TYPES OF CHEMICAL REACTIONS AND SOLUTION STOICHIOMETRY Chapter Four: TYPES OF CHEMICAL REACTIONS AND SOLUTION STOICHIOMETRY

Water, the Common Solvent One of the most important substances on earth Can dissolve many different substances A polar molecule 4.1 Copyright © Houghton Mifflin Company. All rights reserved.

Dissolution of a Solid in a Liquid 4.1 Copyright © Houghton Mifflin Company. All rights reserved.

Nature of Aqueous Solutions Solute – substance being dissolved Solvent – liquid water Electrolyte – substance that when dissolved in water produces a solution that can conduct electricity 4.2 Copyright © Houghton Mifflin Company. All rights reserved.

Electrolytes Strong Electrolytes – conduct current very efficiently (bulb shines brightly) Weak Electrolytes – conduct only a small current (bulb glows dimly) Nonelectrolytes – no current flows (bulb remains unlit) 4.2 Copyright © Houghton Mifflin Company. All rights reserved.

Electrolyte Behavior 4.2 Copyright © Houghton Mifflin Company. All rights reserved.

Chemical Reactions of Solutions We must know: Nature of the reaction Amounts of chemicals present in the solutions 4.3 Copyright © Houghton Mifflin Company. All rights reserved.

Molarity Molarity (M) = moles of solute per volume of solution in liters: 4.3 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check Which of the following solutions contains the greatest number of ions? 400.0 mL of 0.10 M NaCl. 300.0 mL of 0.10 M CaCl2. 200.0 mL of 0.10 M FeCl3. 800.0 mL of 0.10 M sucrose. a) contains 0.080 mol of ions (0.400×0.10×2). b) contains 0.090 mol of ions (0.300×0.10×3). c) contains 0.080 mol of ions (0.200×0.10×4). d) does not contain any ions because sucrose does not break up into ions. Therefore, letter b) is correct. 4.3 Copyright © Houghton Mifflin Company. All rights reserved.

Let’s Think About It Draw molecular level pictures showing each solution. Think about relative numbers of ions. How many moles of each ion are in each solution? 4.3 Copyright © Houghton Mifflin Company. All rights reserved.

Notice The solution with the greatest number of ions is not necessarily the one in which: the volume of the solution is the largest. the formula unit has the greatest number of ions. 4.3 Copyright © Houghton Mifflin Company. All rights reserved.

Dilution The process of adding water to a stock solution to achieve the molarity desired for a particular solution. Dilution with water does not alter the numbers of moles of solute present. Moles of solute before dilution = moles of solute after dilution M1V1 = M2V2 4.3 Copyright © Houghton Mifflin Company. All rights reserved.

Types of Chemical Reactions Precipitation Reactions Acid-Base Reactions Oxidation-Reduction Reactions 4.4 Copyright © Houghton Mifflin Company. All rights reserved.

Precipitation Reaction A double displacement reaction in which a solid forms and separates from the solution. When ionic compounds dissolve in water, the resulting solution contains the separated ions Precipitate – the solid that forms 4.5 Copyright © Houghton Mifflin Company. All rights reserved.

The Reaction of K2CrO4(aq) and Ba(NO3)2(aq) Ba2+(aq) + CrO42–(aq) → BaCrO4(s) solid BaCrO4 formed 4.5 Copyright © Houghton Mifflin Company. All rights reserved.

Precipitation of Silver Chloride 4.5 Copyright © Houghton Mifflin Company. All rights reserved.

Precipitates Soluble – solid dissolves in solution; (aq) is used in reaction Insoluble – solid does not dissolve in solution; (s) is used in reaction Insoluble and slightly soluble are often used interchangeably 4.5 Copyright © Houghton Mifflin Company. All rights reserved.

Simple Rules for Solubility Most nitrate (NO3) salts are soluble. Most alkali (group 1A) salts and NH4+ are soluble. Most Cl, Br, and I salts are soluble (except Ag+, Pb2+, Hg22+). Most sulfate salts are soluble (except BaSO4, PbSO4, Hg2SO4, CaSO4). Most OH salts are only slightly soluble (NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are marginally soluble). Most S2, CO32, CrO42, PO43 salts are only slightly soluble. 4.5 Copyright © Houghton Mifflin Company. All rights reserved.

Describing Reactions in Solution Formula Equation (Molecular Equation) Reactants and products as compounds AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) Complete Ionic Equation All strong electrolytes shown as ions Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq)  AgCl(s) + Na+(aq) + NO3(aq) 4.6 Copyright © Houghton Mifflin Company. All rights reserved.

Describing Reactions in Solution Net Ionic Equation Show only components that actually react Ag+(aq) + Cl(aq)  AgCl(s) Na+ and NO3 are spectator ions 4.6 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check You have two separate beakers with aqueous solutions, one with 4 “units” of potassium sulfate and one with 3 “units” of barium nitrate. Draw molecular level diagrams of both solutions. The first beaker should have 8 K+ ions and 4 SO42- ions. The second beaker should have 3 Ba2+ ions and 6 NO3- ions. 4.6/4.7 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check You have two separate beakers with aqueous solutions, one with 4 “units” of potassium sulfate and one with 3 “units” of barium nitrate. b) Draw a molecular level diagram of the mixture of the two solutions before a reaction has taken place. Now all of the ions should be mixed together in one beaker. 4.6/4.7 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check You have two separate beakers with aqueous solutions, one with 4 “units” of potassium sulfate and one with 3 “units” of barium nitrate. c) Draw a molecular level diagram of the product and solution formed after the reaction has taken place. There should be 8 K+ ions, 6 NO3- ions, 1 SO42- ion, and 3 units of solid BaSO4 formed in the beaker. 4.6/4.7 Copyright © Houghton Mifflin Company. All rights reserved.

