Chapter 4: Reactions in Aqueous Solution

Chapter 4: Reactions in Aqueous Solution
1/12/2019 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Some Ways That Chemical Reactions Occur
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Some Ways That Chemical Reactions Occur Precipitation Reactions: Processes in which soluble reactants yield an insoluble solid product that falls out of solution. 2KNO3(aq) + PbI2(s) Pb(NO3)2(aq) + 2KI(aq) Acid-Base Neutralization Reactions: Processes in which an acid reacts with a base to yield water plus a salt. H2O(l) + NaCl(aq) HCl(aq) + NaOH(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Some Ways That Chemical Reactions Occur
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Some Ways That Chemical Reactions Occur Oxidation-Reduction (Redox) Reactions: Processes in which one or more electrons are transferred between reaction partners (atoms, molecules, or ions). MgCl2(aq) + H2(g) Mg(s) + 2HCl(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Electrolytes in Aqueous Solution
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Electrolytes: Substances which dissolve in water to produce conducting solutions of ions. H2O Na1+(aq) + Cl1-(aq) NaCl(s) A solution of NaCl conducts electricity because of the movement of charged particles (ions). Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Nonelectrolytes: Substances which do not produce ions in aqueous solutions. H2O C12H22O11(aq) C12H22O11(s) A solution of C12H22O11 does not conduct electricity because it contains no charged particles (ions). C12H22O11 is sucrose (table sugar). Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Strong Electrolytes: Compounds that dissociate to a large extent into ions when dissolved in water. KCl(aq) K1+(aq) + Cl1-(aq) Weak Electrolytes: Compounds that dissociate to a small extent into ions when dissolved in water. For a 0.10 M solution, KCl is approximately 98% dissociated while CH3CO2H (acetic acid) is a little more than 1% dissociated. The size of the arrows indicate which side of the chemical equation the equilibrium lies on. The use of the equilibrium arrows is minimal until later chapters on kinetics and chemical equilibrium. H1+(aq) + CH3CO21-(aq) CH3CO2H(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Strong acids are strong electrolytes (which is why they are called strong acids!). These are typically called the 6 common strong acids. There are other strong acids. Strong Acids: hydrochloric acid, hydrobromic acid, hydroiodic acid, perchloric acid, nitric acid, sulfuric acid. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Hydroxides are also ionic compounds. Ionic Compounds Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Any acid that’s not a strong acid is considered to be a weak acid. Weak acids Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Electrolytes in Aqueous Solution Any molecular compounds which are not already classified (acids are molecular). While water does dissociate to a small degree, we consider it to be a nonelectrolyte. The dissociation of water will be dealt with in a later chapter. Molecular Compounds (other than any strong or weak electrolytes) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Aqueous Reactions and Net Ionic Equations
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Aqueous Reactions and Net Ionic Equations Molecular Equation: All substances in the chemical equation are written using their complete formulas as if they were molecules. 2KNO3(aq) + PbI2(s) Pb(NO3)2(aq) + 2KI(aq) strong electrolytes precipitate Granted, all of the substances in the chemical equation are ionic compounds but it’s common to call this the molecular equation. Pictures are useful especially with this reaction. Solutions of lead(II) nitrate and potassium iodide are colorless while the precipitate of lead(II) iodide is yellow. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Aqueous Reactions and Net Ionic Equations Ionic Equation: All of the strong electrolytes are written as ions. Pb(NO3)2(aq) 2KI(aq) Pb2+(aq) + 2NO31- (aq) + 2K1+(aq) + 2I1-(aq) 2K1+(aq) + 2NO31- (aq) + PbI2(s) Note the proper charges (as learned previously when writing ionic compound formulas). 