Chapter 4 Chemical Reactions Contents and Concepts Ions

Chapter 4 Chemical Reactions Contents and Concepts Ions in Aqueous Solution 1. Ionic Theory of Solutions and Solubility Rules 2. Molecular and Ionic Equations Copyright Cengage Learning. All rights reserved. 4|2

Content A. Ions in aqueous solution a) ionic theory b) solubility rules c) molecular/ionic/net equations B. Types of chemical reactions

a) precipitatipon reactions b) acid-base reactions c) redox reactions C. Solutions a) molar concentration b) dilutions D.

Quantitaive analysis a) gravimetric analysis b) volumetric analysis Ions in aqueous solution An electrolyte dissolves to produce ions. The ions, as moving charges, complete the circuit. When a light bulb is attached to the circuit,

it shines. AB A+ + B- A strong electrolyte is an electrolyte that exists in solution

almost entirely as ions. AB A+ + B- A weak electrolyte is an electrolyte that dissolves in water to give a relatively small percentage of ions. As a result, the light bulb shines weakly.

Compounds that dissolve readily are said to be soluble. Compounds that dissolve very little are said to be insoluble. Precipitation reactions: a solid ionic substance forms from the mixture of two solutions of ionic substances . A precipitate is an insoluble solid compound formed during a chemical reaction in solution. Predicting Precipitation Reactions

1. Predict the products (exchange of parts). 2. Determine the state of each product: (s), (l), (g), (aq). Use solubility rules 3. If all products are aqueous (aq), no net reaction occurred. Solubility Rules 1. Group IA and ammonium compounds are 2.

3. soluble. Acetates and nitrates are soluble. Most chlorides, bromides, and iodides are soluble. Exceptions: 4. AgCl, Hg Cl , PbCl ;

2 2 2 AgBr, HgBr , Hg Br , PbBr ; 2 2 2 2 AgI, HgI , Hg I , PbI 2 22 2 Most sulfates are soluble. Exceptions:

CaSO , SrSO , BaSO , 4 4 4 Ag SO , Hg SO , PbSO 2 4 2 4 4 5. Most carbonates are insoluble. Exceptions:

6. Group IA carbonates and (NH ) SO 42 4 Most phosphates are insoluble. Exceptions: 7. Group IA phosphates and (NH ) PO 43 4 Most sulfides are insoluble.

Exceptions: 8. Group IA sulfides and (NH ) S 42 Most hydroxides are insoluble. Exceptions: Group IA hydroxides, Ca(OH) , Sr(OH) , Ba(OH) 2 2 2

Molecular Equation A chemical equation in which the reactants and products are written as if they were molecular substances, even though they may actually exist in solution as ions. State symbols are include: (s), (l), (g), (aq). For example: AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) Although AgNO3, NaCl, and NaNO3 exist as ions in aqueous solutions, they are written as compounds in the molecular equation.

Complete Ionic Equation A chemical equation in which strong electrolytes are written as separate ions in the solution. Other reactants and products are written in molecular form. State symbols are included: (s), (l), (g), (aq). For example: AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) In ionic form: Ag+(aq) + NO3(aq) + Na+(aq)Cl(aq) AgCl(s) + Na+(aq) + NO3(aq) Spectator Ion An ion in an ionic equation that does not take part

in the reaction. It appears as both a reactant and a product. Copyright Cengage Learning. All rights reserved. 4 | 14 Net Ionic Equation A chemical equation in which spectator ions are omitted. It shows the reaction that actually occurs at the ionic level. For example: Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq)

AgCl(s) + Na+(aq) + NO3(aq) In net ionic form: Ag+(aq) + Cl(aq) AgCl(s) ? 1. Decide whether the following reaction occurs. If it does, write the molecular, ionic, and net ionic equations. KBr + MgSO4 Determine the product formulas: K+ and SO 2 make K SO

4 2 4 Mg2+ and Br make MgBr 2 2. Determine whether the products are soluble: K SO is soluble 2 4 MgBr is soluble 2 KBr + MgSO no reaction

4 ? 1. Decide whether the following reaction occurs. If it does, write the molecular, ionic, and net ionic equations. NaOH + MgCl2 Determine the product formulas: Na+ and Cl make NaCl Mg2+ and OH make Mg(OH)

