OxidationReduction Reactions

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Oxidation-Reduction Reactions

• Section 18.1

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What is a Oxidation Reduction Reaction?

• Reactions in which one or more electrons are
  transferred are called reduction-oxidation
  reactions or redox reactions
• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• LEO the lion goes GER
• Usually involves a metal and a nonmetal
• Which one will lose electrons?
• Which one will gain electrons?

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Oxidation Reduction Reactions

• Reaction of sodium metal with chlorine gas
• 2Na (s) Cl2 (g) ? 2NaCl (s)
• Whats really happening in this reaction?
• Na Cl NaCl-
• Na Cl NaCl-

Each sodium gives-up an electron to the chlorine
resulting in ions!
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Oxidation Reduction Reactions

• In our reaction of sodium with chlorine gas
• 2Na (s) Cl2 (g) ? 2NaCl (s)
• What is being oxidized?
• Sodium! (Lose electrons oxidation)
• What is being reduced?
• Chlorine! (Gain electrons reduced)
• In each of the following reactions, identify what
  is being oxidized, and what is being reduced?
• 2Mg (s) O2 (g) ? 2MgO (s)
• 2Al (s) 3I2 (s) ? 2AlI3 (s)

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Oxidation Reduction Reactions

• Can an oxidation reduction reaction occur
  between two nonmetals?
• YES!! Just look for oxygen (O2)
• CH4 (g) 2O2 (g) ? CO2 (g) H2O (g) energy

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Oxidation States

• Oxidation State lets us keep track of electrons
  in redox reactions by assigning charges to each
  atom in a molecule.
• If the compound isnt ionic, the charges arent
  real. (covalent)

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Oxidation States

• These molecules are often not ionic, so the atoms
  do not actually have the charge that we assign.
• It is used as a bookkeeping system to help us see
  the transfer of electron charge and helps us
  balance the equations.

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Oxidation States

• Binary ionic compound it is identical to the
  charge on the ion.
• Uncombined element all atoms are uncharged
  given an oxidation state of zero.
• Covalent compounds (2 nonmetals bonded)
  Assume that the most electronegative atom in the
  bond controls both shared e-.

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Oxidation states

• 3. covalent continued No ions are present and
  e- are shared. It is useful to assign imaginary
  charges to each element.
• Assume that the most electronegative atom in the
  bond controls or has both shared e-.
• Recall trends right increasing down decreasing

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Oxidation States

• ..
• H O O2- H1
• H

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Oxidation States in Covalent Compounds

• Which element is the most electronegative?
• F gt O gt N gt C
• When these are bonded to other nonmetals there
  are no ions.
• You must determine which is the most
  electronegative and its oxidation state will be
  the same as its charge if it were an ion.

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Example

• NO2 Determine the oxidation of each.
• O is most electronegative, so it has an oxid.
  of 2-.
• 0 x 2(-2) x 4
• N is usually 3-, but here it will be 4

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Assign oxidation states to each element in the
following
Sodium metal, Na Sodium fluoride, NaF Magnesium
sulfide, MgS Carbon monoxide, CO Sulfur dioxide,
SO2 Hydrogen peroxide, H2O2 Ammonia, NH3 Sodium
carbonate, Na2CO3
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Self-Check pg 579

• SO3
• SO42-
• N2O5
• PF3
• C2H6

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Oxidation States

• Homework p. 602-603 5, 8, 9, 11, 12, 13 for
  9-13, do a and b only

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