2KNO3(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Aqueous Reactions and Net Ionic Equations Spectator Ions: Ions that undergo no change during the reaction and appear on both sides of the reaction arrow. Pb2+(aq) + 2NO31- (aq) + 2K1+(aq) + 2I1-(aq) 2K1+(aq) + 2NO31- (aq) + PbI2(s) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Aqueous Reactions and Net Ionic Equations Net Ionic Equation: Only the ions undergoing change are shown. PbI2(s) Pb2+(aq) + 2I1-(aq) This doesn’t suggest that you can actually have a solution of iodide ion. The corresponding cations and anions are present but they are spectator ions. An aqueous solution that contains lead(II) ion when mixed with another one that contains iodide ion will typically produce a precipitate of lead(II) iodide. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Precipitation Reactions and Solubility Guidelines
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Precipitation Reactions and Solubility Guidelines A compound is probably soluble if it contains one of the following cations: Group 1A cation: Li1+, Na1+, K1+, Cs1+ Ammonium ion: NH41+ A compound is probably soluble if it contains one of the following anions: Halide: Cl1-, Br1-, I1- except Ag1+, Hg22+, and Pb2+ compounds Nitrate (NO31-), perchlorate (ClO41-), acetate (CH3CO21-), sulfate (SO42-) except Ba2+, Hg22+, and Pb2+ sulfates Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO3 and Na2CO3 are mixed. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO3 and Na2CO3 are mixed. 1. Write the chemical formulas of the products (use proper ionic rules). AgNO3(aq) + Na2CO3(aq) Ag2CO3 + NaNO3 Swap the cations to determine the possible products. Properly written products! Not: AgCO3 and Na2NO3. CB + AD AB + CD double replacement reaction Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO3 and Na2CO3 are mixed. 2. Molecular Equation: Balance the equation and predict the solubility of each possible product. Ag2CO3(s) + 2NaNO3(aq) 2AgNO3(aq) + Na2CO3(aq) Neither the cation nor the anion is in the solubility list. Contains a group 1A cation. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO3 and Na2CO3 are mixed. 3. Ionic Equation: Dissociate the soluble ionic compounds. 2AgNO3(aq) Na2CO3(aq) Watch the coefficients and subscripts. 2Ag1+(aq) + 2NO31- (aq) + 2Na1+(aq) + CO32-(aq) Ag2CO3(s) + 2Na1+(aq) + 2NO31-(aq) 2NaNO3(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO3 and Na2CO3 are mixed. 4. Net Ionic Equation: Eliminate the spectator ions from the ionic equation. 2Ag1+(aq) + 2NO31- (aq) + 2Na1+(aq) + CO32-(aq) Ag2CO3(s) + 2Na1+(aq) + 2NO31-(aq) Ag2CO3(s) 2Ag1+(aq) + CO32-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Acids, Bases, and Neutralization Reactions
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Acid (Arrhenius): A substance that dissociates in water to produce hydrogen ions, H1+: H1+(aq) + A1-(aq) HA(aq) H1+(aq) + Cl1-(aq) HCl(aq) In water, acids produce hydronium ions, H3O1+: “HA” is a generic chemical formula for an acid. Hydrogen ions do not exist in water. They are attached to one or more water molecules (often in clusters). For convenience, we’ll write it as hydrogen ion instead of hydronium ion until a later chapter when a deeper look at acids and bases is undertaken. H3O1+(aq) + Cl1-(aq) HCl(aq) + H2O(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Base (Arrhenius): A substance that dissociates in water to produce hydroxide ions, OH1-: M1+(aq) + OH1-(aq) MOH(aq) Na1+(aq) + OH1-(aq) NaOH(aq) Ammonia, commonly called “ammonium hydroxide” is a base: “M” is shorthand for a metal cation. It’s really not proper to call “aqueous ammonia” “ammonium hydroxide” even though it’s still commonly labeled as such (ordering chemicals, etc.). NH41+(aq) + OH1-(aq) NH3(aq) + H2O(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Strong acids and strong bases are strong electrolytes. Weak acids and weak bases are weak electrolytes. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions These acid-base neutralization reactions are double-replacement reactions just like the precipitation reactions: MA HOH HA MOH or Recall the original definition of an acid-base neutralization written earlier. While it looks goofy to write water as HOH, it can help students remember where the pieces came from: H (acid) and OH (base). It’s also easier to balance. MA H2O HA MOH Acid Base Salt Water Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH)2. 