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Chapter 4:

Reactions in Aqueous Solution

CHM 165 - Spring 2014

Looking ahead.

Different forms of reactions

Precipitation Reactions

Acid Base Reactions

Oxidation Reduction Reactions

Electrolytes & nonelectrolytes

Redox (Oxidation Reduction) Reactions

Redox titrations

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Some Ways That Chemical Reactions Occur

Precipitation Reactions: Processes in which soluble reactants yield an insoluble solid product

2KNO3(aq) + PbI2(s) Pb(NO3)2(aq) + 2KI(aq)

Acid-Base Neutralization Reactions: acid reacts with a base to yield water plus an ionic compound called a salt

H2O(l) + NaCl(aq) HCl(aq) + NaOH(aq)

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)

Electrolytes in Aqueous Solution

Electrolytes: Substances which dissolve in water to produce conducting solutions of ions

H2O

Na+(aq) + Cl-(aq) NaCl(s)

Nonelectrolytes: Substances which do not produce ions in aqueous solutions

H2O

C12H22O11(aq) C12H22O11(s)

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Electrolytes in Aqueous Solution

Weak Electrolytes: Compounds that dissociate to a small extent into ions when dissolved in water

H+(aq) + CH3CO2-(aq) CH3CO2H(aq)

Strong Electrolytes: Compounds that dissociate to a large extent into ions when dissolved in water

K+(aq) + Cl-(aq) KCl(aq)

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Electrolytes in Aqueous Solution

Strong Acids:

Ionic Compounds Weak acids

Molecular Compounds

Aqueous Reactions and Net Ionic Equations

2KNO3(aq) + PbI2(s) Pb(NO3)2(aq) + 2KI(aq)

Molecular Equation: All substances in the chemical equation are written using their complete formulas as if they were molecules.

strong electrolytes precipitate

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Ionic Equation: All of the strong electrolytes are written as ions.

2K+(aq) + 2NO3-(aq) + PbI2(s)

2KNO3(aq)

Pb(NO3)2(aq) 2KI(aq)

Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq)

Spectator Ions: Ions that undergo no change during the reaction and appear on both sides of the reaction arrow.

Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq)

2K+(aq) + 2NO3-(aq) + PbI2(s)

PbI2(s) Pb2+(aq) + 2I-(aq)

Net Ionic Equation: Only the ions undergoing change are shown.

Precipitation Reactions and Solubility Guidelines

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Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO3 and Na2CO3 are mixed.

Ag2CO3(s) 2Ag+(aq) + CO3

2-(aq)

Ag2CO3(s) + 2Na+(aq) + 2NO3

-(aq)

2Ag+(aq) + 2NO3- (aq) + 2Na+(aq) + CO3

2-(aq)

Acids, Bases, and Neutralization Reactions

H3O+(aq) + Cl-(aq) HCl(aq) + H2O(aq)

Acid (Arrhenius): A substance that dissociates in water to produce hydrogen ions, H+:

H+(aq) + A-(aq) HA(aq)

In water, acids produce hydronium ions, H3O+:

H+(aq) + Cl-(aq) HCl(aq)

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Acids, Bases, and Neutralization Reactions

Ammonia is weakly basic because it reacts to a small extent with water to yield ammonium and hydroxide ions:

M+(aq) + OH-(aq) MOH(aq)

Na+(aq) + OH-(aq) NaOH(aq)

NH4+(aq) + OH-(aq) NH3(aq) + H2O(aq)

Base (Arrhenius): A substance that dissociates in water to produce hydroxide ions, OH-:

Acids, Bases, and Neutralization Reactions

Weak acids and weak bases are weak electrolytes.

Strong acids and strong bases are strong electrolytes.

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Acids, Bases, and Neutralization Reactions

ate ic

ite ous

Acids, Bases, and Neutralization Reactions

MA(aq) + H2O(l) HA(aq) + MOH(aq)

These acid-base neutralization reactions are double-replacement reactions just like the precipitation reactions:

Water Acid Base Salt

E.g.s 1. Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH)2.

2. Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF.

