Reactions in Aqueous Solution Chapter 4

A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount Solution Solvent Solute Soft drink (l) H2O Sugar, CO2 Air (g) N2 O2, Ar, CH4 Soft Solder (s) Pb Sn 4.1

An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity. A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity. nonelectrolyte weak electrolyte strong electrolyte 4.1

Ionic compounds that dissolve in water always produce solutions that are strong electrolytes. Molecular compounds that dissolve in water may produce nonelectrolytes, weak electrolytes (weak acids or bases), or strong electrolytes (strong acids).

Conduct electricity in solution? Cations (+) and Anions (-) Strong Electrolyte – 100% dissociation NaCl (s) Na+ (aq) + Cl- (aq) H2O Weak Electrolyte – not completely dissociated CH3COOH CH3COO- (aq) + H+ (aq) Reversible reaction - Chemical equilibrium reversible

Nonelectrolyte does not conduct electricity? No cations (+) and anions (-) in solution C6H12O6 (s) C6H12O6 (aq) H2O Strong Electrolyte Weak Electrolyte Nonelectrolyte HCl CH3COOH (NH2)2CO HNO3 HF CH3OH HClO4 HNO2 C2H5OH NaOH H2O C12H22O11 All Ionic Compounds 4.1

Water’s power as an ionizing solvent results from the distribution of its electrons and its overall shape. A polar molecule is one that has a + end and a - end. Water is one of the most polar molecules.

Hydration is the process in which an ion is surrounded by water molecules arranged in a specific manner. d+ d- H2O

The Solubility of Ionic Compounds in Water The solubility of ionic compounds in water depends upon the relative strengths of the electrical forces between ions in the ionic compound and the attractive forces between the ions and water molecules in the solvent. There is a tremendous range in the solubility of ionic compounds in water! The solubility of so called “insoluble” compounds may be several orders of magnitude less than ones that are called “soluble” in water, for example: Solubility of NaCl in water at 20oC = 365 g/L Solubility of MgCl2 in water at 20oC = 542.5 g/L Solubility of AlCl3 in water at 20oC = 699 g/L Solubility of PbCl2 in water at 20oC = 9.9 g/L Solubility of AgCl in water at 20oC = 0.009 g/L Solubility of CuCl in water at 20oC = 0.0062 g/L

Problems from Chapter 4 Pages 123-127 1-4, 6-12, 14-18, 20-23, 25-36, 40, 42, 46, 48, 50, 54, 56, 58, 60-62, 64-66, 72, 74, 80, 81, 92, 94

Precipitation Reactions Precipitate – insoluble solid that separates from solution PbI2 precipitate Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq) molecular equation Pb2+ + 2NO3- + 2Na+ + 2I- PbI2 (s) + 2Na+ + 2NO3- ionic equation Pb2+ + 2I- PbI2 (s) net ionic equation Na+ and NO3- are spectator ions 4.2

Writing Net Ionic Equations Write the balanced molecular equation. Determine precipitate from solubility rules Write the ionic equation showing the soluble strong electrolytes as ions. Cancel the spectator ions on both sides of the ionic equation Write the net ionic equation for the reaction of silver nitrate solution with sodium chloride solution. AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq) Ag+ + NO3- + Na+ + Cl- AgCl (s) + Na+ + NO3- Ag+ + Cl- AgCl (s) 4.2

Molecular Equation: reactants and products are written as if they were undissociated compounds. Total Ionic Equation: all soluble ionic substances (strong electrolytes) are dissociated into ions. Net Ionic Equation: Eliminate spectator ions. (These are ions which are unchanged in the chemical reaction.)

Solubility Rules for Common Ionic Compounds In water at 250C Soluble Compounds Exceptions Compounds containing alkali metal ions and NH4+ NO3-, HCO3-, ClO3- Cl-, Br-, I- Halides of Ag+, Hg22+, Pb2+ SO42- Sulfates of Ag+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+ Insoluble Compounds CO32-, PO43-, CrO42-, S2- OH- Compounds containing alkali metal ions and Ba2+ 4.2

These precipitation reactions are one type of a class of reactions sometimes called double displacement or metathesis reactions. The neutralization reactions we will study next are also in the class of reactions.

Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Bases Have a bitter taste. Feel slippery. Many soaps contain bases. 4.3

Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water 4.3

A Brønsted acid is a proton donor A Brønsted base is a proton acceptor A Brønsted acid must contain at least one ionizable proton! 4.3

Monoprotic acids Diprotic acids Triprotic acids HCl H+ + Cl- Strong electrolyte, strong acid HNO3 H+ + NO3- Strong electrolyte, strong acid CH3COOH H+ + CH3COO- Weak electrolyte, weak acid Diprotic acids H2SO4 H+ + HSO4- Strong electrolyte, strong acid HSO4- H+ + SO42- Weak electrolyte, weak acid Triprotic acids H3PO4 H+ + H2PO4- Weak electrolyte, weak acid H2PO4- H+ + HPO42- Weak electrolyte, weak acid HPO42- H+ + PO43- Weak electrolyte, weak acid 4.3

Identify each as a Bronsted acid or base or both: a. HI b. CH3COO- c. H2PO4-

EH Homework due Friday Unit 5 Sec 1 Solubility Rules 90 Sec 2 Metathesis Rxns 80 Sec 3 Ionic/Net ionic Rxns 90 Sec 4 Balancing Eq. 90 Sec 5 Predicting Products omit Sec 6 Activity Series 85 Unit 10 Sec 1 Species of Redox Rxns 90 Sec 2 Determining Oxidation No. 90

EH Homework due Sunday Unit 6 Sec 1 Molar Concentrations 90 Sec 2 Titration Problems 90 Sec 3 Volumetric Analysis 90 Sec 4 Molarity of Ions 80 Sec 5 Redox Titrations 70 Test on Chapters 3 and 4

Neutralization Reaction acid + base salt + water HCl (aq) + NaOH (aq) NaCl (aq) + H2O H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O H+ + OH- H2O 4.3

Writing Balanced Equations for Neutralization Reactions Problem: Write balanced chemical equations (molecular, total ionic, and net ionic) for the following chemical reactions: a) barium hydroxide(aq) and hydroiodic acid(aq) b) lithium hydroxide(aq) and nitric acid(aq) c) sodium hydroxide(aq) and sulfuric acid(aq)

Oxidation-Reduction (redox) reactions involve a transfer of electrons Oxidation-Reduction (redox) reactions involve a transfer of electrons. oxidation: loss of electrons reduction: gain of electrons reducing agent: substance oxidized oxidizing agent: substance reduced

Oxidation-Reduction Reactions (electron transfer reactions) 2Mg (s) + O2 (g) 2MgO (s) 2Mg 2Mg2+ + 4e- Oxidation half-reaction (lose e-) O2 + 4e- 2O2- Reduction half-reaction (gain e-) 2Mg + O2 + 4e- 2Mg2+ + 2O2- + 4e- 2Mg + O2 2MgO 4.4

Oxidation number: the charge an atom would have if the shared electrons were transferred completely to the atom with the greater attraction for the electrons. An increase in oxidation number indicates oxidation. A decrease in oxidation number indicates reduction.

Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 Oxidation number The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H2, O2, P4 = 0 In monatomic ions, the oxidation number is equal to the charge on the ion. Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 The oxidation number of oxygen is usually –2. In H2O2 and O22- it is –1. 4.4

The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1. Group 1 metals are +1, group 2 metals are +2 and fluorine is always –1. 6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. Oxidation numbers of all the elements in HCO3-, HNO3, Na3PO4, IF7, KIO3, K2Cr2O7, SO42- ? 4.4

Fig. 4.10

Types of Oxidation-Reduction Reactions Combination Reaction A + B C +4 -2 S + O2 SO2 Oxidized? Reduced? Oxidizing agent? Reducing agent? Decomposition Reaction C A + B +1 +5 -2 +1 -1 2KClO3 2KCl + 3O2 4.4

Types of Oxidation-Reduction Reactions (Single) Displacement Reaction A + BC AC + B +1 +2 Sr + 2H2O Sr(OH)2 + H2 Hydrogen Displacement +4 +2 TiCl4 + 2Mg Ti + 2MgCl2 Metal Displacement -1 -1 Cl2 + 2KBr 2KCl + Br2 Halogen Displacement Oxidized? Reduced? Oxidizing agent? Reducing agent? 4.4

