Chemistry 115: Redox Reactions Lecture Notes

Date: October 5, 2020, Monday

Introduction to Redox Reactions

• Definition: Chemical reactions where electrons are transferred from one species to another.
• Importance:
  • Essential in numerous processes like photosynthesis, cellular respiration, and battery reactions.

Key Characteristics of Redox Reactions

• Oxidation and Reduction: Two halves of a redox reaction
  • Oxidation: Loss of electrons (e.g., Sodium losing an electron to form sodium ion)
  • Reduction: Gain of electrons (e.g., Chlorine gaining an electron to form chloride ion)
• OIL-RIG Acronym:
  • Oxidation Involves Loss of electrons
  • Reduction Involves Gain of electrons

Agents in Redox Reactions

• Reducing Agent: The species that undergoes oxidation and loses electrons.
• Oxidizing Agent: The species that undergoes reduction and gains electrons.
  • Example: In NaCl formation, sodium is the reducing agent and chlorine is the oxidizing agent.

Identifying Oxidized and Reduced Species

• Determining Electron Transfer: Identify species losing or gaining electrons.
• Oxidation States/Numbers: System used to track electron transfer in reactions.

Rules for Assigning Oxidation Numbers

1. Free Elements: Have an oxidation state of 0 (e.g., Na, Cl2).
2. Monatomic Ions: Oxidation state equals the ion charge (e.g., Na⁺ = +1, Cl⁻ = -1).
3. Fluorine in Compounds: Always -1.
4. Oxygen in Compounds: Usually -2, except in peroxides (e.g., H2O2 where it's -1).
5. Hydrogen in Compounds: Usually +1, except in metal hydrides (e.g., NaH where it's -1).
6. Calculation Rule: The sum of oxidation states in a molecule should equal the charge of the group.

Examples of Oxidation Number Calculations

• Carbon Dioxide (CO2): Carbon is +4, Oxygen is -2.
• Sulphate Ion (SO4²⁻): Sulfur is +6.
• Ammonia (NH3): Nitrogen is -3, Hydrogen is +1.

Concept of Unequal Sharing in Molecular Compounds

• Molecular compounds have covalent bonds where sharing of electrons can be unequal.
• Results in assigned oxidation numbers reflecting perceived electron transfer.

Practice and Importance of Assigning Oxidation Numbers

• Essential for determining roles of reactants in redox reactions.
• Practice with textbook exercises for mastery.

Balancing Redox Reactions

• Importance: Provides coefficients for stoichiometry and allows calculation.
• Challenges: More complex than simple equations due to breakdown of polyatomic ions.
• Methods: Several methods exist; focus here on the half-reaction method.

Half-Reaction Method for Balancing Equations

1. Identify Half Reactions: Determine who is turning into what.

• Example: Br2 becoming Br⁻ and U⁴⁺ becoming UO2²⁺.

Conclusion

• Begin practicing splitting redox reactions into half-reactions.
• Continued discussion and balancing techniques to be covered in next lecture.