Redox Reactions and Oxidation Numbers

Redox Reactions

• Aim: Understand "Redox", oxidation, and reduction.

• Oxidation: Initially defined by reactions with oxygen (e.g., $Fe + O \rightarrow FeO$).

• Electron Transfer: Redox is fundamentally about electron movement.

• Oxidizing Agent: The substance that causes oxidation by accepting electrons.

• Reducing Agent: The substance that causes reduction by donating electrons.

Oxidation Numbers

• Purpose: Used to identify oxidized and reduced species.

• Rules for Assigning Oxidation Numbers (Priority Order):

  1. Uncombined neutral element: Oxidation number = 0 (e.g., Fe, $Cl<em>2$, $O</em>3$).

  2. Ions: Oxidation number equals the ion charge (e.g., $Fe^{2+}$ = +2, $Br^-$ = -1).

  3. Compounds: Sum of oxidation numbers = 0 (e.g., NaCl).

  4. Fluorine: Oxidation number = -1 (e.g., NaF, $CaF_2$).

  5. Oxygen: Usually -2 (e.g., MgO, $Al<em>2O</em>3$, $H<em>2O$). Exceptions: $OF</em>2$, $H<em>2O</em>2$, $BaO_2$.

  6. Hydrogen: Usually +1 (e.g., $NH<em>3$, HCl, $H</em>2O$). Exceptions: LiH, $CaH_2$.

  7. Group 1 Metals: Oxidation number = +1 (e.g., NaCl, KOH).

  8. Group 2 Metals: Oxidation number = +2 (e.g., $CaCl<em>2$, $Mg(OH)</em>2$).

  9. Polyatomic Ions: Sum of oxidation numbers equals the ion's charge (e.g., $[PO_4]^{3-}$).

Advanced Definition of Oxidation and Reduction

• Oxidation Number Change:

  • Oxidation: Increase in oxidation number. Example: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$

  • Reduction