Overview
This lecture introduced the concepts of redox reactions and volumetric analysis, covering definitions, oxidation numbers, types of redox reactions, balancing redox equations, and redox titrations.
Oxidation and Reduction Concepts
- Oxidation is the loss of electrons or increase in oxidation state; reduction is the gain of electrons or decrease in oxidation state.
- Ancient definitions: oxidation = addition of oxygen/removal of hydrogen; reduction = addition of hydrogen/removal of oxygen.
- Modern definition relies on electron transfer and change in charge.
Oxidation Number Rules
- Oxidation number of elements in their elemental state is zero.
- Group 1 metals in compounds: +1; Group 2 metals: +2; Group 17 (halogens): -1 (except in interhalogen compounds).
- Hydrogen: +1 when combined with nonmetals, -1 when combined with metals.
- Oxygen: usually -2, -1 in peroxides, -½ in superoxides.
- Sum of oxidation numbers in a neutral molecule is zero; in an ion, it equals ion charge.
Calculating Oxidation Numbers
- Assign oxidation numbers based on rules and solve for unknowns.
- For molecules with coordinate bonds, consider electronegativity and donor/acceptor atoms.
- In exceptional cases, knowledge of structure is necessary to assign correct oxidation states.
Types of Redox Reactions
- Combination: two substances form one product.
- Decomposition: one substance breaks into two or more products.
- Displacement: one element replaces another in a compound.
- Disproportionation: same element is oxidized and reduced in a reaction.
- Comproportionation: two different oxidation states of same element form an intermediate state.
Redox Agents
- Oxidant (oxidizing agent): gains electrons, gets reduced.
- Reductant (reducing agent): loses electrons, gets oxidized.
Balancing Redox Reactions
- Assign oxidation numbers and calculate n-factor (number of electrons exchanged).
- Use n-factor to balance electrons, then balance remaining atoms.
- In acidic medium: use H⁺ and H₂O; in basic medium: use OH⁻ and H₂O to balance.
Redox Titration and Volumetric Analysis
- Titration involves reacting known and unknown solutions until equivalence point.
- Use molarity, normality, and equivalent concepts to calculate concentrations.
- Redox titrations often use agents like potassium dichromate or permanganate.
Range of Oxidation States
- P-block elements show oxidation states from (group number – 10) to + (group number – 10).
- Maximum oxidation states act as oxidants, minimum as reducers, intermediates can do both.
Key Terms & Definitions
- Oxidation — loss of electrons, increase in oxidation number.
- Reduction — gain of electrons, decrease in oxidation number.
- Oxidizing Agent (Oxidant) — substance that gets reduced, oxidizes another.
- Reducing Agent (Reductant) — substance that gets oxidized, reduces another.
- Oxidation Number — charge assigned to an atom in a molecule/ion according to set rules.
- n-factor — number of electrons lost or gained per molecule/ion in a redox process.
- Disproportionation — reaction where the same element is both oxidized and reduced.
- Titration — process to determine concentration of a solution using a reaction with standard solution.
Action Items / Next Steps
- Review practice problems on assigning oxidation numbers and balancing redox equations.
- Complete assigned homework on titration calculations and n-factor determination.
- Study special redox reactions and exceptional cases requiring molecular structure.