Oxidation Numbers and Redox Reactions

Oxidation Numbers

• Definition: Oxidation numbers (also known as oxidation states) are a way to keep track of electrons during a redox reaction.
• Rules for Assigning Oxidation Numbers:
  1. The oxidation number of an atom in its elemental form is zero.
  2. The oxidation number of a monatomic ion is equal to its charge.
  3. In compounds, the oxidation number of oxygen is typically -2, except in peroxides (H2O2) where it is -1.
  4. The oxidation number of hydrogen is typically +1, except in hydrides (NaH) where it is -1.
  5. The oxidation number of fluorine is always -1.
  6. The sum of the oxidation numbers of all atoms in a neutral compound is zero.
  7. The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.
• Examples:
  • NaCl: Na (+1), Cl (-1)
  • H2O: H (+1), O (-2)
  • CaO: Ca (+2), O (-2)

Half-cell Reactions

• Definition: A half-cell reaction is a reduction or oxidation reaction that occurs at an electrode during a redox reaction.
• Characteristics:
  • Half-cell reactions involve the transfer of electrons.
  • They can be either oxidation (loss of electrons) or reduction (gain of electrons) reactions.
  • They occur at an electrode, which can be an anode (oxidation) or a cathode (reduction).
• Notation:
  • Half-cell reactions are often written in the format: oxidized species → reduced species
  • The reaction is often divided into two parts: the oxidation half-cell and the reduction half-cell.
• Examples:
  • Oxidation half-cell: Zn → Zn²⁺ + 2e⁻
  • Reduction half-cell: Cu²⁺ + 2e⁻ → Cu
  • Overall reaction: Zn + Cu²⁺ → Zn²⁺ + Cu

Oxidation Numbers

• Oxidation numbers track electrons during redox reactions.

Rules for Assigning Oxidation Numbers

• Atoms in elemental form have an oxidation number of zero.
• Monatomic ions have an oxidation number equal to their charge.
• Oxygen has an oxidation number of -2 in compounds, except -1 in peroxides.
• Hydrogen has an oxidation number of +1 in compounds, except -1 in hydrides.
• Fluorine always has an oxidation number of -1.
• In neutral compounds, the sum of oxidation numbers is zero.
• In polyatomic ions, the sum of oxidation numbers equals the ion's charge.

Oxidation Number Examples

• NaCl: Na is +1, Cl is -1.
• H2O: H is +1, O is -2.
• CaO: Ca is +2, O is -2.

Half-cell Reactions

• Half-cell reactions are reduction or oxidation reactions at an electrode during redox reactions.

Characteristics of Half-cell Reactions

• They involve electron transfer.
• They can be oxidation (electron loss) or reduction (electron gain) reactions.
• They occur at an electrode: anode (oxidation) or cathode (reduction).

Notation for Half-cell Reactions

• Format: oxidized species → reduced species.
• Divided into oxidation and reduction half-cells.

Half-cell Reaction Examples

• Oxidation half-cell: Zn → Zn²⁺ + 2e⁻.
• Reduction half-cell: Cu²⁺ + 2e⁻ → Cu.
• Overall reaction: Zn + Cu²⁺ → Zn²⁺ + Cu.