Balancing Chemical Equations
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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.
Practice Tips
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.
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LABORATORY
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DEMONSTRATIONS
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Core Concept
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.
Understanding Equation Components
| 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