Chemical Quantities and Aqueous Reactions Chapter 4 Chemical Quantities and Aqueous Reactions

What Happens When a Solute Dissolves? there are attractive forces between the solute particles holding them together there are also attractive forces between the solvent molecules when we mix the solute with the solvent, there are attractive forces between the solute particles and the solvent molecules if the attractions between solute and solvent are strong enough, the solute will dissolve

Salt vs. Sugar Dissolved in Water molecular compounds do not dissociate when they dissolve ionic compounds dissociate into ions when they dissolve

Electrolytes and Nonelectrolytes materials that dissolve in water to form a solution that will conduct electricity are called electrolytes materials that dissolve in water to form a solution that will not conduct electricity are called nonelectrolytes

Electrolytes in Aqueous Solution Chapter 4: Reactions in Aqueous Solution Electrolytes in Aqueous Solution 5/29/2018 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. H1+(aq) + CH3CO21-(aq) CH3CO2H(aq) 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. Nonelectrolytes: Substances which do not produce ions in aqueous solutions. H2O C12H22O11(aq) C12H22O11(s) Copyright © 2008 Pearson Prentice Hall, Inc.

HCl(aq)  H+(aq) + Cl-(aq) Acids acids are molecular compounds that ionize when they dissolve in water the molecules are pulled apart by their attraction for the water when acids ionize, they form H+ cations and anions the percentage of molecules that ionize varies from one acid to another acids that ionize virtually 100% are called strong acids HCl(aq)  H+(aq) + Cl-(aq) acids that only ionize a small percentage are called weak acids HF(aq) H+(aq) + F-(aq)

Acids, Bases, and Neutralization Reactions Chapter 4: Reactions in Aqueous Solution 5/29/2018 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.

Solubility of Ionic Compounds compounds that dissolve in a solvent are said to be soluble, while those that do not are said to be insoluble NaCl is soluble in water, AgCl is insoluble in water the degree of solubility depends on the temperature even insoluble compounds dissolve, just not enough to be meaningful

Precipitation Reactions and Solubility Guidelines Chemistry: McMurry and Fay, 6th Edition Chapter 4: Reactions in Aqueous Solution Precipitation Reactions and Solubility Guidelines 5/29/2018 5:53:57 AM Copyright © 2011 Pearson Prentice Hall, Inc.

Precipitation Reaction Chapter 4: Reactions in Aqueous Solution Precipitation Reaction 5/29/2018 Precipitation Reactions: Processes in which soluble reactants yield an insoluble solid product that falls out of solution. NaNO3(aq) + AgCl(s) AgNO3(aq) + NaCl(aq) Copyright © 2008 Pearson Prentice Hall, Inc.

Aqueous Reactions and Net Ionic Equations Chapter 4: Reactions in Aqueous Solution Aqueous Reactions and Net Ionic Equations 5/29/2018 Molecular Equation: All substances in the chemical equation are written using their complete formulas as if they were molecules. NaNO3(aq) + AgCl(s) AgNO3(aq) + NaCl(aq) strong electrolytes indicated as (aq) soluble Precipitate insoluble 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. Ionic Equation: All of the strong electrolytes are written as ions. Ag1+(aq) + NO31- (aq) + Na1+(aq) + Cl1-(aq) Na1+(aq) + NO31- (aq) + AgCl(s) Copyright © 2008 Pearson Prentice Hall, Inc.

Aqueous Reactions and Net Ionic Equations Chapter 4: Reactions in Aqueous Solution Aqueous Reactions and Net Ionic Equations 5/29/2018 Spectator Ions: Ions that undergo no change during the reaction and appear on both sides of the reaction arrow. Ag1+(aq) + NO31- (aq) + Na1+(aq) + Cl1-(aq) Na1+(aq) + NO31- (aq) + AgCl(s) Net Ionic Equation: Only the ions undergoing change are shown. AgCl(s) Ag+(aq) + Cl1-(aq) Copyright © 2008 Pearson Prentice Hall, Inc.

Example Predict the products and write a balanced molecular equation, ionic equation and net ionic equation for the following reactions: K2S(aq) + Fe(NO3)2(aq)  NaCl(aq) +(NH4)2(aq)CO3 

Acid-Base Reaction 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) Acid + Base  Water + Salt

Acids, Bases, and Neutralization Reactions Chapter 4: Reactions in Aqueous Solution Acids, Bases, and Neutralization Reactions 5/29/2018 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+: H3O1+(aq) + Cl1-(aq) HCl(aq) + H2O(aq) “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. Copyright © 2008 Pearson Prentice Hall, Inc.

Acids, Bases, and Neutralization Reactions Chapter 4: Reactions in Aqueous Solution 5/29/2018 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.

Acids, Bases, and Neutralization Reactions Chapter 4: Reactions in Aqueous Solution Acids, Bases, and Neutralization Reactions 5/29/2018 Write the molecular, ionic, and net ionic equations for the reaction of aqueous H2SO4 and aqueous KOH Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. 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.

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. Titrant: solution that is added from the buret Equivalence Point: The point at which stoichiometrically equivalent quantities of acid and base have been mixed together. Endpoint: is the point at which the titration is complete (usually by a sudden color change), as determined by an indicator, Types of acid-base titration Strong acid-strong acid Strong acid-weak acid Weak acid-strong base Weak acid – weak base

Titration The base solution is the titrant in the burette. As the base is added to the acid, the H+ reacts with the OH– to form water. But there is still excess acid present so the color does not change. At the titration’s endpoint, just enough base has been added to neutralize all the acid. At this point the indicator changes color.

