Overview
Redox chemistry involves electron transfer between species and is consistently challenging. Exam questions are difficult but reasonable; marks are achievable through persistent problem-solving and methodical application of rules.
Redox Fundamentals
- OIL RIG mnemonic: Oxidation Involves Loss; Reduction Involves Gain (of electrons)
- Oxidation and reduction always occur together in a redox reaction
- Electrons lost by one species must be gained by another (electron conservation)
- Half equations show each process separately but reactions happen simultaneously
- Chemical equations don't explicitly show electron movement; oxidation states track electron transfer
Oxidant and Reductant Terminology
- Oxidant: Causes oxidation of another species but is itself reduced
- Reductant: Causes reduction of another species but is itself oxidized
- Analogy: Teacher teaches students (teacher causes teaching, students are taught)
- When A sends electron to B: A is reductant (undergoes oxidation); B is oxidant (undergoes reduction)
- Avoid overthinking this concept; write it down and reference as needed
Oxidation States
Core Rules:
| Species Type | Oxidation State | Example |
|---|
| Free elements | 0 | F₂, He, Na(s), Fe(s) |
| Monoatomic ions | Charge of ion | Cl⁻ = -1, Na⁺ = +1, Mg²⁺ = +2 |
| Main group metals | Charge of ion | Na = +1, Mg = +2 |
| Hydrogen | Almost always +1 | Exception exists but rarely tested |
| Oxygen | Almost always -2 | Exception: H₂O₂ (peroxide) = -1 |
Additional Rules:
- Sum of all oxidation states equals overall charge on species
- For CO₂: O = -2 (×2 = -4), so C = +4 (total = 0)
- For PO₄³⁻: O = -2 (×4 = -8), charge = -3, so P = +5
- H₂O₂ (hydrogen peroxide): Each H = +1, each O = -1; poison used historically in WWII
Identifying Redox Reactions
- Reaction is redox if oxidation states change between reactants and products
- Oxidation: Oxidation state increases (species loses electrons)
- Reduction: Oxidation state decreases (species gains electrons)
- Compare oxidation states on both sides; no change means not redox
- Example: C₂H₆ + O₂ → CO₂ + H₂O is redox (C changes 0 to +4; O changes 0 to -2)
- Reductant/oxidant identified as whole molecule even if only one atom undergoes change
Balancing Half Equations - KOHS Method
KOHS Steps:
- K - Key atoms: Balance all atoms except H and O first
- O - Oxygen: Add H₂O to opposite side to balance oxygen atoms
- H - Hydrogen: Add H⁺ ions to balance hydrogen atoms
- S - Charge: Add electrons to balance overall charge
- Add states of matter to final equation
Example: Cr₂O₇²⁻ → Cr³⁺
- K: Balance Cr → Cr₂O₇²⁻ → 2Cr³⁺
- O: Add 7 H₂O to right → Cr₂O₇²⁻ → 2Cr³⁺ + 7H₂O
- H: Add 14 H⁺ to left → Cr₂O₇²⁻ + 14H⁺ → 2Cr³⁺ + 7H₂O
- S: Left charge = +12, right = +6; add 6e⁻ to left → Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
- Add states (electrons have no state)
States Tips:
- When uncertain, default to aqueous (aq)
- Water is always liquid unless specified as gas
- For combustion reactions, check data book for fuel states
- Incorrect states may result in lost marks
Key Terms & Definitions
- Oxidation: Loss of electrons; oxidation state increases
- Reduction: Gain of electrons; oxidation state decreases
- Oxidant: Species that accepts electrons and causes oxidation
- Reductant: Species that donates electrons and causes reduction
- Half equation: Shows oxidation or reduction process separately with electron transfer explicit
- Oxidation state: Tool to track electron movement; numerical value assigned to atoms in compounds
Action Items / Next Steps
- Practice KOHS method repeatedly to achieve sub-1-minute completion for two-mark questions
- Review oxidation state rules, especially oxygen exception in H₂O₂
- Complete practice problems identifying reductants and oxidants
- Focus on persistence when solving difficult redox problems; answers are achievable with sustained effort
- Memorize KOHS acronym and apply systematically to all half equation questions