2. 2 Determine whether the products are soluble: NaCl is soluble Mg(OH) is insoluble 2 Molecular Equation (Balance the reaction and include state symbols) 2NaOH(aq) + MgCl2(aq) 2NaCl(aq) + Mg(OH)2(s)

Ionic Equation 2Na+(aq) + 2OH(aq) + Mg2+(aq) + 2Cl(aq) 2Na+(aq) + 2Cl(aq) + Mg(OH)2(s) Net Ionic Equation 2OH(aq) + Mg2+(aq) Mg(OH)2(s) Copyright Cengage Learning. All rights reserved. 4 | 18 ? 1. Decide whether the following reaction

occurs. If it does, write the molecular, ionic, and net ionic equations. K3PO4 + CaCl2 Determine the product formulas: K+ and Cl make KCl Ca2+ and PO 3 make Ca (PO ) 4 3 42 2. Determine whether the products are soluble:

KCl is soluble Ca (PO ) is insoluble 3 42 Molecular Equation (Balance the reaction and include state symbols) 2K3PO4(aq) + 3CaCl2(aq) 6KCl(aq) + Ca3(PO4)2(s) Ionic Equation 6K+(aq) + 2PO43(aq) + 3Ca2+(aq) + 6Cl(aq) 6K+(aq) + 6Cl(aq) + Ca3(PO4)2(s) Net Ionic Equation

2PO43(aq) + 3Ca2+(aq) Ca3(PO4)2(s) Copyright Cengage Learning. All rights reserved. 4 | 20 Acidbase reactions: reactions that involve the transfer of a proton (H +) between reactants. Arrhenius Acid-base definitions Arrhenius Acid A substance that produces hydrogen ions, H+,

when it dissolves in water. Arrhenius Base A substance that produces hydroxide ions, OH , when it dissolves in water. Brnsted-lowry Acid-base definitions BrnstedLowry Acid The species (molecule or ion) that donates a proton to another species in a protontransfer reaction.

BrnstedLowry Base The species (molecule or ion) that accepts a proton from another species in a protontransfer reaction. Household acids and bases Hydrochloric acid AcidBase Indicator A dye used to distinguish between an acidic and basic solution by means of the color changes it undergoes in these solutions.

The sample beakers show a red cabbage indicator in beakers varying in acidity from highly acidic (left) to highly basic (right). Acid Nomenclature If the anion in the acid ends in -ide, change the ending to -ic acid and add the prefix hydro-.

If the anion ends in -ite, change the ending to -ous acid. HClO: hypochlorous acid HClO : chlorous acid 2

If the anion ends in -ate, change the ending to -ic acid. HClO : chloric acid 3 HClO : perchloric acid 4 HCl: hydrochloric acid HBr: hydrobromic acid

HI: hydroiodic acid Strong Acid An acid that ionizes completely (or at least 1 proton equivalent for polyprotic acids) in water. It is present entirely as ions; it is a strong electrolyte. Common strong acids: HNO3 H2SO4 HClO4 HCl HBr

HI Weak Acid An acid that only partly ionizes in water. It is present primarily as molecules and partly as ions; it is a weak electrolyte. If an acid is not strong, it is weak. In Figure A, a solution of HCl (a strong acid) illustrated on a molecular/ionic level,

shows the acid as all ions. In Figure B, a solution of HF (a weak acid) also illustrated on a molecular/ionic level, shows mostly molecules with very few ions. Strong Base A base that ionizes completely in water. It is present entirely as ions; it is a strong electrolyte. Common strong bases: LiOH

NaOH KOH Ca(OH)2 Sr(OH)2 Ba(OH)2 Weak Base A base that is only partly ionized in water. It is present primarily as molecules and partly as ions; it is a weak electrolyte. These are often nitrogen bases such as NH3:

NH3(aq) + H2O(l) NH4+(aq) + OH(aq) If a base is not strong, it is weak. ? Classify the following as strong or weak acids or bases: a. KOH b. H2S c. CH3NH2 d. HClO4 a.

KOH is a strong base. b. H S is a weak acid. 2 CH NH is a weak base. 3 2 HClO is a strong acid. 4 c. d.

Copyright Cengage Learning. All rights reserved. 4 | 32 Polyprotic Acid An acid that results in two or more acidic hydrogens per molecule. For example: H2SO4, sulfuric acid H3PO4, phosphoric acid Neutralization Reaction

A reaction of an acid and a base that results in an ionic compound (a salt) and possibly other product(s). Other product(s) are dependent on the nature of the base Base Other product(s) in addition to a salt OH- H2O

S2- H2S SO32- H2O + SO2 ( H2SO3 unstable) CO32- H2O + CO2 (H2CO3 unstable) ?

