Reduction- Oxidation Reactions (1) 213 PHC 8th lecture Dr. mona alshehri (1) Gary D. Christian, Analytical Chemistry, 6 th edition. 1

By the end of the lecture the student should be able to: Define oxidation, reduction, Define oxidizing agent, and reducing agent. Define Oxidation state. 2

WHAT IS A REDOX REACTION? It is a reaction occurs between an oxidizing agent and a reducing agent involves transfer of electrons. 3

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OXIDATIONREDUCTION A loss of electrons to give a higher oxidation state Zn  Zn e - A gain of electrons to give a lower oxidation state Cu e -  Cu 5

Oxidizing AgentReducing Agent Take on electrons Gets reduced Cu e -  Cu Give up electrons Gets oxidized Zn  Zn e - 6

Zn + Cu 2+  Zn 2+ + Cu    reducing oxidizing oxidizing reducing agent agent agent agent (reduced form) (oxidized form) (oxidized form) (reduced form) 7

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Oxidation state The oxidation state, often called the oxidation number, is an indication of the degree of oxidation of an atom in a chemical compound. Oxidation states are typically represented by integers, which can be positive, negative, or zero. 9

Rules: The oxidation state of a free element (monatomic or diatomic) is zero e.g. Zn, O 2, Cl 2 For a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion e.g. Cu 2+, I -, S 2- Hydrogen has an oxidation state of +1 and oxygen has an oxidation state of − 2 when they are present in most compounds. 10

The sum of oxidation states of all atoms in a neutral molecule must be zero e.g. Consider a neutral molecule of carbon dioxide, CO 2. Oxygen is assumed to have its usual oxidation state of − 2, and so the sum of the oxidation states of all the atoms can be expressed as: X + 2( − 2) = 0 X − 4 = 0 where X is the unknown oxidation state of carbon. Thus, it can be seen that the oxidation state of carbon in the molecule is +4 11

In polyatomic ions, the sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. e.g. consider the sulfate anion, which has the formula SO The total charge of this ion is − 2. Because all four oxygen atoms are assumed to have their usual oxidation state of − 2, and the sum of the oxidation states of all the atoms is equal to the charge of the ion, the sum of the oxidation states can be represented as: Y + 4( − 2) = − 2 Y − 8 = − 2 where Y is the unknown oxidation state of sulfur. Thus, it can be computed that Y = +6 12

CompoundOxidation state CH 4 -4 CCl 4 +4 CH 2 Cl 2 0 C2H6C2H6 -3 CHCl 3 +2 C2H2C2H2 13

Questions? 14

Homework: Calculate the oxidation state of the underlined atoms: Fe 3 O 4 N 2 H 4 O 3 S 2 O 3 2 − Cr(OH) 3 NH 4 + C 2 Cl 6 Na 2 SO 4 15

Summary: Definition of oxidation and reduction reactions. Definition of oxidizing and reducing agents. Definition of oxidation state. 16

Thank you 17