Overview
This lecture introduces redox (reduction-oxidation) reactions, explains how electrons move during these reactions, and demonstrates how to assign and use oxidation numbers for balancing equations.
Importance of Electrons in Chemistry
- Electrons enable atoms to bond and form molecules, often exchanging energy.
- Redox reactions involve the transfer of electrons between atoms, making them central to many important chemical processes.
Redox Reactions: Definitions and Naming
- Redox is short for reduction and oxidation, referring to electron transfer.
- Oxidation is the loss of electrons, and reduction is the gain of electrons ("OILRIG").
- These terms are historical and not entirely logical, but remain in use.
Assigning Oxidation Numbers
- The oxidation number tracks the hypothetical charge if electrons were fully transferred.
- For any pure element (monoatomic, diatomic, or polyatomic), the oxidation number is 0.
- For a monatomic ion, the oxidation number equals its charge.
- Oxygen is usually -2 (except in peroxides), hydrogen is +1, and fluorine is -1.
- The sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion, it equals the ion’s charge.
- Elements like sulfur can have varying oxidation numbers depending on their chemical environment.
Balancing Redox Reactions
- Track electron movement by writing half-reactions for oxidation and reduction separately.
- Balance atoms and electrons in each half-reaction, then combine them so electrons cancel out.
- Used examples: the Haber process (N2 + H2 → NH3) and silver extraction using an aldehyde.
Key Terms & Definitions
- Electron — A subatomic particle involved in chemical bonding and redox reactions.
- Redox Reaction — A chemical reaction involving the transfer of electrons.
- Oxidation — Loss of electrons by a substance during a reaction.
- Reduction — Gain of electrons by a substance during a reaction.
- Oxidation Number (State) — The hypothetical charge an atom would have if all bonds were ionic.
- OILRIG — Mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons).
Action Items / Next Steps
- Practice assigning oxidation numbers to various compounds and ions.
- Balance redox reactions using half-reactions.
- Review key rules for determining oxidation states (elements, ions, oxygen, hydrogen, halogens).