1. Write the chemical formulas of the products (use proper ionic rules for the salt). HBr(aq) + Ba(OH)2(aq) H2O + BaBr2 Acid Base Water Salt The order the products are written in is irrelevant. The products will be water and a salt. The salt is written using the rules for naming ionic compounds as previously used. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH)2. 2. Molecular Equation: Balance the equation and predict the solubility of the salt in the products. 2H2O(l) + BaBr2(aq) 2HBr(aq) + Ba(OH)2(aq) Use the solubility rules. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH)2. 3. Ionic Equation: Dissociate a strong acid and the soluble ionic compounds. 2HBr(aq) Ba(OH)2(aq) 2H1+(aq) + 2Br1-(aq) + Ba2+(aq) + 2OH1-(aq) HBr is one of the 6 strong acids. 2H2O(l) + Ba2+(aq) + 2Br1-(aq) BaBr2(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH)2. 4. Net Ionic Equation: Eliminate the spectator ions from the ionic equation. 2H1+(aq) + 2Br1-(aq) + Ba2+(aq) + 2OH1-(aq) 2H2O(l) + Ba2+(aq) + 2Br1-(aq) 2H2O(l) 2H1+(aq) + 2OH1-(aq) or H2O(l) H1+(aq) + OH1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. 1. Write the chemical formulas of the products (use proper ionic rules for the salt). HF(aq) + NaOH(aq) H2O + NaF Acid Base Water Salt Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. 2. Molecular Equation: Balance the equation and predict the solubility of the salt in the products. H2O(l) + NaF(aq) HF(aq) + NaOH(aq) Use the solubility rules. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. 3. Ionic Equation: Dissociate a strong acid and the soluble ionic compounds. NaOH(aq) HF(aq) + Na1+(aq) + OH1-(aq) H2O(l) + Na1+(aq) + F1-(aq) HF is a weak acid. Since it dissociates only to a small degree in solution, it is not dissociated in the ionic equation. NaF(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. 4. Net Ionic Equation: Eliminate the spectator ions from the ionic equation. HF(aq) + Na1+(aq) + OH1-(aq) H2O(l) + Na1+(aq) + F1-(aq) H2O(l) + F1-(aq) HF(aq) + OH1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Oxidation-Reduction (Redox) Reactions
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Oxidation-Reduction (Redox) Reactions Rusting of iron: an oxidation of Fe 2Fe2O3(s) 4Fe(s) + 3O2(g) Manufacture of iron: a reduction of Fe 4Fe(s) + 3CO2(g) 2Fe2O3(s) + 3C(s) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Oxidation-Reduction (Redox) Reactions Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron-poor. Rules for Assigning Oxidation Numbers An atom in its elemental state has an oxidation number of 0. Na H2 Br2 S Ne Oxidation number 0 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Oxidation-Reduction (Redox) Reactions A monatomic ion has an oxidation number identical to its charge. Na1+ +1 Ca2+ +2 Al3+ +3 Cl1- -1 O2- -2 This is why charges are supposed to be written as “1-”. The corresponding oxidation number is written “-1”. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Oxidation-Reduction (Redox) Reactions An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion. Hydrogen can be either +1 or -1. H O 1- H Ca +1 -2 -1 +2 -1 H bonded to a nonmetal has an oxidation number of +1. H bonded to a metal has an oxidation number of -1. In peroxides, O22-, oxygen has an oxidation number of -1. Oxygen usually has an oxidation number of -2. H O H O O H +1 -2 +1 +1 -1 -1 +1 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Oxidation-Reduction (Redox) Reactions Halogens usually have an oxidation number of -1. 3. H Cl Cl O +1 -1 +1 -2 +1 A major exception is when the halogen is bonded to oxygen. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Oxidation-Reduction (Redox) Reactions The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion. H2SO3 2(+1) + x + 3(-2) = 0 (net charge) x = +4 +1 x -2 Cr2O72- 2(x) + 7(-2) = -2 (net charge) x = +6 x -2 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Identifying Redox Reactions
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Identifying Redox Reactions Oxidizing Agent Causes reduction Loses one or more electrons Undergoes oxidation Oxidation number of atom increases Reducing Agent Causes oxidation Gains one or more electrons Undergoes reduction Oxidation number of atom decreases An “agent” causes something to happen. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Identifying Redox Reactions Oxidizing Agent 2Fe2 3O2(g) + 4Fe(s) O3 (s) +3 oxidation reduction -2 Reducing Agent Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Identifying Redox Reactions Oxidizing Agent reduction +3 2Fe2O3 (s) + 3 C (s) 4Fe(s) + 3 C O2 (g) +4 Reducing Agent oxidation Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