Write the molecular, ionic, and net ionic equations for the complete neutralization of sulfurous acid, H2SO3, by potassium hydroxide, KOH. Copyright Cengage Learning. All rights reserved. 4 | 35 Molecular Equation (Balance the reaction and include state symbols) H2SO3(aq) + 2KOH(aq) 2H2O(l) + K2SO3(aq)

Ionic Equation H2SO3(aq) + 2K+(aq) + 2OH(aq) 2H2O(l) + 2K+(aq) + SO32(aq) Net Ionic Equation H2SO3(aq) + 2OH(aq) 2H2O(l) + SO32(aq) AcidBase Reaction with Gas Formation Some salts, when treated with an acid, produce a gas. Typically sulfides, sulfites, and carbonates behave in this way producing hydrogen sulfide, sulfur trioxide, and

carbon dioxide, respectively. The photo to the right shows baking soda (sodium hydrogen carbonate) reacting with acetic acid in vinegar to give bubbles of carbon dioxide. Acidbase reactions with gasformation Sulfides, carbonates, sulfites react with acid to form a gas. Na2S(aq) + 2HCl(aq) 2NaCl(aq) + H2S(g) Na2CO3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + CO2(g) Na2SO3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + SO2(g)

? Write the molecular, ionic, and net ionic equations for the reaction of copper(II) carbonate with hydrochloric acid. Copyright Cengage Learning. All rights reserved. 4 | 39 Molecular Equation (Balance the reaction and include state symbols)

CuCO3(s) + 2HCl(aq) CuCl2(aq) + H2O(l) + CO2(g) Ionic Equation CuCO3(s) + 2H+(aq) + 2Cl(aq) Cu2+(aq) + 2Cl(aq) + H2O(l) + CO2(g) Net Ionic Equation CuCO3(s) + 2H+(aq) Cu2+(aq) + H2O(l) + CO2(g) Copyright Cengage Learning. All rights reserved. 4 | 40 Oxidation-reduction (redox) reactions

Reduction: gain of electrons Oxidation: loss of electrons Oxidation Number For a monatomic ion, the actual charge of the atom or a hypothetical charge assigned to the atom in the substance using simple rules. Assume ionic bonding in all compounds with the more Rules for Assigning Oxidation Numbers 1. Elements: The oxidation number of an atom in an element is zero.

2. Monatomic ions: The oxidation number of an atom in a monatomic ion equals the charge on the ion. 3. Oxygen: The oxidation number of oxygen is 2 in most of its compounds. (An exception is O in H2O2 and other peroxides, where the oxidation number is 1.) 4. Hydrogen: The oxidation number of hydrogen is +1 in most of its compounds. (The oxidation number of hydrogen is 1 in binary compounds with a metal such as CaH2.) 5. Halogens: The oxidation number of fluorine is

1. Each of the other halogens (Cl, Br, I) has an oxidation number of 1 in binary compounds, except when the other element is another halogen above it in the periodic table or the other element is oxygen. 6. Compounds and ions: The sum of the oxidation numbers of the atoms in a compound is zero. The sum of the oxidation numbers of the atoms in a polyatomic ion equals the charge on the ion. Copyright Cengage Learning. All rights reserved. 4 | 43

? Potassium permanganate, KMnO4, is a purplecolored compound; potassium manganate, K2MnO4, is a greencolored compound. Obtain the oxidation numbers of the manganese in these compounds. K Mn

1(+1) + 1(oxidation number of Mn) + 4(2) = 0 1 + 1(oxidation number of Mn) + (8) = 0 (7) + (oxidation number of Mn) = 0 Oxidation number of Mn = +7 O K Mn O 2(+1) + 1(oxidation number of Mn) + 4(2) = 0 2 + 1(oxidation number of Mn) + (8) = 0 (6) + (oxidation number of Mn) = 0 Oxidation number of Mn = +6 In KMnO4, the oxidation number of Mn is +7.

