Overview
This lecture covers oxidation-reduction (redox) reactions, how to identify them, determine oxidation states, and distinguish redox from non-redox reactions using various examples and rules.
Oxidation and Reduction Basics
- Redox reactions involve the transfer of electrons between elements.
- Oxidation is the loss of electrons; reduction is the gain of electrons.
- The oxidation number of any pure element is zero.
- If an atom's oxidation number increases, it is oxidized; if it decreases, it is reduced.
Identifying Agents
- The substance oxidized is the reducing agent.
- The substance reduced is the oxidizing agent.
- Metals typically act as reducing agents; non-metals (like oxygen, fluorine) are usually oxidizing agents.
Half-Reactions
- Oxidation half-reactions have electrons on the right (product) side.
- Reduction half-reactions have electrons on the left (reactant) side.
Determining Oxidation States in Reactions
- Hydrogen bonded to non-metals is usually +1; oxygen in compounds is usually –2.
- Calculate oxidation states to identify which substance is oxidized or reduced.
- In compounds, assign oxidation numbers so the sum equals the compound's net charge.
Identifying Redox Reactions
- Look for pure elements on one side and the same element in a compound on the other to quickly spot redox reactions.
- All single replacement and combustion reactions are redox.
- Double replacement reactions (including acid-base and precipitation reactions) are never redox.
- Synthesis/combination and decomposition reactions may or may not be redox, depending on the presence of pure elements and changes in oxidation states.
Key Terms & Definitions
- Oxidation — Loss of electrons; increase in oxidation number.
- Reduction — Gain of electrons; decrease in oxidation number.
- Oxidizing agent — Substance that is reduced (gains electrons).
- Reducing agent — Substance that is oxidized (loses electrons).
- Half-reaction — Part of a redox reaction showing only oxidation or reduction.
Action Items / Next Steps
- Practice assigning oxidation states and identifying redox reactions in sample equations.
- Review the rules for oxidation numbers, especially for complex ions and exceptions.