Balancing Redox Reactions Using the Ion-Electron Method

Introduction

• Today's topic: Balancing reactions in acid medium using the ion-electron method.
• The ion-electron method involves a systematic step-by-step process.

Steps to Balance Redox Reactions

Step 1: Write and Split the Reaction

• Example reaction:
  • Unbalanced: Fe^2+ + Cr2O7^2– → Fe^3+ + Cr^3+
• Split into half-reactions:
  • Oxidation half-reaction
  • Reduction half-reaction

Step 2: Determine Oxidation States

• Identify oxidation states of elements in the reactants and products.
  • For Fe^2+: Oxidation state = +2
  • For Fe^3+: Oxidation state = +3
  • For Cr2O7^2–:
    • Oxygen: Oxidation state = –2
    • Chromium: Determine using the overall charge

Step 3: Write Half-Reactions

• Oxidation half-reaction: Fe^2+ → Fe^3+ + e–
• Reduction half-reaction:
  • Cr2O7^2– + 14H+ + 6e– → 2Cr^3+ + 7H2O

Step 4: Balance Each Half-Reaction

• Balance atoms other than H and O first.
• Then balance O by adding H2O and H by adding H+.
• Balance charges by adding electrons (e–).

Step 5: Combine Half-Reactions

• Make the number of electrons equal in both half-reactions by multiplying appropriately.
• Combine and cancel out common species.
  • Example combined reaction:
    • 6Fe^2+ + Cr2O7^2– + 14H+ → 6Fe^3+ + 2Cr^3+ + 7H2O

Step 6: Verify the Balanced Equation

• Ensure both mass and charge are balanced on both sides.

Key Concepts

• Oxidation involves an increase in oxidation state.
• Reduction involves a decrease in oxidation state.
• The method requires balancing atoms and charges stepwise before combining half-reactions.

Summary

• Follow a systematic approach for balancing redox reactions in acidic medium.
• Ensure the final equation is balanced in terms of atoms and charge.