Redox Reactions (Oxidation-Reduction Reactions)

Definitions

• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Mnemonics:
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain.
  • LEO the lion says GER: Lose Electrons Oxidation, Gain Electrons Reduction.

Oxidation States

• Elemental Form: Always zero oxidation state (e.g., Zn, Cu).
• Monatomic Ions: Oxidation state corresponds to their charge (e.g., Zn²⁺ = +2).
• Understanding Charge Changes:
  • Losing electrons (negatively charged) makes oxidation state go up.
  • Gaining electrons makes oxidation state go down.

Examples and Assigning Oxidation States

• Zinc and Copper: Zn → Zn²⁺ (+2), Cu²⁺ → Cu (0).
  • Zn: Oxidation (loses electrons), Cu: Reduction (gains electrons).
• NaCl: Na = +1, Cl⁻ = -1.
• CuCl₂: Cu = +2 (balanced by 2 Cl⁻ ions).

Rules for Assigning Oxidation States

1. Elemental Form: Oxidation state = 0.
2. Monatomic Ions: Oxidation state = ionic charge.
3. Oxygen: Usually -2. Peroxides: -1.
4. Hydrogen: +1 with non-metals, -1 with metals.
5. Halogens: Usually -1, except in oxoanions (e.g., Chlorate).
6. Sum Rule: Sum of oxidation states in a compound or ion must equal overall charge.

Examples:

• NaHCO₃: Na = +1, H = +1, O = -2, C = +4 (balancing to 0).
• CO₂: O = -2, C = +4.
• H₂O₂: O = -1, H = +1.
• SO₄²⁻: O = -2, S = +6 (balancing to -2).

Identifying Redox Reactions

• Change in Oxidation State: Indicates an electron transfer (redox reaction).
• Elements in Elemental Form: Look for zero oxidation state transforming into another state.
• Tips:
  • Single Replacement Reactions: Always redox.
  • Double Replacement Reactions: Never redox.

Single Replacement Reactions

• Activity Series: List of metals arranged by ease of oxidation.
  • Higher on the list = more easily oxidized.
• Example:
  • Mg + HCl → MgCl₂ + H₂: Mg is higher than H₂, therefore spontaneous.
  • Cu + Zn(NO₃)₂ → No Reaction: Cu is lower than Zn, therefore non-spontaneous.
• Formal Understanding:
  • Reaction spontaneity depends on the element getting oxidized being higher on the activity series.

Practice and Understanding

• Assigning Oxidation States: Practice is key.
  • Expect questions on both singular oxidation state determination and reaction identification.
• Understanding Reaction Mechanics: Helps in exams and practical applications.

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