe2+(aq) and requires 26.42 mL of a KMnO4(aq) solution for its titration. What is the molarity of the KMnO4(aq)?

5 Fe2+(aq) + MnO4-(aq) + 8 H+(aq) 4 H2O(l) + 5 Fe3+(aq) + Mn2+(aq) Slide 41 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Example 5-10 5 Fe2+(aq) + MnO4-(aq) + 8 H+(aq) 4 H2O(l) + 5 Fe3+(aq) + Mn2+(aq) Determine KMnO4 consumed in the reaction: nH 2O 1 mol Fe

1 mol Fe 2 0.1568 g Fe 55.847 g Fe 1 mol Fe 1 mol MnO4 1 mol KMnO4 4 5 . 615 10 mol KMnO4

2 5 mol Fe 1 mol MnO4 Determine the concentration: 5.615 10 4 mol KMnO4 [ KMnO4 ] 0.02140 M KMnO4 0.02624 L Slide 42 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002 Chapter 5 Questions

1, 2, 3, 5, 6, 8, 14, 17, 19, 24, 27, 33, 37, 41, 43, 51, 53, 59, 68, 71, 82, 96. Slide 43 of 43 General Chemistry: Chapter 5 Prentice-Hall 2002

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