The Activity Series of the Elements
Chapter 4: Reactions in Aqueous Solution 1/12/2019 The Activity Series of the Elements Cu2+(aq) + 2Ag(s) Cu(s) + 2Ag1+(g) Which one of these reactions will occur? 2Ag1+(aq) + Cu(s) 2Ag(s) + Cu2+(g) These are net ionic equations. The spectator ions (nitrate, for one example) are not involved in the main reaction. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 The Activity Series of the Elements The elements that are higher up in the table are more likely to be oxidized. Thus, any element higher in the activity series will reduce the ion of any element lower in the activity series. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 The Activity Series of the Elements Cu2+(aq) + 2Ag(s) Cu(s) + 2Ag1+(g) Which one of these reactions will occur? 2Ag1+(aq) + Cu(s) 2Ag(s) + Cu2+(g) This is a nice one to do as a demonstration. Place a coil of copper wire in a solution of silver nitrate at the beginning of lecture or lab and show it to the students. Show it again at the end of the period. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Balancing Redox Reactions: The Oxidation-Number Method
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Oxidation-Number Method Redox reactions can be too difficult to balance by inspection (as in Ch. 3). The oxidation-number method focuses more on the chemical changes by analyzing species which are changing their oxidation numbers. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Oxidation-Number Method Balance the following net ionic equation in acidic solution: Cr3+(aq) + IO31-(aq) I1-(aq) + Cr2O72-(aq) Balance the equation for all atoms other than H and O. 2Cr3+(aq) + IO31-(aq) I1-(aq) + Cr2O72-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Oxidation-Number Method Assign oxidation numbers to all atoms. 2Cr3+(aq) + IO31-(aq) I1-(aq) + Cr2O72-(aq) -1 +6 -2 +3 +5 -2 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Oxidation-Number Method Decide which atoms have changed oxidation number, and by how much. I1- (aq) + Cr2 O72- (aq) 2Cr3+ (aq) + I O31- (aq) -1 +6 +3 +5 oxidation reduction Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Oxidation-Number Method Make the total increase in oxidation number equal to the total decrease in oxidation number. I1- (aq) + Cr2 O72- (aq) 2Cr3+ (aq) + I O31- (aq) -1 +6 +3 +5 Net increase = +6 Net decrease = -6 2 Cr+3 ions: 2(-3)=-6 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Oxidation-Number Method Balance the equation for by adding H2O, and then balance for H by adding H1+. 8H1+(aq) + I1-(aq) + Cr2O72-(aq) 2Cr3+(aq) + IO31-(aq) + 4H2O(l) Do an atom check and a charge balance check at the end. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Balancing Redox Reactions: The Half-Reaction Method
Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method The half-reaction method focuses on the individual half-reactions which is important for electrochemistry. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance the following net ionic equation in acidic solution: Cr3+(aq) + IO31-(aq) I1-(aq) + Cr2O72-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Write the two unbalanced half-reactions. Cr3+(aq) Cr2O72-(aq) IO31-(aq) I1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance both half-reactions for all atoms except O and H. 2Cr3+(aq) Cr2O72-(aq) IO31-(aq) I1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance each half-reaction for O by adding H2O, and then balance for H by adding H1+. 2Cr3+(aq) + 7H2O(l) 14H1+(aq) + Cr2O72-(aq) IO31-(aq) + 6H1+(aq) 3H2O(l) + I1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance each half-reaction for charge by adding electrons to the side with greater positive charge. reduction: 2Cr3+(aq) + 7H2O(l) + 6e- 14H1+(aq) + Cr2O72-(aq) oxidation: IO31-(aq) + 6H1+(aq) 6e- + 3H2O(l) + I1-(aq) LEO says GER. Think of the lion… LOSS of ELECTRONS is OXIDATION GAIN of ELECTRONS is REDUCTION Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Multiply each half-reaction by a factor to make the electron count the same in both half-reactions. reduction: 2Cr3+(aq) + 7H2O(l) + 6e- 14H1+(aq) + Cr2O72-(aq) oxidation: IO31-(aq) + 6H1+(aq) 6e- + 3H2O(l) + I1-(aq) Nothing to do since the number of electrons is the same for each half-reaction. