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2. Redox Reactions Section 19.1Oxidation and Reduction Section 19.2Balancing Redox Equations Click a hyperlink or folder tab to view the corresponding slides. Exit Chapter Menu

3. 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

4. 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

5. 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

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

7. Electron Transfer and Redox Reactions (cont.) • Oxidationis 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Redox and Electronegativity (cont.) Section 19-1

15. 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

16. Determining Oxidation Numbers (cont.) Section 19-1

17. 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

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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

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

27. 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

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

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