In K2MnO4, the oxidation number of Mn is +6. Copyright Cengage Learning. All rights reserved. 4 | 45 ? What is the oxidation number of Cr in dichromate, Cr2O72? Cr

O 2(oxidation number of Cr) + 7(2) = 2 2(oxidation number of Cr) + (14) = 2 2(oxidation number of Cr) = +12 Oxidation number of Cr = +6 Copyright Cengage Learning. All rights reserved. 4 | 46 Halfreaction One of two parts of an oxidationreduction

reaction, one part of which involves a loss of electrons (or increase in oxidation number) and the other part of which involves a gain of electrons (or decrease in oxidation number). Oxidation The halfreaction in which there is a loss of electrons by a species (or an increase in oxidation number). Reduction The halfreaction in which there is a gain of electrons by a species (or a decrease in oxidation

number). Oxidizing Agent A species that oxidizes another species; it is itself reduced. Reducing Agent A species that reduces another species; it is itself oxidized. Common OxidationReduction Reactions 1. 2.

3. 4. Combination reaction Decomposition reaction Displacement reaction Combustion reaction Combination Reaction A reaction in which two substances combine to form a third substance. For example: 2Na(s) + Cl2(g) 2NaCl(s)

Decomposition Reaction A reaction in which a single compound reacts to give two or more substances. For example: 2HgO(s) 2Hg(l) + O2(g) Displacement Reaction A reaction in which an element reacts with a compound, displacing another element from it. For example:

Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq) The Activity Series (metals and hydrogen) -Ordering of metals and hydrogen by ease of oxidation establishes the activity series -A more active (easily oxidized) metal will displace a less active one in a chemical reaction Copyright Cengage Learning. All rights reserved. 4 | 55 Combustion Reaction

A reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame. Copyright Cengage Learning. All rights reserved. 4 | 56 For example: 4Fe(s) + 3O2(g) 2Fe2O3(s) Copyright Cengage Learning. All rights reserved.

4 | 57 Balancing Simple OxidationReduction Reactions: HalfReaction Method First, identify what is oxidized and what is reduced by determining oxidation numbers. For the reaction Zn(s) + Ag+(aq) Zn2+(aq) + Ag(s) 0 +1 +2 0 Zn is oxidized from 0 to +2. Ag+ is reduced from +1 to 0.

Next, write the unbalanced halfreactions. Zn(s) Zn2+(aq) (oxidation) Ag+(aq) Ag(s) (reduction) Now, balance the charge in each half reaction by adding electrons. Zn(s) Zn2+(aq) + 2e (oxidation) e + Ag+(aq) Ag(s) (reduction) Copyright Cengage Learning. All rights reserved. 4 | 60 Since the electrons lost in oxidation are the same

as those gained in reduction, we need each halfreaction to have the same number of electrons. To do this, multiply each halfreaction by a factor so that when the halfreactions are added, the electrons cancel. Zn(s) Zn2+(aq) + 2e (oxidation) 2e + 2Ag+(aq) 2Ag(s) (reduction) Lastly, add the two halfreactions together. Zn(s) + 2Ag+(aq) Zn2+(aq) + 2Ag(s) ?

Balance the following oxidationreduction reaction: FeI3(aq) + Mg(s) Fe(s) + MgI2(aq) The oxidation numbers are given below the reaction. +3 1 FeI (aq) + Mg(s) Fe(s) + MgI (s) 3

2 0 0 +2 1 Now, write the halfreactions. Since Iodide is a spectator ion it is omitted at this point. Mg(s) Mg2+(aq) (oxidation) Fe3+(aq) Fe(s)

(reduction) Balancing the halfreactions: Mg(s) Mg2+(aq) + 2e (oxidation) Fe3+(aq) + 3e Fe(s) (reduction) Multiply the oxidation halfreaction by 3 and the reduction halfreaction by 2. 3Mg(s) 3Mg2+(aq) + 6e (oxidation) 2Fe3+(aq) + 6e 2Fe(s) (reduction) Add the halfreactions together.

2Fe3+(aq) + 3Mg(s) 2Fe(s) + 3Mg2+(aq) Now, return the spectator ion, I. 2FeI3(aq) + 3Mg(s) 2Fe(s) + 3MgI2(aq) Molar Concentration, Molarity, (M) Moles of solute per liter of solution moles of solute Molarity (M ) liters of solution To prepare a solution, add the measured amount of solute to a volumetric flask, then add water to

bring the solution to the mark on the flask. Dilution When a higher concentration solution is used to make a lessconcentration solution, the moles of solute are determined by the amount of the higherconcentration solution. The number of moles of solute remains constant. MiVi = MfVf Note: The units on Vi and Vf must match. Copyright Cengage Learning. All rights reserved.

