Chapter 4: Chemical Quantities

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Chapter 4 Chemical Quantities Aqueous Reactions

• CHE 123 General Chemistry I
• Dr. Jerome Williams, Ph.D.
• Saint Leo University

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Overview

• Oxidation Reduction Principles
• Oxidation Numbers
• Activity Series of Elements

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Oxidation Reduction Principles

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
• Oxidation and reduction must occur together. They
  cannot exist alone.
• Recall definition using pneumonic OIL RIG.

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Oxidation Reduction Principles

• Oxidizing Agent is the species that undergoes
  reduction.
• Reducing Agent is the species that undergoes
  oxidation.

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Oxidation Numbers

• Assigning Oxidation Numbers All atoms have an
  oxidation number regardless of whether it
  carries an ionic charge.
• 1. An atom in its elemental state has an
  oxidation number of zero.
• 2. An atom in a monatomic ion has an oxidation
  number identical to its charge.

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Oxidation Numbers

• 3. An atom in a polyatomic ion or in a molecular
  compound usually has the same oxidation number it
  would have if it were a monatomic ion.
• A. Hydrogen can be either 1 or 1.
• B. Oxygen usually has an oxidation number of 2.
• In peroxides, oxygen is 1.
• C. Halogens usually have an oxidation number of
  1.
• When bonded to oxygen, chlorine, bromine, and
  iodine have positive oxidation numbers.

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Oxidation Numbers

• 4. The sum of the oxidation numbers must be zero
  for a neutral compound and must be equal to the
  net charge for a polyatomic ion.
• A. H2SO4 2(1) (?) 4(2) 0 net charge
• ? 0 2(1) 4(2) 6
• B. ClO4 (?) 4(2) 1 net charge
• ? 1 4(2) 7

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Number Line Analogy

• 5. Whenever one atom loses electrons (is
  oxidized), another atom must gain those electrons
  (be reduced).
• A substance which loses electrons (oxidized) is
  called a reducing agent. Its oxidation number
  increases.
• A substance which gains electrons (reduced) is
  called the oxidizing agent. Its oxidation number
  decreases.

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Oxidation Numbers

• Assign oxidation numbers to each atom in the
  following substances
• A. CdS B. AlH3 C. Na2Cr2O7 D. SnCl4
• E. CrO3 F. VOCl3 G. V2O3 H. HNO3
• I. FeSO4 J. Fe2O3 K. H2PO4 L. MnO4
• M. Cr2O72

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Oxidation Numbers

• For each of the following, identify which species
  is the reducing agent and which is the oxidizing
  agent.
• Ca(s) 2 H(aq) ? Ca2(aq) H2(g)
• 2 Fe2(aq) Cl2(aq) ? 2 Fe3(aq) 2 Cl(aq)
• SnO2(s) 2 C(s) ? Sn(s) 2 CO(g)
• Sn2(aq) 2 Fe3(aq) ? Sn4(aq) 2 Fe2(aq)

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Activity Series of Elements