Overview
This lecture explains oxidation-reduction (redox) reactions, how to identify them, determine oxidation states, and distinguish redox from non-redox reactions using key rules and examples.
Oxidation and Reduction Concepts
- Redox reactions involve the transfer of electrons between elements.
- The oxidation number of a pure element is always zero.
- Oxidation occurs when an element’s oxidation state increases (loss of electrons).
- Reduction occurs when an element’s oxidation state decreases (gain of electrons).
- Metals tend to lose electrons and become cations (oxidized).
- Non-metals like oxygen gain electrons and become anions (reduced).
Agents and Identifying Changes
- The oxidized substance is called the reducing agent.
- The reduced substance is called the oxidizing agent.
- Metals usually act as reducing agents; nonmetals are often oxidizing agents.
- In half-reactions, electrons on the right indicate oxidation; on the left indicate reduction.
Example Reactions and Analysis
- Magnesium + O₂ → MgO: Magnesium is oxidized, oxygen is reduced.
- Zn + HCl → H₂ + ZnCl₂: Zinc is oxidized, HCl is reduced. Identify changes based on reactants.
- CH₄ + O₂ → CO₂ + H₂O: Methane is oxidized (carbon), oxygen is reduced. The whole compound is considered, not just the element.
Identifying Redox vs Non-Redox Reactions
- If a pure element is on one side and the same element in a compound on the other, it is a redox reaction.
- All single replacement and combustion reactions are redox reactions.
- Double replacement and acid-base neutralization reactions are never redox reactions.
- Combination (synthesis) and decomposition reactions can be redox or not, depending on the presence of pure elements.
- If oxidation states do not change, it is not a redox reaction.
Key Terms & Definitions
- Redox reaction — a chemical reaction involving electron transfer.
- Oxidation — loss of electrons; increase in oxidation state.
- Reduction — gain of electrons; decrease in oxidation state.
- Oxidizing agent — the substance reduced, causing oxidation.
- Reducing agent — the substance oxidized, causing reduction.
- Half-reaction — equation showing either oxidation or reduction part of a redox process.
Action Items / Next Steps
- Practice assigning oxidation numbers to elements in compounds and reactions.
- Identify oxidation, reduction, oxidizing agent, and reducing agent in given reactions.
- Review examples of redox, non-redox, and double replacement reactions.