4 | 68 Diluting a solution quantitatively requires specific glassware. The photo at the right shows a volumetric flask used in dilution.

? You place a 1.52g of potassium dichromate, K2Cr2O7, into a 50.0mL volumetric flask. You then add water to bring the solution up to the mark on the neck of the flask. What is the molarity of K2Cr2O7 in the solution? Molar mass of K Cr O is 294 g. 2 2 7 1 mol

1.52 g 294 g 3 50.0 10 L 0.103 M ? A solution of sodium chloride used for intravenous transfusion (physiological saline solution) has a concentration of

0.154 M NaCl. How many moles of NaCl are contained in 500.mL of physiological saline? How many grams of NaCl are in the 500.mL of solution? mol M L 0.154 M 0.500 L 0.0770 mol NaCl Molar mass NaCl 58.4 g 58.4 g 0.0770 mol 1 mol 4.50 g NaCl

? A saturated stock solution of NaCl is 6.00 M. How much of this stock solution is needed to prepare 1.00L of physiological saline soluiton (0.154 M)? M iVi M fVf M fVf Vi Mi Copyright Cengage Learning. All rights reserved.

(0.154 M )(1.00 L) Vi 6.00 M Vi 0.0257 L or 25.7 mL 4 | 73 Quantitative Analysis The determination of the amount of a substance or species present in a material. Gravimetric Analysis

A type of quantitative analysis in which the amount of a species in a material is determined by converting the species to a product that can be isolated completely and weighed. Copyright Cengage Learning. All rights reserved. 4 | 75 The figure on the right shows the reaction of Ba(NO3)2 with K2CrO4 forming

the yellow BaCrO4 precipitate. The BaCrO4 precipitate is being filtered in the figure on the right. It can then be dried and weighed. Copyright Cengage Learning. All rights reserved. 4 | 77

? A soluble silver compound was analyzed for the percentage of silver by adding sodium chloride solution to precipitate the silver ion as silver chloride. If 1.583 g of silver compound gave 1.788 g of silver chloride, what is the mass percent of silver in the compound? Copyright Cengage Learning. All rights reserved.

4 | 78 Molar mass of silver chloride (AgCl) = 143.32 g 1mol AgCl 1mol Ag 107.9 g Ag 1.788 g AgCl 143.32 g AgCl 1mol AgCl 1mol Ag = 1.346 g Ag in the compound

1.346 g Ag 100% 1.583 g silver compound = 85.03% Ag Copyright Cengage Learning. All rights reserved. 4 | 79 Titration A procedure for determining the amount of substance A by adding a carefully measured volume with a known concentration of B until the

reaction of A and B is just complete. Copyright Cengage Learning. All rights reserved. 4 | 80 In the titration above, the indicator changes color to indicate when the reaction is just complete. Volumetric Analysis A type of quantitative analysis based on titration. ?

Zinc sulfide reacts with hydrochloric acid to produce hydrogen sulfide gas: ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g) How many milliliters of 0.0512 M HCl are required to react with 0.392 g ZnS? Copyright Cengage Learning. All rights reserved. 4 | 83 Molar mass of ZnS = 97.45 g

1mol ZnS 2 mol HCl 1L solution 0.392 g ZnS 97.45 g ZnS 1mol ZnS 0.0512 mol HCl = 0.157 L = 157 mL HCl solution Copyright Cengage Learning. All rights reserved. 4 | 84

? A dilute solution of hydrogen peroxide is sold in drugstores as a mild antiseptic. A typical solution was analyzed for the percentage of hydrogen peroxide by titrating it with potassium permanganate: 5H O (aq) + 2KMnO (aq) + 6H+(aq) 2 2 4

8H O(l) + 5O (g) + 2K+(aq) + 2Mn2+(aq) 2 2 What is the mass percent of H O in a solution if 57.5 g of solution required 2 2 38.9 mL of 0.534 M KMnO for its titration? 4 Copyright Cengage Learning. All rights reserved. 4 | 85 Molar mass of H O = 34.01 g 2 2

0.534 mol KMnO4 5 mol H2O2 34.01g H2O2 38.9 10 L 1L 2 mol KMnO4 1mol H2O2 3 = 1.77 g H O

2 2 1.77 g H 2 O 2 100% 57.5 g solution = 3.07% H O 2 2

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