Overview
This lecture explains how to balance redox (redux) equations in acidic solutions using the half-reaction method, focusing on atom and charge balance.
Assigning Oxidation Numbers
- Assign oxidation numbers to each atom to track electrons gained or lost.
- Use rules: elements alone are zero, oxygen is usually -2, sum of polyatomic ion’s numbers equals its charge.
- Example: In NO3^-, oxygen is -2, nitrogen adjusts so the sum equals -1 (N = +5).
Identifying Oxidation and Reduction
- Oxidation: Oxidation number increases (loss of electrons).
- Reduction: Oxidation number decreases (gain of electrons).
- Example: Ag goes from 0 to +1 (oxidized), N from +5 to +2 (reduced).
Writing Half-Reactions
- Write separate half-reactions for oxidation and reduction.
- Oxidation: Ag → Ag^+.
- Reduction: NO3^- → NO.
Balancing Atoms in Half-Reactions
- First, balance atoms except O and H.
- Balance O with H2O, and balance H with H^+.
- Example: To balance O, add H2O; to balance H, add H^+.
Balancing Charges in Half-Reactions
- Calculate total charge on both sides.
- Add electrons (e^-) to the more positive side to equalize charges.
- Example: Add electrons to the side with higher positive charge.
Combining Half-Reactions
- Multiply half-reactions so electrons lost = electrons gained.
- Add half-reactions together; cancel electrons and any species appearing on both sides.
Final Equation Check
- Ensure all atoms and charges are balanced in the final equation.
- Example: Count each element and total charge on both sides to confirm balance.
Key Terms & Definitions
- Oxidation Number — a value assigned to an atom to track electron transfer.
- Redox Reaction — a chemical reaction involving both reduction and oxidation.
- Half-Reaction — a part of a redox reaction showing either oxidation or reduction alone.
Action Items / Next Steps
- Practice solving various redox equations using the half-reaction method to build proficiency.