OxidationReductions: Part I: Oxidation Numbers from Chapter 5 Sec 1 of Brady

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Oxidation-ReductionsPart I Oxidation
Numbers(from Chapter 5 Sec 1of Brady Senese
5TH ed)

• Dr. C. Yau
• Fall 2009

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What are oxidation-reduction reactions?
"Oxidation-reduction" reactions are commonly
called "redox" reactions. Remember Acid-base
rxns involve a transfer of H or proton. Redox
rxns involve a transfer of electrons.
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Oxidation Numbers

• To understand redox, first you must learn to
  assign oxidation numbers.
• Oxidation numbers are hypothetical charges
  assigned to each atom.
• Each atom, even in molecular substances that have
  no ions, is assigned a charge.
• Note The charge is hypothetical. Oxidation
  numbers is only for "book keeping" to keep track
  of electrons.

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Assigning Oxidation Numbers
LEARN THESE RULES IN ORDER! 1. Oxidation of any
free element is zero. e.g. H in H2, P in P4 are
assigned zero. 2. Oxid of simple monatomic ions
is the charge of the ion. e.g. In MgCl2, Mg is
2. Cl is -1. In SnS, Sn is 2 and S is
-2.
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Assigning Oxidation Numbers
4. In its cmpd, F is assigned -1. 5. In its
cmpds, H is 1 unless it is bonded to a metal
where it is -1. e.g. In HCl, H is 1 and Cl is
-1.In MgH2, Mg is 2 and H is -1. 6. In its
cmpds, O is -2 unless it is a peroxide. eg. in
MgO, Mg is 2, O is -2. In hydrogen peroxide
(H2O2), H is 1 and O is -1.
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Assigning Oxidation Numbers

• 6. Oxidation of others are generally calculated
  from knowing that the sum of all charges must add
  up to charge of particle. For example,
• In HClO, sum of charges 0
• 1 ? -2
• H is assigned 1, O is assigned -2. Cl is
  calculated to be 1 in order for the sum to be
  zero.
• In the chlorite ion, ClO2-, sum of charges -1
• ? -2
• Oxygen is assigned -2, what must Cl be in order
  for net charge to be -1?
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  Ans. Cl 3

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• Example 5.2
• Titanium oxide, TiO2, is a white pigment used in
  making paint. A now outmoded process of making
  TiO2 from its ore involved Ti(SO4)2.
• What is the oxidation number of titanium in
  Ti(SO4)2?
• Example
• Determine the oxidation numbers of atoms in
  oxygen difluoride.
• When there is conflict, apply the rules starting
  from the top (Rule 1 and down).

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• Example 5.4
• The air bags used as safety devices in modern
  autos are inflated by the very rapid
  decomposition of the ionic compound sodium azide,
  NaN3. The rxn gives elemental sodium and gaseous
  nitrogen.
• What is the average oxidation of the nitrogen
  in sodium azide?
• How can we have a fractional charge?
• Remember these are hypothetical charges only.

Do Pract Exer 3, 4, 5 p.181
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Assign Oxidation

• a) SO42-
• b) NH4
• c) CoBr3
• d) Fe(NO3)2
• e) CrClO3
• f) V2(OH)3

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Oxidation is to identify oxidation reduction.

• Redox MUST have a change in oxidation numbers in
  at least two of the atoms.
• One MUST increase (oxidation).
• The other MUST decrease (reduction).
• H2 Cl2 2 HCl
• 0 0 1 -1
• Oxid of H increased from 0 to 1 (oxidation)
• Oxid of Cl decreased from 0 to -1 (reduction)
• We say H2 is oxidized and Cl is reduced.
• We say H2 has undergone oxidation and
• Cl has undergone reduction.
• An oxidizing agent causes others to be oxidized
  and is itself reduced. (Cl is reduced and
  therefore it is the oxidizing agent.)
• A reducing agent causes others to be reduced and
  is itself oxidized. (H is oxidized and therefore
  it is the reducing agent.)

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Recognizing Redox Rxns

• Example 5.5 p.181
• Is the following a redox rxn? If so, identify the
  substance oxidized and the substance reduced as
  well as the oxidizing and reducing agents.
• 2KCl MnO2 2H2SO4
• K2SO4 MnSO4 Cl2 2H2O

Do Pract Exer 6, 7, 8 on p.182.