The Activity Series for Metals (Single) Displacement Reaction M + BC MC + B Reaction will occur only if M is above B. Ca + 2H2O Ca(OH)2 + H2 Pb + 2H2O Pb(OH)2 + H2 4.4

Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s) Zn Zn2+ + 2e- Zn is oxidized Zn is the reducing agent Cu2+ + 2e- Cu Cu2+ is reduced Cu2+ is the oxidizing agent Nickel wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Ni (s) + 2AgNO3 (aq) Ni(NO3)2 (aq) + 2Ag (s) Ni Ni2+ + 2e- Ni is oxidized Ni is the reducing agent. Ag+ + 1e- Ag Ag+ is reduced Ag+ is the oxidizing agent 4.4

4.4

For displacement reactions involving halogens the reactivity is F2> Cl2>Br2>I2 Cl2(aq) + NaBr(aq)  ? Cl2(aq) + NaF(aq)  ?

Types of Oxidation-Reduction Reactions Disproportionation Reaction Element is simultaneously oxidized and reduced. +1 -1 Cl2 + 2OH- ClO- + Cl- + H2O Chlorine Chemistry 4.4

Classify the following reactions. Ca2+ + CO32- CaCO3(s) Precipitation NH3 + H+ NH4+ Acid-Base Zn + 2HCl ZnCl2 + H2 Redox (H2 Displacement) Ca + F2 CaF2 Redox (Combination) 4.4

Solution Stoichiometry The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. M = molarity = moles of solute liters of solution What mass of KI is required to make 500. mL of a 2.80 M KI solution? M KI M KI volume KI moles KI grams KI 1 L 1000 mL x 2.80 mol KI 1 L soln x 166 g KI 1 mol KI x 500. mL = 232 g KI 4.5

4.5

500 mL of solution contains 0. 730 moles of glucose 500 mL of solution contains 0.730 moles of glucose. What is the molarity? What volume of 2.00 M CuSO4 will contain 10.0 g of CuSO4?

Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. Dilution Add Solvent Moles of solute before dilution (i) after dilution (f) = MiVi MfVf = 4.5

How would you prepare 60.0 mL of 0.20 M HNO3 from a stock solution of 4.00 M HNO3? MiVi = MfVf Mi = 4.00 Mf = 0.20 Vf = 0.06 L Vi = ? L Vi = MfVf Mi = 0.20 M x 60.0 mL 4.00 M = 3.0 mL 3.0 mL of acid to 60 mL of solution 4.5

Gravimetric Analysis Dissolve unknown substance in water React unknown with known substance to form a precipitate Filter and dry precipitate Weigh precipitate Use chemical formula and mass of precipitate to determine amount of unknown ion 4.6

Practice Exercise page 113 A sample of 0.3220 g of an unknown compound containing Br- ion is dissolved in water and treated with an excess of AgNO3 solution. If the mass of AgBr ppt that forms is 0.6964 g, what is the percent by mass of Br in the unknown sample?

Volumetric Analysis - Titrations In a titration a solution of accurately known concentration is added gradually added to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL the indicator changes color 4.7

What volume of a 1.420 M NaOH solution is Required to titrate 25.00 mL of a 4.50 M H2SO4 solution? WRITE THE CHEMICAL EQUATION! H2SO4 + 2NaOH 2H2O + Na2SO4 M acid rx coef. M base volume acid moles acid moles base volume base 4.50 mol H2SO4 1000 mL soln x 2 mol NaOH 1 mol H2SO4 x 1000 ml soln 1.420 mol NaOH x 25.00 mL = 158 mL 4.7

(What is net ionic equation?) A standard solution is one of known concentration. NaOH solutions are frequently standardized with potassium hydrogen phthalate (KHP). NaOH(aq) + KHC8H4O2(aq)  KNaC8H4O2(aq) + H2O (What is net ionic equation?) In a titration 23.48 mL of NaOH solution are required to neutralize 0.5468 g of KHP. What is the concentration of the NaOH solution?