Overview
This lecture covers the core concepts of Redox Reactions, including oxidation numbers, rules for assigning oxidation numbers, redox titration calculations, and common exam tricks and formulas.
Oxidation Number: Concept & Assignment
- Oxidation number represents the real or imaginary charge an atom would have if electrons were transferred completely.
- Assign oxidation numbers based on electronegativity; the more electronegative atom gets the negative charge.
- For neutral molecules, the sum of oxidation numbers is zero; for ions, it's equal to the ion's charge.
- Oxidation number for elemental state (e.g., H₂, O₂, S₈) is always zero.
- Alkali metals always have +1, alkaline earth metals always +2 in compounds.
- Hydrogen is +1 (except in metal hydrides, where it's -1); oxygen is -2 (except in peroxides: -1, superoxides: -1/2).
- Halogens usually -1, except with oxygen or higher halogens.
- The oxidation state of the same element can vary in a compound (e.g. S₂O₃²⁻).
Calculating Average and Individual Oxidation Numbers
- To find an unknown oxidation number, set up an equation for the compound’s total charge.
- In polyatomic ions, consider total numbers and charges to solve for unknowns.
- For coordination compounds, account for overall and individual charges.
Redox Agents: Oxidizing and Reducing Agents
- Oxidizing agent: substance that gets reduced (decreases its oxidation number).
- Reducing agent: substance that gets oxidized (increases its oxidation number).
- If an atom’s oxidation number is at maximum, it acts as an oxidizing agent; if at minimum, as a reducing agent; if intermediate, can act as both.
Normality & Equivalent Concept
- Normality (N) = gram equivalents/liter of solution.
- Gram equivalent = mass/equivalent weight; equivalent weight = molecular weight/n-factor.
- n-factor for acids: number of H⁺ ions ionized; for bases: number of OH⁻ ions provided.
- n-factor for redox reactions: total change in oxidation numbers multiplied by number of atoms involved.
Redox Titration Calculations
- In titration: gram equivalents of acid = gram equivalents of base.
- For redox: n₁V₁ = n₂V₂, or N₁V₁ = N₂V₂, or (M₁n₁V₁) = (M₂n₂V₂), with n = n-factor.
- n-factor in redox = change in oxidation number per atom × number of atoms changed.
Key Terms & Definitions
- Oxidation Number — charge assigned to an atom in a compound, assuming complete electron transfer.
- n-factor — number representing total electrons gained or lost per formula unit in a reaction.
- Oxidizing agent — species that gains electrons (is reduced).
- Reducing agent — species that loses electrons (is oxidized).
- Normality (N) — measure of concentration in equivalents/liter.
- Equivalent weight — mass of substance that reacts with or supplies 1 mole of H⁺ or e⁻.
Action Items / Next Steps
- Practice assigning oxidation numbers to various compounds and polyatomic ions.
- Solve redox titration problems using n-factor and normality formulas.
- Review rules and exceptions for oxidation numbers in compounds.
- Complete any given homework or example questions using these methods.