Chemical Reactions & Balancing Equations
Key terms, reaction types, and the balancing routine from this lesson.
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Questions Covered in This Set
11 cards to master
Why must chemical equations be balanced?
Because of conservation of mass — atoms are never created or destroyed in a reaction, only rearranged.
What is a subscript, and can you change it?
The small number inside a formula (the 4 in CH₄) showing atoms per molecule. Never change it — it would change the substance (H₂O vs H₂O₂).
What is a coefficient?
The large number in front of a formula (the 2 in 2O₂) showing how many molecules. Coefficients are the ONLY thing you adjust when balancing.
State symbols: (s), (l), (g), (aq)
Solid, liquid, gas, and dissolved in water (aqueous).
List the five reaction types with their patterns.
Synthesis A+B→AB; Decomposition AB→A+B; Single displacement A+BC→AC+B; Double displacement AB+CD→AD+CB; Combustion fuel+O₂.
What are the products of complete combustion of a hydrocarbon?
Always CO₂ and H₂O. Incomplete combustion gives CO or soot (C).
In what order should you balance elements?
Elements appearing in only one compound on each side first; leave H and O for later; leave lone elements (O₂, Fe) for last as the free adjuster.
How do you handle polyatomic ions like SO₄²⁻ or NO₃⁻?
Treat them as a single block if they survive intact on both sides.
What do you do if balancing gives a fraction?
Multiply every coefficient in the equation through to clear it (e.g. ×2 for 3½O₂).
Name the diatomic elements.
H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ — they must be written as pairs.
Balance: C₃H₈ + O₂ → CO₂ + H₂O
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
