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Reactions in Aqueous Solution Chapter 4 For test 3: Sections 3.7 and 4.1 to 4.5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 aqueous solutions of KMnO4 2 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 3 Conduct electricity in solution? Cations (+) and Anions (-) Strong Electrolyte – 100% dissociation NaCl(s) H 2O Na+(aq) + Cl-(aq) Weak Electrolyte – not completely dissociated CH3COOH CH3COO-(aq) + H+(aq) 4 Ionization of acetic acid CH3COOH CH3COO-(aq) + H+(aq) A reversible reaction. The reaction can occur in both directions. Acetic acid is a weak electrolyte because its ionization in water is incomplete. 5 Hydration is the process in which an ion is surrounded by water molecules arranged in a specific manner. d- d+ H2O 6 Nonelectrolyte does not conduct electricity? No cations (+) and anions (-) in solution C6H12O6(s) H 2O C6H12O6(aq) 7 A process in which one or more substances is changed into one or more new substances is a chemical reaction A chemical equation uses chemical symbols to show what happens during a chemical reaction reactants products 3 ways of representing the reaction of H2 with O2 to form H2O From Chapter 3 section 3.7 8 How to “Read” Chemical Equations 2 Mg + O2 2 MgO 2 atoms Mg + 1 molecule O2 makes 2 formula units MgO 2 moles Mg + 1 mole O2 makes 2 moles MgO 48.6 grams Mg + 32.0 grams O2 makes 80.6 g MgO NOT 2 grams Mg + 1 gram O2 makes 2 g MgO From Chapter 3 section 3.7 9 Balancing Chemical Equations 1. Write the correct formula(s) for the reactants on the left side and the correct formula(s) for the product(s) on the right side of the equation. Ethane reacts with oxygen to form carbon dioxide and water C2H6 + O2 CO2 + H2O 2. Change the numbers in front of the formulas (coefficients) to make the number of atoms of each element the same on both sides of the equation. Do not change the subscripts. 2C2H6 NOT C4H12 From Chapter 3 section 3.7 10 Balancing Chemical Equations 3. Start by balancing those elements that appear in only one reactant and one product. C2H6 + O2 2 carbon on left C2H6 + O2 6 hydrogen on left C2H6 + O2 CO2 + H2O start with C or H but not O 1 carbon on right multiply CO2 by 2 2CO2 + H2O 2 hydrogen on right 2CO2 + 3H2O multiply H2O by 3 11 From Chapter 3 section 3.7 Balancing Chemical Equations 4. Balance those elements that appear in two or more reactants or products. C2H6 + O2 2 oxygen on left 2CO2 + 3H2O multiply O2 by 7 2 4 oxygen + 3 oxygen = 7 oxygen (3x1) on right (2x2) C2H6 + 7 O2 2 2CO2 + 3H2O 2C2H6 + 7O2 4CO2 + 6H2O remove fraction multiply both sides by 2 From Chapter 3 section 3.7 12 Balancing Chemical Equations 5. Check to make sure that you have the same number of each type of atom on both sides of the equation. 2C2H6 + 7O2 4CO2 + 6H2O 4 C (2 x 2) 4C 12 H (2 x 6) 12 H (6 x 2) 14 O (7 x 2) 14 O (4 x 2 + 6) Reactants 4C 12 H 14 O From Chapter 3 section 3.7 Products 4C 12 H 14 O 13 Precipitation Reactions Precipitate – insoluble solid that separates from solution precipitate Pb(NO3)2(aq) + 2NaI(aq) PbI2(s) + 2NaNO3(aq) molecular equation Pb2+ + 2NO3- + 2Na+ + 2I- PbI2(s) + 2Na+ + 2NO3- ionic equation Pb2+ + 2IPbI2 PbI2(s) net ionic equation Na+ and NO3- are spectator ions 14 Precipitation of Lead Iodide Pb2+ + 2I- PbI2(s) PbI2 15 Solubility is the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. 16 Examples of Insoluble Compounds CdS PbS Ni(OH)2 Al(OH)3 17 Writing Net Ionic Equations 1. Write the balanced molecular equation. 2. Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions. 3. Cancel the spectator ions on both sides of the ionic equation 4. Check that charges and number of atoms are balanced in the net ionic equation Write the net ionic equation for the reaction of silver nitrate with sodium chloride. AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) Ag+ + NO3- + Na+ + Cl- AgCl(s) + Na+ + NO3- Ag+ + Cl- AgCl(s) 18 Properties of Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. Cause color changes in plant dyes. React with certain metals to produce hydrogen gas. 2HCl(aq) + Mg(s) MgCl2(aq) + H2(g) React with carbonates and bicarbonates to produce carbon dioxide gas 2HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l) Aqueous acid solutions conduct electricity. 19 Properties of Bases Have a bitter taste. Feel slippery. Many soaps contain bases. Cause color changes in plant dyes. Aqueous base solutions conduct electricity. Examples: 20 Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water 21 Hydronium ion, hydrated proton, H3O+ 22 A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid acid base A Brønsted acid must contain at least one ionizable proton! 