Oxidation Numbers in Chemistry

Oxidation Numbers

• Definition: A number assigned to an atom in a molecule that represents the number of electrons it has gained or lost.
• Rules for assigning oxidation numbers:
  • Free elements have an oxidation number of 0.
  • Monatomic ions have an oxidation number equal to their charge.
  • In compounds, the oxidation number of a Group 1A element is +1, Group 2A element is +2, and a Group 17 element is -1.
  • Hydrogen has an oxidation number of +1, except in metal hydrides where it is -1.
  • Oxygen has an oxidation number of -2, except in peroxides where it is -1 and in oxygen fluorides where it is +2.

Reduction Reactions

• Definition: A reaction in which an atom, molecule, or ion gains one or more electrons.
• Characteristics:
  • Involves a gain of electrons.
  • Results in a decrease in oxidation number.
  • Often accompanied by the loss of oxygen or the gain of hydrogen.
  • Can be represented by the half-reaction: Ox + e⁻ → Red

Strong Vs Weak Acids

• Strong Acids:
  • Completely dissociate in water to produce H₃O⁺ ions.
  • Examples: HCl, HNO₃, H₂SO₄, HClO₃, and HClO₄.
  • High acidity, low pH.
• Weak Acids:
  • Only partially dissociate in water to produce H₃O⁺ ions.
  • Examples: HF, CH₃COOH, HCO₃⁻, and HPO₄²⁻.
  • Low acidity, higher pH.
  • Can be represented by the equilibrium: HA + H₂O ⇌ H₃O⁺ + A⁻

Oxidation Numbers

• Oxidation numbers are assigned to atoms in a molecule to represent the number of electrons gained or lost.
• Free elements have an oxidation number of 0.
• Monatomic ions have an oxidation number equal to their charge.
• Group 1A elements have an oxidation number of +1 in compounds.
• Group 2A elements have an oxidation number of +2 in compounds.
• Group 17 elements have an oxidation number of -1 in compounds.
• Hydrogen has an oxidation number of +1, except in metal hydrides where it is -1.
• Oxygen has an oxidation number of -2, except in peroxides where it is -1 and in oxygen fluorides where it is +2.

Reduction Reactions

• Reduction reactions involve a gain of electrons.
• Reduction reactions result in a decrease in oxidation number.
• Reduction reactions often involve the loss of oxygen or the gain of hydrogen.
• Reduction reactions can be represented by the half-reaction: Ox + e⁻ → Red.

Strong vs Weak Acids

Strong Acids

• Strong acids completely dissociate in water to produce H₃O⁺ ions.
• Examples of strong acids include HCl, HNO₃, H₂SO₄, HClO₃, and HClO₄.
• Strong acids have high acidity and low pH.

Weak Acids

• Weak acids only partially dissociate in water to produce H₃O⁺ ions.
• Examples of weak acids include HF, CH₃COOH, HCO₃⁻, and HPO₄²⁻.
• Weak acids have low acidity and higher pH.
• Weak acids can be represented by the equilibrium: HA + H₂O ⇌ H₃O⁺ + A⁻.