Oxidation Numbers in Chemistry

Study Notes

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⁻.

Description

Learn about the rules for assigning oxidation numbers to atoms in molecules, including free elements, monatomic ions, and compounds. Understand how to determine the oxidation number of different elements.