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Oxidation-Reduction (Redox) Reactions

Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)

MnO4-(aq) + 5Fe2+(aq) + 8H+(aq)

2MgO(s) 2Mg(s) + O2(g)

Oxidation: The loss of one or more electrons by a substance whether element, compound or ion Reduction: The gain of one or more electrons by a substance Oxidation-Reduction (Redox) Reaction: Any process in which electrons are transferred from one substance to another

1. An atom in its elemental state has an oxidation number of 0.

Rules for Assigning Oxidation Numbers

Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron-poor

Na H2 Br2 S Ne

Oxidation number 0

2. An atom in a monatomic ion has an oxidation number

identical to its charge.

Na+

+1

Ca2+

+2

Al3+

+3

Cl-

-1

O2-

-2

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b) Oxygen usually has an oxidation number of -2.

H O 1-

-2 +1

H H Ca

-1 -1 +2

3. 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.

a) Hydrogen can be either +1 or -1.

O H O

-1 +1 -1

H H O

+1 +1 -2

H

+1

c) Halogens usually have an oxidation number of -1.

H Cl +1 -1

Cr2O72-

-2 +1 ?

4. The sum of the oxidation numbers is 0 for a neutral

compound and is equal to the net charge for a polyatomic

ion.

? = +6

2(?) + 7(-2) = -2 (net charge)

H2SO4

? -2

? = +6

2(+1) + (?) + 4(-2) = 0 (net charge)

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Identifying Redox Reactions

Reducing Agent

Causes reduction Loses one or more electrons Undergoes oxidation Oxidation number of atom increases

Oxidizing Agent

Causes oxidation Gains one or more electrons Undergoes reduction Oxidation number of atom decreases

Reducing Agent

Oxidizing Agent

2Fe2 3O2(g) + 4Fe(s) O3 (s)

+3 0

oxidation

reduction

-2 0

The Activity Series of the Elements

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.

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The Activity Series of the Elements

Reaction of an aqueous cation (metal ion) with a free element

Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)

Balancing Redox Reactions: The Half-Reaction Method

Balance the following net ionic equation in acidic solution:

Cr3+(aq) + Cl2(aq) Cr2O72-(aq) + Cl-(aq)

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Write the two unbalanced half-reactions.

Cl2(aq) Cl-(aq)

Cr3+(aq) Cr2O72-(aq)

Balance both half-reactions for all atoms except O and H.

Cl2(aq) 2Cl-(aq)

2Cr3+(aq) Cr2O72-(aq)

Balance each half-reaction for O by adding H2O, and then balance for H by adding H+.

Cl2(aq) 2Cl-(aq)

2Cr3+(aq) + 7H2O(l) 14H+(aq) + Cr2O7

2-(aq)

Balance each half-reaction for charge by adding electrons to the side with greater positive charge.

Cl2(aq) + 2e- 2Cl-(aq)

2Cr3+(aq) + 7H2O(l) 6e- + 14H+(aq) + Cr2O7

2-(aq) reduction:

oxidation:

oxidation:

reduction:

Cl2(aq) + 2e- 3 2Cl-(aq)

2Cr3+(aq) + 7H2O(l) 6e- + 14H+(aq) + Cr2O7

2-(aq)

2Cr3+(aq) + 7H2O(l) + 3Cl2(aq)

14H+(aq) + Cr2O72-(aq) + 6Cl-(aq)

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Redox Titrations

5H2C2O4(aq) + 2MnO4-(aq) + 6H+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

Redox titration (also called oxidation-reduction titration) is a type of titration based on a redox reaction between the analyte and titrant

Chapter 4/28 2012 Pearson Education, Inc.

Redox Titrations

5H2C2O4(aq) + 2MnO4-(aq) + 6H+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

A solution is prepared with 0.2585 g of oxalic acid, H2C2O4. 22.35 mL of an unknown solution of potassium permanganate are needed to titrate the solution. What is the concentration (molarity) of the potassium permanganate solution?

Moles of H2C2O4

Mass of H2C2O4

Moles of KMnO4

Molarity of KMnO4

Mole Ratio Molarity of KMnO4

Molar Mass of H2C2O4

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5H2C2O4(aq) + 2MnO4-(aq) + 6H+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

Moles of H2C2O4 available:

90.04 g

1 mol = 0.002871 mol H2C2O4

0.2585 g H2C2O4

Moles of KMnO4 reacted:

5 mol H2C2O4

2 mol KMnO4 = 0.001148 mol KMnO4

0.002871 mol H2C2O4

x

x

1 L

1000 mL

Concentration of KMnO4 solution:

= 0.05136 M KMnO4 22.35 mL

x 0.001148 mol KMnO4