Overview
This lecture explains the concepts of oxidation and reduction, oxidation numbers, redox reactions, and provides methods for identifying oxidizing and reducing agents with examples and typical exam questions.
Oxidation and Reduction Basics
- Oxidation is the gain of oxygen or loss of electrons by a substance.
- Reduction is the loss of oxygen or gain of electrons by a substance.
- "OIL RIG" stands for Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
- A reaction where oxidation and reduction occur simultaneously is a redox (reduction-oxidation) reaction.
Oxidation Numbers: Rules and Calculation
- The oxidation number of an atom in its free state is zero (e.g., Na, Cl2).
- For ions, the oxidation number equals the charge of the ion.
- In compounds, the sum of oxidation numbers for all atoms is zero.
- In polyatomic ions (radicals), the sum of oxidation numbers equals the ion's charge.
- Hydrogen usually has +1 oxidation number, except in hydrides (−1).
- Oxygen usually has −2 oxidation number, except in peroxides (−1).
Practice Examples for Finding Oxidation Numbers
- To find an element’s oxidation number, set up an equation adding all atom oxidation numbers and solve for the unknown.
- Example: In K2Cr2O7, let Cr be x: 2(+1) + 2x + 7(−2) = 0 ⇒ x = +6 (for Cr).
- Example: In SO4^2−, let S be x: x + 4(−2) = −2 ⇒ x = +6 (for S).
- Always show the sign (+ or −) when writing the oxidation number.
Oxidizing and Reducing Agents
- An oxidizing agent is reduced during the reaction (it gains electrons).
- A reducing agent is oxidized during the reaction (it loses electrons).
- Oxidizing agents: acidified KMnO4 (turns purple to colorless), acidified K2Cr2O7 (orange to green), Fe3+ salts (yellow to pale green), concentrated H2SO4 (produces SO2), nitric acid (produces NO2), O2, Cl2.
- Reducing agents: potassium iodide (colorless to brown), Fe2+ salts (pale green to yellow), hydrogen sulfide (yellow sulfur forms), concentrated HCl (chlorine gas forms), carbon, CO, H2.
- Some substances like SO2 and hydrogen peroxide can act as both oxidizing and reducing agents.
Testing for Oxidizing and Reducing Agents
- To test for oxidizing agents: add KI (colorless to brown), Fe2+ salt (pale green to yellow), or H2S (sulfur forms).
- To test for reducing agents: add acidified KMnO4 (purple to colorless), acidified K2Cr2O7 (orange to green), or Fe3+ (yellow to pale green).
- Memorize at least two tests and note the color changes for exams.
Identifying Oxidation and Reduction in Equations
- Oxidation involves an increase in oxidation number; reduction involves a decrease.
- Identify the substance being oxidized by looking for an increase in its oxidation number in the reaction.
- The oxidizing agent is reduced (decrease in oxidation number); the reducing agent is oxidized (increase in oxidation number).
- In redox equations, match changes in oxidation numbers to identify agents.
Key Terms & Definitions
- Oxidation — Gain of oxygen or loss of electrons.
- Reduction — Loss of oxygen or gain of electrons.
- Oxidizing agent — Substance that is reduced and causes oxidation.
- Reducing agent — Substance that is oxidized and causes reduction.
- Oxidation number — Value indicating the degree of oxidation of an atom in a compound or ion.
- Redox reaction — A reaction involving both oxidation and reduction.
Action Items / Next Steps
- Practice calculating oxidation numbers in various compounds and ions.
- Memorize key tests and expected color changes for identifying oxidizing and reducing agents.
- Review and answer sample exam questions on redox identification.