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Section 19.1 Oxidation and Reduction Describe the processes of oxidation and reduction. spectator ion: an ion that does not participate in a reaction and is not usually shown in an ionic equation Identify oxidizing and reducing agents. Determine the oxidation number of an element in a compound. Interpret redox reactions in terms of change in oxidation state. Section 19-1

Section 19.1 Oxidation and Reduction (cont.) oxidation-reduction reaction redox reaction oxidation reduction oxidizing agent reducing agent Oxidation and reduction are complementary—as an atom is oxidized, another atom is reduced. Section 19-1

Electron Transfer and Redox Reactions An oxidation-reduction reaction, or redox reaction involves the transfer of electrons from one atom to another. Section 19-1

Electron Transfer and Redox Reactions (cont.) Section 19-1

Electron Transfer and Redox Reactions (cont.) Oxidation is defined as the loss of electrons from atoms of a substance. Na → Na+ + e– Reduction is defined as the gain of electrons by the atoms of a substance. Cl2 + 2e– → 2Cl– Section 19-1

Electron Transfer and Redox Reactions (cont.) The oxidation number of an atom in an ionic compound is the number of electrons lost or gained by the atom when it forms an ion. When an atom or ion is reduced, the numerical value of its oxidation number decreases. When an atom or ion is oxidized, its oxidation number increases. Section 19-1

Electron Transfer and Redox Reactions (cont.) Oxidation numbers are tools that scientists use to keep track of the movement of electrons in a redox reaction. Section 19-1

Oxidizing and Reducing Agents The substance that oxidizes another substance by accepting its electrons is called an oxidizing agent. The oxidizing agent is the substance that is reduced in a redox reaction. Section 19-1

Oxidizing and Reducing Agents (cont.) The substance that reduces another substance by losing its electrons is the reducing agent. The reducing agent is the substance that is oxidized in a redox reaction. Section 19-1

Redox and Electronegativity Redox reactions are not limited to atoms of an element changing to ions. Some redox reactions involve changes in molecular substances or polyatomic ions. N2(g) +3H2(g) → NH3(g) N is reduced and H is oxidized. Section 19-1

Redox and Electronegativity (cont.) To determine which was oxidized and which was reduced, you must know which atom is more electronegative. Elements with high electronegativity are strong oxidizing agents. Section 19-1

Redox and Electronegativity (cont.) Section 19-1

Determining Oxidation Numbers To understand all types of redox reactions, the oxidation number of the atoms involved in the reaction must be determined. Section 19-1

Determining Oxidation Numbers (cont.) Section 19-1

Oxidation Numbers in Redox Reactions Oxidation-reduction reactions are changes in oxidation number. Atoms that are reduced have their oxidation number decreased. Atoms that are oxidized have their oxidation number increased. Section 19-1

Oxidation Numbers in Redox Reactions (cont.) Section 19-1

A B C D Section 19.1 Assessment In a redox reaction, the reducing agent is: A. the substance that is reduced B. the substance that is oxidized C. the substance that gains electrons D. none of the above A B C D Section 19-1

A B C D Section 19.1 Assessment In redox reactions, more electronegative elements tend to: A. be reduced B. be reducing agents C. lose electrons D. not change A B C D Section 19-1

End of Section 19-1

Section 19.2 Balancing Redox Reactions Relate changes in oxidation number to the transfer of electrons. net ionic equation: an ionic equation that includes only the particles that participate in the reaction Use changes in oxidation number to balance redox equations. Balance net ionic redox equations using the oxidation-number method. Section 19-2

Section 19.2 Balancing Redox Reactions (cont.) oxidation-number method species half-reaction Redox equations are balanced when the total increase in oxidation numbers equals the total decrease in oxidation numbers of the atoms involved in the reaction. Section 19-2

The Oxidation-Number Method Chemical equations must be balanced to show the correct quantities of reactants and products. The number of electrons transferred from atoms must equal the number of electrons accepted by other atoms. Section 19-2

The Oxidation-Number Method (cont.) The total increase in oxidation numbers must equal the total decrease in oxidation numbers in the reaction. This method is called the oxidation number method. Section 19-2

