Na 0 and Cl2 0 in 2 Na s Cl2 g ALWAYS 1Monatomic ions have an oxidation state

Na 0 and cl2 0 in 2 na s cl2 g always 1monatomic ions

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Na = 0 and Cl 2 = 0 in 2 Na( s ) + Cl 2 ( g ) ALWAYS! 1. Monatomic ions have an oxidation state equal to their charge. Na = +1 and Cl = –1 in NaCl 1. (a) Sum of oxidation states of all atoms in a “no- net charge” compound is 0. ALWAYS! Na = +1 and Cl = –1 in NaCl, (+1) + (–1) = 0
30 Rules for Assigning Oxidation States 3. (b) The sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion. N = +5 and O = −2 in NO 3 , (+5) + 3(−2) = −1 4. (a) Group I metals have an oxidation state of +1 in all their compounds. Na = +1 in NaCl (b) Group II metals have an oxidation state of +2 in all their compounds. Mg = +2 in MgCl 2
31 Rules for Assigning Oxidation States 5. In their compounds, nonmetals have oxidation states according to the table below. Nonmetals higher on the table take priority. Nonmetal Oxidation State Example F −1 CF 4 H +1 CH 4 O −2 CO 2 Group 7A −1 CCl 4 Group 6A −2 CS 2 Group 5A −3 NH 3
32 Determine the oxidation states of all the atoms in a propanoate ion, C 3 H 5 O 2 . Sum of oxidation states must be charge on the chemical species. Not a compound, is an ion. (C 3 ) + (H 5 ) + (O 2 ) = −1 Some elements have the same oxidation state in most of their compounds: H = +1 O = −2 The one unknown oxidation state is found using an algebraic expression: (C 3 ) + 5(+1) + 2(−2) = −1 (C 3 ) = −2 C = − Note: Unlike charges, oxidation states can be fractions!
33 Examples: Assign an oxidation state to each element in the following. Br 2 K + LiF CO 2 SO 4 2− Na 2 O 2
34 Oxidation and Reduction Another Definition Oxidation : an atom’s oxidation state increases during a reaction. Reduction: an atom’s oxidation state decreases during a reaction. CH 4 + 2 O 2 CO 2 + 2 H 2 O –4 +1 0 +4 –2 +1 –2 oxidation reduction
35 Oxidation–Reduction Oxidation and reduction must occur simultaneously. If an atom loses electrons another atom must take them. The reactant that reduces another element is the reducing agent . The reducing agent is or contains the element that is oxidized. The reactant that oxidizes another element is the oxidizing agent . The oxidizing agent is or contains the element that is reduced. 2 Na( s ) + Cl 2 ( g ) → 2 Na + Cl ( s ) Na is oxidized, Cl is reduced Na is the reducing agent, Cl 2 is the oxidizing agent
36 (i) Assign oxidation states (ii) determine elements oxidized and reduced (iii) determine oxidizing agent and reducing agent in the following reaction. Fe + MnO 4 + 4 H + → Fe 3+ + MnO 2 + 2 H 2 O 0 −2 +7 +1 +3 −2 +4 +1 −2 Oxidation Reduction Oxidizing Agent Reducing Agent
37 (i) Assign oxidation states, (ii) determine the elements oxidized and reduced, and (iii) determine the oxidizing agent and reducing agent in the following reaction.

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• Spring '08
• LEMASTER

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