OxidationReduction Reactions

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Oxidation-ReductionReactions

• REDOX

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Oxidation

• Used to be the addition of oxygen to a metal to
  form an oxide
• Example the rusting of iron.
• 4 Fe 3 O2 -----gt 2 Fe2O3

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Reduction

• Used to be considered the removal of oxygen from
  a metal oxide to result in an elemental metal.
• Example the production of iron from iron ore.
• 2 Fe2O3 3 C -----gt 3 CO2 4 Fe

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• Redox reactions used to b e consider reactions in
  which oxygen was transferred but now it is
  defined as a reaction in which electrons are
  transferred.

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• Redox reactions are phenomena in which electrons
  are transferred from one substance to another.

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Whats the difference?

• Oxidation
• Loss of electrons

• Reduction
• Gain of electrons

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Try This

• 2K Br2 ? 2KBr
• K looses an electron
• K ? K e-
• Bromine gaines an electron
• Br2 2e- ? 2Br-

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Thus

• Potassium is oxidized and bromine is reduced.
• Potassium lost an electron to become a cation.
• Bromine gained an electron to become an anion.

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Still more

• Potassium is oxidizedit gives up an electron and
  causes bromine to be reduced. Thus, potassium is
  the reducing agent.
• The substance that is oxidized is the reducing
  agent.

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and yetmore

• Bromine is reducedit gains an electron and
  causes potassium to be oxidized. Thus, it is the
  oxidizing agent.
• The substance that is reduced is the oxidizing
  agent.

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Try an example

• 2Mg O2 ? 2MgO
• Write the half-reactions
• What is reduced?
• What is oxidized?
• What is the reducing agent?
• What is the oxidizing agent?

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Oxidation Numbers

• A bookkeeping system that allows you to keep
  track of electron transfer.
• These number allow you to evaluate a chemical
  reaction for oxidation and reduction.

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Rules for Oxidation Numbers

• Atoms in elemental form O.N. 0
• Diatomic molecules O.N.0
• Monatomic ions O.N. equals ionic charge

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Rules again

• The sum of O.N. is a compounds in zero.
• The sum of O.N. on a polyatomic ion is equal to
  the ionic charge of the ion.

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and still more rules

• Group 1 O.N. 1
• Group 2 O.N. 2
• Hydrogen - 1 with non-metal and -1 with metals
  as a hydride and boron as an anion.
• Fluorine O.N. -1

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and the rules go on

• Oxygen - -1 in peroxides and -2 with everything
  else.
• Group 7 O.N. -1

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Balancing With Redox

• Ion-electron method
• In acid media
• In basic media

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Ion-electron method

• Take skeleton equation
• Divide equation into half reaction.
• Balance half reactions atomically and
  electrically.
• Balance electrons
• Add equations and cancel electrons.

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In Acid Media

• Skeleton equation
• Break down to half-rxns
• Balance metals
• Balance oxygen by adding waters
• Balance Hydrogen with H
• Balance charges by adding electrons
• Equalize electrons and add

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In Basic Media

• Follow steps for acid media and then add the
  following
• Add OH- to both sides to neutralize H
• Combine and cancel waters as needed.