Balancing Net Ionic Redox Equations Sometimes it is preferred to express redox reactions in the simplest possible terms, showing only the oxidation and reduction processes. When balancing equations in acidic solution, hydrogen ions (H+) or water molecules can be added to either side of the equation. When balancing equations in basic solution, hydroxide ions (OH–) or water molecules can be added to either side of the equation. Section 19-2

Balancing Redox Equations Using Half-Reactions In chemistry, a species is any kind of chemical unit involved in a process. Oxidation-reduction reactions occur whenever a species that can give up electrons comes in contact with another species that can accept them. Section 19-2

Balancing Redox Equations Using Half-Reactions (cont.) A half-reaction is one of the two parts of a redox reaction—the oxidation half of the reduction half. Section 19-2

Balancing Redox Equations Using Half-Reactions (cont.) Section 19-2

A B C D Section 19.2 Assessment A redox reaction split into two parts is called ____. A. net reaction B. oxidation-reaction C. half-reaction D. reduction-reaction A B C D Section 19-2

A B C D Section 19.2 Assessment In acid solution, what would you use to balance the charge in a redox-reaction? A. electrons B. hydrogen ions and water C. hydroxide ions and water D. hydrogen ions and hydroxide ions A B C D Section 19-2

End of Section 19-2

Standardized Test Practice Image Bank Concepts in Motion Chemistry Online Study Guide Chapter Assessment Standardized Test Practice Image Bank Concepts in Motion Resources Menu

Section 19.1 Oxidation and Reduction Key Concepts Oxidation-reduction reactions involve the transfer of electrons from one atom to another. When an atom or ion is reduced, its oxidation number is lowered. When an atom or ion is oxidized, its oxidation number is raised. In oxidation-reduction reactions involving molecular compounds (and polyatomic ions with covalent bonds), the more-electronegative atoms are treated as if they are reduced. The less-electronegative atoms are treated as if they are oxidized. Study Guide 1

Section 19.1 Oxidation and Reduction (cont.) Key Concepts Study Guide 1

Section 19.2 Balancing Redox Equations Key Concepts Redox equations in which the same element appears in several reactants and products can be difficult to balance using the conventional method. The oxidation-number method is based on the number of electrons transferred from atoms equaling the number of electrons accepted by other atoms. To balance equations for reactions in an acid solution, add enough hydrogen ions and water molecules to balance the equation. Study Guide 2

Section 19.2 Balancing Redox Equations (cont.) Key Concepts To balance equations for reactions in a basic solution, add enough hydroxide ions and water molecules to balance the equation. A half-reaction is one of the two parts of a redox reaction. Study Guide 2

What type of reaction involves the transfer of electrons from one atom to another? A. synthesis B. decomposition C. double replacement D. redox A B C D Chapter Assessment 1

Less electronegative atoms in redox reactions are most often ____. A. oxidized B. reduced C. oxidizing agents D. neutral A B C D Chapter Assessment 2

A B C D Any chemical unit involved in a process is called a(n) ____. A. atom B. type C. species D. ion A B C D Chapter Assessment 3

A B C D Determine the oxidation number of oxygen in HNO3. A. +3 B. –3 Chapter Assessment 4

A B C D Determine the oxidation number of nitrogen in HNO3. A. +1 Chapter Assessment 5

In the reaction MgI2 + Br2 → MgBr2 + I2, what is the oxidation number of I2? STP 1

A B C D Which is NOT an oxidizing agent in a redox reaction? A. substance reduced B. electron acceptor C. oxidizer of another substance D. electron donor A B C D STP 2

How does the oxidation number change in sodium in the following equation? 2NaI(aq) + Cl2(aq) → 2NaCl(aq) + I2(aq) A. It changes from 0 to –1. B. It changes from –1 to 0. C. It changes from 2 to –2. D. no change A B C D STP 3

If the temperature in a system is held constant, the pressure of a gas must do what when the volume is increased? A. no change B. increase C. decrease D. unable to determine A B C D STP 4

A B C D What is the oxidizing agent in the following equation? Na2SO4 + 4C → Na2S + 4CO A. C B. S C. O D. Na A B C D STP 5

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Table 19.1 Summary of Redox Reactions Figure 19.2 Redox Reaction Table 19.1 Summary of Redox Reactions CIM

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