Lecture on Oxidation Numbers and States

Jul 1, 2024

Lecture on Oxidation Numbers and States

Introduction to Oxidation Numbers

  • Definition: Oxidation number (or oxidation state) is a concept in chemistry that describes the degree of oxidation of an atom in a compound.
  • Importance: Helps in understanding redox reactions and the transfer of electrons.

General Rules for Assigning Oxidation Numbers

  1. Free Elements: The oxidation number is zero (e.g., O2, H2, N2, etc.).
  2. Monoatomic Ions: Oxidation number is equal to the charge of the ion (e.g., Na+ = +1, Cl- = -1).
  3. Fluorine: Always -1 in compounds.
  4. Oxygen: Usually -2, but in peroxides (e.g., H2O2), it is -1.
  5. Hydrogen: +1 when bonded with non-metals, -1 when bonded with metals.
  6. Sum of Oxidation Numbers: In a neutral compound, the sum of all oxidation numbers is zero. In a polyatomic ion, the sum is equal to the charge of the ion.

Examples and Practice Problems

  • Water (H2O):

    • Hydrogen = +1
    • Oxygen = -2
    • Sum = 2*(+1) + (-2) = 0

  • Sulfur in H2SO4:

    • Hydrogen = +1
    • Oxygen = -2
    • H2SO4 is neutral, so sum = 0
    • 2*(+1) + S + 4*(-2) = 0
    • Solving for S: S = +6

  • Nitrogen in NH4+:

    • Hydrogen = +1
    • NH4+ has a total charge of +1
    • N + 4*(+1) = +1
    • Solving for N: N = -3

  • Iron in Fe2O3:

    • Oxygen = -2
    • Fe2O3 is neutral
    • 2*(Fe) + 3*(-2) = 0
    • Solving for Fe: Fe = +3

Special Cases

  • Peroxides (general formula ROOR):

    • Oxygen = -1

  • Superoxides (general formula MO2):

    • Oxygen ~ -1/2

  • Hydrogen in Metal Hydrides:

    • e.g., NaH, CaH2
    • Hydrogen is -1

  • Complex Ions:

    • e.g., [Fe(CN)6]4-
    • Sum of oxidation numbers equals the ion charge (e.g., Fe + 6*(-1) = -4; Solving for Fe = +2).

  • Transition Metals:

    • Multiple possible oxidation states (e.g., Fe2+, Fe3+)
    • Determined based on chemical context and specific compound analysis.

Practical Application in Redox Reactions

  • Identify Oxidation and Reduction: Assign oxidation numbers to identify which atoms are oxidized and reduced.
  • Calculate Changes: Determine the changes in oxidation numbers to track electron flow.

Homework Problems

  1. Determine the oxidation number: KMnO4.
  2. Calculate the oxidation state of Cr in Cr2O7^2-.
  3. Identify oxidized and reduced elements in the reaction: Zn + CuSO4 → ZnSO4 + Cu.

Conclusion

  • Mastery of oxidation numbers is crucial for understanding redox reactions and chemical bonding.
  • Always follow the rules methodically to avoid mistakes.
  • Practice with different compounds to become more comfortable with determining oxidation states.