23 Monoprotic acids HCl H+ + Cl- HNO3 H+ + NO3H+ + CH3COO- CH3COOH Strong electrolyte, strong acid Strong electrolyte, strong acid Weak electrolyte, weak acid Diprotic acids H2SO4 H+ + HSO4- Strong electrolyte, strong acid HSO4- H+ + SO42- Weak electrolyte, weak acid Triprotic acids H3PO4 H2PO4HPO42- H+ + H2PO4H+ + HPO42H+ + PO43- Weak electrolyte, weak acid Weak electrolyte, weak acid Weak electrolyte, weak acid 24 Identify each of the following species as a Brønsted acid, base, or both. (a) HI, (b) CH3COO-, (c) H2PO4- HI (aq) H+ (aq) + I- (aq) CH3COO- (aq) + H+ (aq) H2PO4- (aq) Brønsted acid CH3COOH (aq) H+ (aq) + HPO42- (aq) H2PO4- (aq) + H+ (aq) H3PO4 (aq) Brønsted base Brønsted acid Brønsted base 25 Neutralization Reaction acid + base salt + water HCl(aq) + NaOH(aq) NaCl(aq) + H2O H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O H+ + OH- H2O 26 Neutralization Reaction Involving a Weak Electrolyte weak acid + base HCN(aq) + NaOH(aq) HCN + Na+ + OH- HCN + OH- salt + water NaCN(aq) + H2O Na+ + CN- + H2O CN- + H2O 27 Neutralization Reaction Producing a Gas acid + base 2HCl(aq) + Na2CO3(aq) 2H+ + 2Cl- + 2Na+ + CO32- 2H+ + CO32- salt + water + CO2 2NaCl(aq) + H2O +CO2 2Na+ + 2Cl- + H2O + CO2 H2O + CO2 28 Oxidation-Reduction Reactions (electron transfer reactions) 2Mg O2 + 4e- 2Mg2+ + 4e- Oxidation half-reaction (lose e-) 2O2Reduction half-reaction (gain e-) 2Mg + O2 + 4e2Mg2+ + 2O2- + 4e29 2Mg + O2 2MgO 30 Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s) Zn2+ + 2e- Zn is oxidized Zn Cu2+ + 2e- Zn is the reducing agent Cu Cu2+ is reduced Cu2+ is the oxidizing agent Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Cu(s) + 2AgNO3(aq) Cu Ag+ + 1e- Cu(NO3)2(aq) + 2Ag(s) Cu2+ + 2eAg Ag+ is reduced Ag+ is the oxidizing agent 31 Oxidation number The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. 1. Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H2, O2, P4 = 0 2. In monatomic ions, the oxidation number is equal to the charge on the ion. Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 3. The oxidation number of oxygen is usually –2. In H2O2 and O22- it is –1. 32 4.4 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. 5. Group IA metals are +1, IIA 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. 7. Oxidation numbers do not have to be integers. Oxidation number of oxygen in the superoxide ion, O2-, is –½. - HCO3 What are the oxidation numbers of all the elements in HCO3- ? O = –2 H = +1 3x(–2) + 1 + ? = –1 C = +4 33 The Oxidation Numbers of Elements in their Compounds 34 What are the oxidation numbers of all the elements in each of these compounds? NaIO3 IF7 K2Cr2O7 NaIO3 IF7 F = -1 7x(-1) + ? = 0 I = +7 Na = +1 O = -2 3x(-2) + 1 + ? = 0 I = +5 K2Cr2O7 O = -2 K = +1 7x(-2) + 2x(+1) + 2x(?) = 0 Cr = +6 35 Types of Oxidation-Reduction Reactions Combination Reaction A+B 0 0 3Mg + N2 C +2 -3 MgN2 Decomposition Reaction C +1 +5 -2 2KClO3 A+B +1 -1 0 2KCl + 3O2 36 Types of Oxidation-Reduction Reactions Combustion Reaction A + O2 B 0 0 S + O2 0 0 2Mg + O2 +4 -2 SO2 +2 -2 2MgO 37 Types of Oxidation-Reduction Reactions Displacement Reaction A + BC 0 +1 +2 Sr + 2H2O +4 0 TiCl4 + 2Mg 0 AC + B -1 Cl2 + 2KBr 0 Sr(OH)2 + H2 Hydrogen Displacement 0 +2 Ti + 2MgCl2 -1 Metal Displacement 0 2KCl + Br2 Halogen Displacement 38 The Activity Series for Metals Hydrogen Displacement Reaction M + BC MC + B M is metal BC is acid or H2O B is H2 Ca + 2H2O Ca(OH)2 + H2 Pb + 2H2O Pb(OH)2 + H2 39 The Activity Series for Halogens F2 > Cl2 > Br2 > I2 Halogen Displacement Reaction 0 -1 Cl2 + 2KBr I2 + 2KBr -1 0 2KCl + Br2 2KI + Br2 40 Classify each of the following reactions. Ca2+ + CO32NH3 + H+ Zn + 2HCl Ca + F2 CaCO3 NH4+ ZnCl2 + H2 CaF2 Precipitation Acid-Base Redox (H2 Displacement) Redox (Combination) 41 Gravimetric Analysis 1. Dissolve unknown substance in water 2. React unknown with known substance to form a precipitate 3. Filter and dry precipitate 4. Weigh precipitate 5. Use chemical formula and mass of precipitate to determine amount of unknown ion 42 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 43 Titrations can be used in the analysis of Acid-base reactions H2SO4 + 2NaOH 2H2O + Na2SO4 Redox reactions 5Fe2+ + MnO4- + 8H+ Mn2+ + 5Fe3+ + 4H2O 44 CH-3 (part) and 4 HW Questions and Problems Page 91 3.60 & 3.64 Questions and Problems Pages 129 - 131 4.8, 4.10, 4.12, 4.14, 4.18, 4.20, 4.22, 4.24, 4.32 , 4.34, 4.40, 4.42, 4.44, 4.46, 4.48, 4.50 45