Chemistry Tutorial: Balancing Half Equations

Introduction

• Suitable for NCEA Level 2 and NCEA Level 3 chemistry students
• Focus: Balancing half equations in redox reactions

Importance of Half Equations

• Essential for understanding redox reactions
• Redox involves the movement of electrons
• Important for electricity generation and batteries

Steps for Balancing Half Equations

1. Balance unique elements (except H and O):
   • Ensure the same number of each element on both sides.
2. Balance oxygen by adding water (H₂O):
   • Add the necessary amount of H₂O to balance the O atoms.
3. Balance hydrogen by adding H⁺ ions:
   • Add H⁺ to balance the H atoms.
4. Balance charge by adding electrons (e⁻):
   • Ensure the net charge is the same on both sides of the equation.
   • Electrons should be added to the more positive side to balance the charge.

Example: Balancing Dichromate and Chromium Half Equation

1. Balance Cr atoms:
   • Start: Cr₂O₇²⁻ → Cr³⁺
   • Observation: 2 Cr on left, 1 Cr on right
   • Solution: Add coefficient 2 before Cr³⁺
2. Balance Oxygen:
   • Observation: 7 O on left, add 7 H₂O on the right
3. Balance Hydrogen:
   • Observation: Added 7 H₂O means 14 H on the right
   • Solution: Add 14 H⁺ on the left
4. Balance Charge:
   • Calculate total charge on both sides and add electrons to balance
   • Example: Left side (Cr₂O₇²⁻ + 14 H⁺) = +12, Right side (2 Cr³⁺) = +6
   • Add 6 electrons to the left to balance
   • Electrons on the left indicate a reduction reaction

Further Examples

1. Permanganate to Manganese (MnO₄⁻ → Mn²⁺)

• Balance Mn: 1 Mn on both sides
  • Balance O: Add 4 H₂O to the right
  • Balance H: Add 8 H⁺ to the left
  • Balance Charge: Left = +7, Right = +2
  • Add 5 electrons to the left
  • Electrons on the left indicate a reduction reaction

2. Iodide to Iodine (I⁻ → I₂)

• Balance I: 2 I⁻ on left, 1 I₂ on right
  • Skip steps 2 and 3 (no O or H involved)
  • Balance Charge: Left = -2, Right = 0
  • Add 2 electrons to the right (oxidation reaction)

3. Iron (II) to Iron (III) (Fe²⁺ → Fe³⁺)

• Balance Fe: 1 Fe on both sides
  • Skip steps 2 and 3 (no O or H involved)
  • Balance Charge: Left = +2, Right = +3
  • Add 1 electron to the right
  • Indicates oxidation reaction

Combining Half Equations

Example: Aluminum and Bromine

• Reaction: Al → Al³⁺ and Br₂ → 2 Br⁻
• Balance electrons for full equation:
  • Al → Al³⁺ + 3e⁻
  • 2 Br⁻ → Br₂ + 2e⁻
  • Adjust multiples to balance electrons
  • Combine and simplify the full equation

Simplified Full Equation

• Make sure to combine and cancel out common elements
• Ensure final simplified equation is accurate and balanced

Conclusion

• Revision and practice are essential
• Key takeaways: Remember the steps and balance method
• Future lessons will focus on advanced concepts for Year 13 and quantitative analysis

Note: Like and subscribe for more tutorials; stay tuned for updates!