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Add the two balanced half-reactions together and cancel species that appear on both sides of the equation. reduction: 2Cr3+(aq) + 7H2O(l) + 6e- 14H1+(aq) + Cr2O72-(aq) oxidation: IO31-(aq) + 6H1+(aq) 6e- + 3H2O(l) + I1-(aq) 8H1+(aq) + I1-(aq) + Cr2O72-(aq) IO31-(aq) + 2Cr3+(aq) + 4H2O(l) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance the following net ionic equation in basic solution: MnO2(s) + BrO31-(aq) MnO41-(aq) + Br1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Write the two unbalanced half-reactions. BrO31-(aq) Br1-(aq) MnO2(s) MnO41-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance both half-reactions for all atoms except O and H. BrO31-(aq) Br1-(aq) MnO2(s) MnO41-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance each half-reaction for O by adding H2O, and then balance for H by adding H1+. BrO31-(aq) + 6H1+(aq) 3H2O(l) + Br1-(aq) MnO2(s) + 2H2O(l) 4H1+(aq) + MnO41-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Balance each half-reaction for charge by adding electrons to the side with greater positive charge. BrO31-(aq) + 6H1+(aq) + 6e- 3H2O(l) + Br1-(aq) MnO2(s) + 2H2O(l) 3e- + 4H1+(aq) + MnO41-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Multiply each half-reaction by a factor to make the electron count the same in both half-reactions. BrO31-(aq) + 6H1+(aq) + 6e- 3H2O(l) + Br1-(aq) 2 MnO2(s) + 2H2O(l) 3e- + 4H1+(aq) + MnO41-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Add the two balanced half-reactions together and cancel species that appear on both sides of the equation. BrO31-(aq) + 6H1+(aq) + 6e- 3H2O(l) + Br1-(aq) 2MnO2(s) + 4H2O(l) 6e- + 8H1+(aq) + 2MnO41-(aq) 2H1+(aq) + 2MnO41-(aq) + Br1-(aq) 2MnO2(s) + H2O(l) + BrO31-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution 1/12/2019 Balancing Redox Reactions: The Half-Reaction Method Since the reaction occurs in a basic solution, “neutralize” the excess H1+ by adding OH1- and cancel any water (if possible) 2H2O 2OH1-(aq) + 2H1+(aq) + 2MnO41-(aq) + Br1-(aq) 2MnO2(s) + H2O(l) + BrO31-(aq) + 2OH1-(aq) H2O(l) + 2MnO41-(aq) + Br1-(aq) 2MnO2(s) + BrO31-(aq) + 2OH1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

1/12/2019 Redox Titrations Titration: A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known. 5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq) 10CO2(g) + 2Mn2+(aq) + 8H2O(l) The net ionic equation is shown. Potassium permanganate is dark purple. As it’s added to a solution of oxalic acid, the permanganate reacts to form Mn2+ and remains colorless (dilute solutions of Mn2+ are colored but it can be difficult to see). When a slight excess of permanganate is added, the solution turns faint purple. If the unknown concentration is the potassium permanganate solution, MnO41-, it can be slowly added to a known amount of oxalic acid, H2C2O4, until a faint purple color persists. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

1/12/2019 Redox Titrations A solution is prepared with g of oxalic acid, H2C2O mL of an unknown solution of potassium permanganate are needed to titrate the solution. What is the concentration of the potassium permanganate solution? 5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq) 10CO2(g) + 2Mn2+(aq) + 8H2O(l) Mass of H2C2O4 Moles of H2C2O4 Moles of KMnO4 Molarity of KMnO4 Molar Mass of H2C2O4 Mole Ratio Molarity of KMnO4 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

1/12/2019 Redox Titrations 5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq) 10CO2(g) + 2Mn2+(aq) + 8H2O(l) Moles of H2C2O4 available: g H2C2O4 1 mol x = mol H2C2O4 90.04 g 4 significant figures are used in the molar mass since the mass is given to 4 significant figures. Moles of KMnO4 reacted: mol H2C2O4 2 mol KMnO4 x = mol KMnO4 5 mol H2C2O4 Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

1/12/2019 Redox Titrations 5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq) 10CO2(g) + 2Mn2+(aq) + 8H2O(l) Concentration of KMnO4 solution: mol KMnO4 1 L 1000 mL x = M KMnO4 22.35 mL Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 4: Reactions in Aqueous Solution

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