Balancing Chemical Equations

Core Concept

Balancing chemical equations is essential for understanding reactions because it ensures that the law of conservation of mass is satisfied — the total number of atoms for each element must be the same on both sides of the equation.

Only Adjust Coefficients: Never change subscripts in a chemical formula to balance an equation, as this changes the substance.
Start with Elements in a Single Compound: Begin by balancing elements that appear in only one reactant and one product to simplify the process.
Check for the Lowest Whole-Number Coefficients: If all coefficients can be divided by a common factor, simplify them.
Polyatomic Ions as Units: If a polyatomic ion appears unchanged on both sides, treat it as a single unit to balance it more easily.

Test Yourself

Assorted Multiple Choice

A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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Balancing Chemical Equations

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Balancing Simple Inorganic Reactions by Inspection

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Balancing Reactions Involving Polyatomic Ions

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Balancing Combustion Reactions

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Writing and Balancing Equations from Descriptions

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Conceptual and Analytical Problems

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Why Balance Chemical Equations?

In a chemical reaction, atoms are neither created nor destroyed; they are simply rearranged. Balancing an equation ensures that each atom in the reactants has a corresponding atom in the products.

The 4-Step Balancing Method

Step 1: Count atoms of each element on both sides.

Example: Make a table: H: 2 left, 2 right; O: 2 left, 1 right

Step 2: Start with the most complex molecule (most elements).

Example: Choose H₂O over H₂ or O₂.

Step 3: Balance one element at a time using coefficients.

Example: Add coefficient 2 to H₂O: → 2H₂O

Step 4: Check your work and reduce to lowest terms.

Example: Verify all atoms balance; ensure smallest whole numbers.

  
      Component
      What It Is
      Can You Change It?
      Example
    
      Subscripts
      Small numbers in formulas showing atoms per molecule
      NO - Changes the compound
      H2O (2 hydrogens, 1 oxygen)
    
      Coefficients
      Numbers in front of formulas showing number of molecules
      YES - This is how we balance
      2H2O (2 water molecules)
    
      Arrow (→)
      Separates reactants from products
      NO - Shows reaction direction
      Reactants → Products

Special Strategies

Polyatomic Ions as Units

When polyatomic ions (like SO₄²⁻, OH⁻, NO₃⁻) appear unchanged on both sides, treat them as single units rather than counting individual atoms.
Example: Ca(OH)₂ + HCl → CaCl₂ + H₂O
Count OH as one unit: 2 OH units on left → need 2 H₂O on right
Balanced: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Fractional Coefficients (Temporary)

Sometimes you’ll get fractions during balancing. Use them temporarily, then multiply everything by the denominator to clear fractions.
Example: If you get C₂H₆ + 3.5O₂ → 2CO₂ + 3H₂O
Multiply all coefficients by 2: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Understanding Equation Components

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