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Unit 11: Redox

Unit 11: Redox . Oxidation numbers. What is an Oxidation number?. Oxidation numbers are assigned to an atom in a molecule based on the distribution of electrons. Rules for assigning Oxidation Numbers. An atom in its elemental state has an oxidation number of 0.

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Unit 11: Redox

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  1. Unit 11: Redox Oxidation numbers

  2. What is an Oxidation number? • Oxidation numbers are assigned to an atom in a molecule based on the distribution of electrons.

  3. Rules for assigning Oxidation Numbers • An atom in its elemental state has an oxidation number of 0. • Ex) Mg(s)0 Al(s)0, H2(g)0, Br2(g)0 • The oxidation number of a monatomic ion is equal to the charge of the ion. • Ex) K+, Mg2+, Br-, S2- • In compounds, atoms of Group 1, 2, and aluminum have oxidation numbers of +1,+2,+3 respectively. • Ex) NaCl  Na+1Cl-1, • MgO  Mg+2O-2, • AlCl3 Al+3Cl3-1

  4. Rules for assigning Oxidation Numbers • The oxidation number of hydrogen in a compound is +1, unless it is combined with a metal atom, then it is -1. • Ex) HCl H+1Cl-1, CaH2 Ca+2H2-1 • The oxidation number of fluorine is always -1 • Ex) Ca+2F2-1, Na+1F-1 • The oxidation number of oxygen is -2, except with compounds of fluorine where it is +2, and peroxides such as H2O2 it is -1. • Ex) Mg+2O-2 • OF2 O+2F2-1

  5. Rules for Assigning Oxidation Numbers • The sum of all oxidation numbers in a compound is 0. • Ex) H2O H2+1O-2, (2)(+1) + (-2) = 0 • NaCl  Na+1Cl-1 (+1) + (-1) = 0 • The sum of all oxidation numbers in a polyatomic ion equals the charge of that ion. • Ex) NO3- (N+5O3-2)- (+5) + 3(-2) = -1 • OH- (O-2H+1)- (-2) + (+1) = -1

  6. Practice… • Assign oxidation numbers to the elements in each of the following: a) N2 b) NCl3 c) SF6 d) MgBr2 e) SO42- f) Mg2+ • Answers: • N20 • N+3Cl3-1 • S+6F6-1 • Mg+2Br2-1 • e) (S+6O4-2)2- • f) Mg2+

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