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  • Home / Ask Me Stuff
  • Lessons
    • Thermodynamics >
      • Thermochemistry Worksheet + Answers
      • First Law of Thermodynamics
      • Pressure-Volume Work
      • Enthalpy
      • Hess' Law
      • Enthalpy of Formation
      • Heat Capacity
      • Calorimetry
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      • Third Law of Thermodynamics
      • Spontaneity: Gibbs Free Energy
      • Second Law of Thermodynamics
      • Spontaneity at Different Temperatures
    • Electrochemistry >
      • Redox Reactions
      • Introduction to Half-Reactions
      • Calculating Oxidation Number
      • Has a Redox Reaction Occurred?
      • How to Balance Redox Reactions (Acidic Solution)
      • How to Balance Redox Reactions (Basic Solution)
      • Galvanic Cells
      • Standard Reduction Potentials
      • Electrolytic Cells
      • Nernst Equation
    • Kinetics >
      • Introduction
      • Relative Rates of Reaction
      • Rate Laws
      • Zero-Order Reactions
      • First-Order Reactions
      • Second-Order Reactions
      • Half-Life Expressions
      • Arrhenius Equation
      • How Long will it Take to Decay?
      • What Order is this Reaction?
      • Find the Rate Constant
      • Find the Activation Energy
    • Gases >
      • Pressure, Volume, Temperature
      • Ideal Gas Law
      • Density of Gases
      • Ideal Gas Law and Changes in P, V, T
      • Kinetic Molecular Theory
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      • Partial Pressures
      • Kinetic Energy and Temperature
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      • Writing Equilibrium Expressions
      • Le Chatelier's Principle
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      • Acids and Bases Worksheet + Answers
      • Arrhenius vs Bronsted-Lowry vs Lewis Acids
      • Solve Titration Questions
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      • Phase Diagrams
      • Phase Changes
      • Intermolecular Forces
      • Effects of Intermolecular Forces
      • Ranking by Boiling/Melting Point
      • Clausius-Clapeyron Equation
      • Heating Curves
    • Solids >
      • Ionic/Metallic/Covalent
      • Symmetry of Solids
      • Simple Cubic, fcc and bcc
      • How to Find Edge Length
    • Organic Reactions >
      • Br2 + Alkene (Adding across a double bond)
      • HCl + Alkene (Adding across a double bond)
      • Reaction of OH with Alkyl Halide
      • What is Regioselectivity?
    • Moles and Mass >
      • Average Atomic Mass
      • Solve for Isotopic Abundance
      • Limiting Reagents
      • Percent Yield
      • Actual Yield and Percentage Yield
      • Percent Composition
    • Atomic Structure >
      • What's in an Atom?
      • Quantum Numbers
      • Pauli, Aufbau, Hund
      • Light: E h ν λ
      • Energy Levels of Hydrogen
      • Energy Levels of Non-Hydrogen Atoms
    • Organic Naming >
      • Naming Organic Molecules
      • How to Name Amines
      • How to Name Amides
  • Privacy Policy
  • Lewis Structures

How to Balance Redox Reactions (Acidic Solution)


Since the total number of electrons gained by one atom must be the same as the number of electrons lost by another, we can use this knowledge to balance complicated chemical equations.  Here's a challenge: Balance the following equation.  Remember that the number of each type of atom has to be the same on each side, and so does the total charge!
Picture
This is how it's done, folks:

Cl's oxidation number changes from +5 to -1 (a change of 6)
I's oxidation number changes from 0 to +5 (a change of 5)

The lowest common multiple between these two is 30. So, 6 I atoms must give up 5 electrons each, and 5 Cl atoms must gain 6 each.
Picture
Notice that the coefficient on I2 is “3” since this makes for 6 I atoms total.

Unfortunately, O is not balanced. Add H2O to whichever side is short of O, and add H+ to the other side to counteract the H that you just added.
Picture
Note: After all this work, the charges balance too! WHAT UP.

Also Note: Don't be afraid to add H2O to either side, especially if any of the reactants or products are aqueous. You may end up added OH- to one side, in lieu of adding H+, if the question explicitly tells you that the reaction is occurring in basic solution.  I've done an example of that HERE.
Created by Nathan Oldridge (aka ChemistNATE).  All lessons here are Public Domain.