Solution Stoichiometry Problems Identify the species present in the combined solution. Write the balanced net ionic equation. Calculate the moles of reactants. Determine limiting reactant. Calculate the moles of product(s). Convert to grams or other units. 4.7 Copyright © Houghton Mifflin Company. All rights reserved.

Acid-Base Reactions (Brønsted-Lowry) Acid – proton donor Base – proton acceptor For a strong acid and base reaction: H+(aq) + OH–(aq)  H2O(l) 4.8 Copyright © Houghton Mifflin Company. All rights reserved.

Neutralization of a Strong Acid by a Strong Base 4.8 Copyright © Houghton Mifflin Company. All rights reserved.

Key Titration Terms Titrant – solution of known concentration used in titration Analyte – substance being analyzed Equivalence point – enough titrant added to react exactly with the analyte Endpoint – the indicator changes color so you can tell the equivalence point has been reached 4.8 Copyright © Houghton Mifflin Company. All rights reserved.

Performing Calculations for Acid-Base Reactions List initial species and predict reaction. Write balanced net ionic reaction. Calculate moles of reactants. Determine limiting reactant. Calculate moles of required reactant or product. Convert to grams or volume, as required. 4.8 Copyright © Houghton Mifflin Company. All rights reserved.

Oxidation-Reduction Reactions (Redox Reactions) Reactions in which one or more electrons are transferred. 4.9 Copyright © Houghton Mifflin Company. All rights reserved.

Reaction of Sodium and Chlorine 4.9 Copyright © Houghton Mifflin Company. All rights reserved.

Rules for Assigning Oxidation States Oxidation state of an atom in an element = 0 Oxidation state of monatomic element = charge Oxygen = 2 in covalent compounds (except in peroxides where it = 1) Hydrogen = +1 in covalent compounds Fluorine = 1 in compounds Sum of oxidation states = 0 in compounds Sum of oxidation states = charge of the ion 4.9 Copyright © Houghton Mifflin Company. All rights reserved.

Exercise Find the oxidation states for each of the elements in each of the following compounds: K2Cr2O7 CO32- MnO2 PCl5 SF4 K2Cr2O7; K = +1; Cr = +6; O = -2 CO32-; C = +4; O = -2 MnO2; Mn = +4; O = -2 PCl5; P = +5; Cl = -1 SF4; S = +4; F = -1 4.9 Copyright © Houghton Mifflin Company. All rights reserved.

Redox Characteristics Transfer of Electrons Transfer may occur to form ions Oxidation – increase in oxidation state (loss of electrons); reducing agent Reduction – decrease in oxidation state (gain of electrons); oxidizing agent 4.9 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check Which of the following are oxidation-reduction reactions? Identify the oxidizing agent and the reducing agent. a) Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) b) Cr2O72-(aq) + 2OH-(aq)  2CrO42-(aq) + H2O(l) c) 2CuCl(aq)  CuCl2(aq) + Cu(s) Zn – reducing agent; HCl – oxidizing agent c) CuCl acts as the reducing and oxidizing agent 4.9 Copyright © Houghton Mifflin Company. All rights reserved.

Balancing Oxidation-Reduction Reactions Cr2O72-(aq) + SO32-(aq)  Cr3+(aq) + SO42-(aq) How can we balance this equation? First Steps: Separate into half-reactions Balance elements except H and O 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Method of Half Reactions Cr2O72-(aq)  2Cr3+(aq)   SO32-(aq)  SO42-(aq) How many electrons are involved in each half reaction? 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Method of Half Reactions (continued) 6e- + Cr2O72-(aq)  2Cr3+(aq)  SO32-(aq)  + SO42-(aq) + 2e- How can we balance the oxygen atoms? 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Method of Half Reactions (continued) 6e- + Cr2O72-(aq)  Cr3+(aq) + 7H2O  H2O +SO32-(aq)  + SO42-(aq) + 2e- How can we balance the hydrogen atoms? 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Method of Half Reactions (continued) This reaction occurs in an acidic solution. 14H+ + 6e- + Cr2O72-  2Cr3+ + 7H2O  H2O +SO32-  SO42- + 2e- + 2H+ How can we balance the electrons? 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Method of Half Reactions (continued) 14H+ + 6e- + Cr2O72-  2Cr3+ + 7H2O 3[H2O +SO32-  SO42- + 2e- + 2H+] Final Balanced Equation: Cr2O72- + 3SO32- + 8H+  2Cr3+ + 3SO42- + 4H2O 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Exercise Balance the following oxidation-reduction reaction that occurs in acidic solution. Br-(aq) + MnO4-(aq)  Br2(l)+ Mn2+(aq) 10Br-(aq) + 16H+(aq) + 2MnO4-(aq)  5Br2(l)+ 2Mn2+(aq) + 8H2O(l) 4.10 Copyright © Houghton Mifflin Company. All rights reserved.

Half-Reaction Method – Balancing in Base Balance as in acid. Add OH that equals H+ ions (both sides!) Form water by combining H+, OH. Cancel any water molecules that appear on both sides. Check elements and charges for balance. 4.10 Copyright © Houghton Mifflin Company. All rights reserved.