Oxidation-Reduction Reactions Chemistry Chapter 20

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Oxidation-Reduction ReactionsChemistryChapter 20
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What are Oxidation Reduction?(a history lesson)

• Oxidation originally was defined as a process
  (reaction) in which a substance gains oxygen
  (oxide).

CH4 2O2 -gt CO2 2H2O
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What are Oxidation Reduction?(a history lesson
Cont)

• No oxidation can occur without another substance
  losing oxygen the substance losing oxygen is
  reduced (reduction).
• Reactions that involve the transfer of oxygen
  were known as oxidation-reduction reactions (or
  redox reactions).
• Oxidation and reduction occur simultaneously

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Electron Shift in Redox Reactions

• The MODERN concept of oxidation / reduction
  also includes reactions that do not include
  oxygen.
• Redox reactions are currently defined as a
  reaction that involves the transfer of electrons
  between reactants.
• Oxidation is the complete or partial loss of
  electrons or gain of oxygen.
• Reduction is the complete or partial gain of
  electrons or loss of oxygen.

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Oxidation
Reduction
Loss of electrons (Gain of oxygen)
Gain of electrons (Loss of oxygen)
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Redox Reactions That Form Ions(Ionic Compounds)

• In metal / nonmetal reactions, electrons are
  transferred from the metal atom to the nonmetal.

• Losing electrons is oxidation. Gaining electrons
  is reduction.
• The substance that loses electrons is known as
  the reducing agent.
• The substance that gains the electrons is known
  as the oxidizing agent.

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oxidized
Mg(s) S(s) MgS(s)
Magnesium reducing agent
reduced
Sulfur Oxidizing agent
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LEO the lion goes GER.
Losing Electrons is Oxidation
Gaining Electrons is Reduction
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Redox with Covalent Compounds

• Electrons are not completely lost (because pairs
  are shared) however there is a shift (partial
  loss) of electrons toward the greater
  electronegative atom.
• 2H2(g) O2(g) ? 2H2O(l)

The partial electron shift may be less obvious in
some reactions involving covalent compounds.
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General Guidelines forIdentifying Redox Reactions
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Corrosion

• Iron, a common construction metal often used in
  the form of the alloy steel, corrodes by being
  oxidized to ions of iron by oxygen.
• Water speeds up the rate of corrosion
• Oxygen (oxidizing agent) is reduced to oxide ions
• 2Fe(s) O2(g) 2H2O(l) ? 2Fe(OH)2(s)
• 4Fe(OH)2(s) O2(g) 2H2O(l) ? 4Fe(OH)3(s)

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Assigning Oxidation Numbers

• An oxidation number is a positive or negative
  number assigned to an atom to indicate its degree
  of oxidation or reduction.
• As a general rule, a bonded atoms oxidation is
  the charge that it would have if the electrons in
  the bond were assigned to the atom of the more
  electronegative element.

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Give d. a try on your own! Helpful hint, what is
the charge of the NH4 ion.
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Oxidation Changes in Chemical Reactions

• An increase in the oxidation number of an atom or
  ion indicates oxidation. A decrease in the
  oxidation number of an atom or ion indicates
  reduction.
• 2AgNO3(aq) Cu(s) ? CU(NO3)2 2Ag(s)

1 5 -2 0
2 5 -2 0
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Identifying Redox Reactions

• A REDOX reaction involves a change in the
  oxidation number of two or more elements
• Is the following reaction a redox reaction?
• N2(g) O2(g) ? 2NO(g)
• How can you tell if the above reaction is a redox?

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Balancing Redox Equations

• Two main methods
• Oxidation-number change method
• Half-Reactions

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• Using the oxidation-number change method,
  involves balancing a redox equation by comparing
  the increases decreases in oxidation s.
• Fe2O3(s) CO(g) ? Fe(s) CO2(g) (unbalanced)
• Step 1 assign oxidation s to all the atoms in
  the equation.
• Step 2 ID atoms oxidized and reduced.
• Step 3 Use one bracketing line to connect the
  atoms that undergo oxidation another to connect
  reduced.
• Step 4 Make the total increase in oxidation
  equal to the total decrease in oxidation by
  using appropriate coefficients.

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• A half-reaction is an equation showing just the
  oxidation or just the reduction that takes place
  in a redox reaction.

Work through example on pages 650 651 in text
book.