Example The titration of 10.00 mL of HCl solution of unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution? The titration of a 20.0 mL sample of an H2SO4 solution of an unknown concentration requires 22.87 mL of a 0.158M KOH solution to reach the equivalent point. What is the concentration of the unknown H2SO4 solution?

Gas Evolving Reactions Some reactions form a gas directly from the ion exchange K2S(aq) + H2SO4(aq)  K2SO4(aq) + H2S(g) Other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water K2SO3(aq) + H2SO4(aq)  K2SO4(aq) + H2SO3(aq) H2SO3  H2O(l) + SO2(g)

Compounds that Undergo Gas Evolving Reactions Reactant Type Reacting With Ion Exchange Product Decom-pose? Gas Formed Example metalnS, metal HS acid H2S no K2S(aq) + 2HCl(aq)  2KCl(aq) + H2S(g) metalnCO3, metal HCO3 H2CO3 yes CO2 K2CO3(aq) + 2HCl(aq)  2KCl(aq) + CO2(g) + H2O(l) metalnSO3 metal HSO3 H2SO3 SO2 K2SO3(aq) + 2HCl(aq)  2KCl(aq) + SO2(g) + H2O(l) (NH4)nanion base NH4OH NH3 KOH(aq) + NH4Cl(aq)  KCl(aq) + NH3(g) + H2O(l)

Examples When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves Write a molecular equation for the gas-evolution that occurs when you mix aqueous hydrobromic acid and aqueous potassium sulfite

Oxidation-Reduction (Redox) Reactions Chapter 4: Reactions in Aqueous Solution 5/29/2018 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.

Oxidation-Reduction (Redox) Reactions Chapter 4: Reactions in Aqueous Solution Oxidation-Reduction (Redox) Reactions 5/29/2018 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 A monatomic ion has an oxidation number identical to its charge. Na1+ +1 Ca2+ +2 Al3+ +3 Cl1- -1 O2- -2 Copyright © 2008 Pearson Prentice Hall, Inc.

Oxidation-Reduction (Redox) Reactions Chapter 4: Reactions in Aqueous Solution 5/29/2018 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 Oxygen usually has an oxidation number of -2. H O H O O H 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. +1 -2 +1 +1 -1 -1 +1 Copyright © 2008 Pearson Prentice Hall, Inc.

Oxidation-Reduction (Redox) Reactions Chapter 4: Reactions in Aqueous Solution Oxidation-Reduction (Redox) Reactions 5/29/2018 3. Halogens usually have an oxidation number of -1. H Cl Cl O +1 -1 +1 -2 +1 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) A major exception is when the halogen is bonded to oxygen. x = +4 +1 x -2 Cr2O72- 2(x) + 7(-2) = -2 (net charge) x = +6 x -2 Copyright © 2008 Pearson Prentice Hall, Inc.

Example Determine the oxidation number for each atom in the following compounds/molecules CO2 CCl4 CoSO4 K2O2

Types of Redox reaction Combustion Reaction of nonmetal and metal 2 H2(g) + O2(g)  2 H2O(g) 2 Na(s) + Cl2(g)  2 NaCl(s)

Identifying Redox Reactions Chapter 4: Reactions in Aqueous Solution 5/29/2018 Identifying Redox Reactions Reduction: gaining one or more electron increasing in oxidation number  Oxidizing agent 2Fe2 3O2(g) + 4Fe(s) O3 (s) +3 oxidation reduction -2 Oxidation: losing one or more electrons decreasing in oxidation number  Reducing agent Copyright © 2008 Pearson Prentice Hall, Inc.

Oxidation and Reduction Another Definition oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction CH4 + 2 O2 → CO2 + 2 H2O -4 +1 0 +4 –2 +1 -2 oxidation reduction

Example Identify each of half reactions as 1) oxidation or 2) reduction. __A. Sn(s) Sn4+(aq) + 4e− __B. Fe3+(aq) + 1e− Fe2+(aq) __C. Cl2(g) + 2e− 2Cl-(aq)

Writing Oxidation and Reduction Reactions Write the separate half oxidation and reduction reactions for the following equation. 2Cs(s) + F2(g) 2CsF(s) 3 Na(l) + AlCl3(l)  3 NaCl(l) + Al(l)

Identify the Oxidizing and Reducing Agents in Each of the Following 3 H2S(aq) + 2 NO3– (aq) + 2 H+ (aq)  3 S(s) + 2 NO(g) + 4 H2O(l) MnO2(s) + 4 HBr(aq)  MnBr2(aq)+ Br2 (g)+ 2 H2O(l)

Combustion Reactions Reactions in which O2(g) is a reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidation-reduction reactions 2 C8H18(g) + 25 O2(g)  16 CO2(g) + 18 H2O(g) Tro, Chemistry: A Molecular Approach

Combustion Products Reactant Combustion Product contains C CO2(g) Reactions in which O2(g) is a reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidation-reduction reactions to predict the products of a combustion reaction, combine each element in the other reactant with oxygen Reactant Combustion Product contains C CO2(g) contains H H2O(g) contains S SO2(g) contains N NO(g) or NO2(g) contains metal M2On(s)

Practice – Complete the Reactions combustion of C3H7OH(l) combustion of CH3NH2(g)