Overview
This lecture explains oxidation-reduction (redox) reactions, how to identify them using oxidation numbers, and how to determine the oxidizing and reducing agents in various types of chemical reactions.
Redox Reactions Basics
- Redox (oxidation-reduction) reactions involve the transfer of electrons between elements.
- The oxidation number of any pure element is zero.
- Oxidation is an increase in oxidation number (loss of electrons).
- Reduction is a decrease in oxidation number (gain of electrons).
- Metals tend to lose electrons and become cations; non-metals (like oxygen) gain electrons and become anions.
Agents in Redox Reactions
- The substance oxidized is called the reducing agent.
- The substance reduced is called the oxidizing agent.
- Metals typically act as reducing agents; non-metals like oxygen and fluorine act as oxidizing agents.
Identifying Oxidation and Reduction
- In half-reactions, electrons on the right indicate oxidation, and electrons on the left indicate reduction.
- For each reaction, determine the oxidation states of elements in reactants and products to identify what is oxidized or reduced.
- When a substance’s oxidation number increases, it is oxidized; when it decreases, it is reduced.
Examples of Redox Analysis
- Magnesium + oxygen → magnesium oxide: Magnesium is oxidized, oxygen is reduced.
- Zinc + hydrochloric acid → hydrogen + zinc chloride: Zinc is oxidized, hydrogen in HCl is reduced.
- Methane + oxygen → carbon dioxide + water: Carbon in methane is oxidized, oxygen is reduced.
Identifying Redox Reactions in Reactions Types
- If a pure element turns into a compound, or vice versa, the reaction is redox (electron transfer occurs).
- Single replacement and combustion reactions are always redox reactions.
- Double replacement reactions, including acid-base neutralizations and precipitation, are never redox reactions.
- Combination (synthesis) and decomposition reactions may or may not be redox, depending on the presence of pure elements and changes in oxidation states.
Quick Redox Identification Tips
- If a pure element appears on one side and that element is in a compound on the other, it’s a redox reaction.
- No change in oxidation state between reactants and products means no redox reaction.
Key Terms & Definitions
- Oxidation — loss of electrons and increase in oxidation number.
- Reduction — gain of electrons and 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 that shows either oxidation or reduction alone.
Action Items / Next Steps
- Practice assigning oxidation states and identifying redox reactions in sample equations.
- Review types of chemical reactions and classify them as redox or not using provided rules.