Balancing Chemical Equations
Master the art of balancing chemical equations, understand the law of conservation of mass, and identify different types of chemical reactions.
Why Must Equations Be Balanced?
The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. This means the total number of atoms of each element must be the same on both sides of the equation.
Unbalanced (incorrect):
H₂ + O₂ → H₂O
Left: 2H, 2O | Right: 2H, 1O — Oxygen is not balanced!
Balanced (correct):
2H₂ + O₂ → 2H₂O
Left: 4H, 2O | Right: 4H, 2O — All atoms balanced!
Step-by-Step Balancing Method
Write the unbalanced equation
Identify all reactants and products with their correct chemical formulas. Do NOT change any formulas.
Count atoms on each side
List every element and count how many atoms appear on the reactant (left) and product (right) sides.
Add coefficients to balance
Place numbers (coefficients) in front of formulas to equalise atom counts. Start with the most complex molecule. Never change subscripts.
Check your work
Recount all atoms. Both sides must be equal. Ensure coefficients are in the lowest whole-number ratio.
Important: You can only add coefficients (numbers in front of formulas). You must never change the subscripts in a chemical formula, as that would change the substance itself.
Types of Chemical Reactions
Synthesis (Combination)
Two or more reactants combine to form a single product.
A + B → AB
Example: 2Na + Cl₂ → 2NaCl
Decomposition
A single compound breaks down into two or more simpler substances.
AB → A + B
Example: 2H₂O → 2H₂ + O₂ (electrolysis of water)
Single Displacement (Substitution)
One element replaces another in a compound.
A + BC → AC + B
Example: Zn + CuSO₄ → ZnSO₄ + Cu
Double Displacement (Exchange)
Two compounds exchange ions or atoms to form two new compounds.
AB + CD → AD + CB
Example: NaCl + AgNO₃ → NaNO₃ + AgCl
Combustion
A substance reacts with oxygen, releasing energy as heat and light.
Fuel + O₂ → CO₂ + H₂O + energy
Example: CH₄ + 2O₂ → CO₂ + 2H₂O (burning methane)
Introduction to the Mole Concept
Coefficients in balanced equations also represent the ratio of moles of each substance. A mole is a unit that measures the amount of substance, defined as exactly 6.022 × 10²³ particles (Avogadro's number).
2H₂ + O₂ → 2H₂O
This tells us: 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
Molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol). For example, water (H₂O) has a molar mass of approximately 18 g/mol (2 × 1 for H + 16 for O).
Key Vocabulary
| Term | Definition |
|---|---|
| Reactant | A substance that takes part in and undergoes change during a chemical reaction (left side). |
| Product | A substance formed as a result of a chemical reaction (right side). |
| Coefficient | A number placed before a chemical formula to indicate the number of molecules or moles. |
| Subscript | A number written below and to the right of an element symbol showing the number of atoms. |
| Mole | A unit for measuring amount of substance; 1 mole = 6.022 × 10²³ particles. |
| Conservation of mass | Mass is neither created nor destroyed in a chemical reaction. |
Worked Examples
Balance: Fe + O₂ → Fe₂O₃
Step 1: Count atoms (unbalanced).
Left: 1 Fe, 2 O | Right: 2 Fe, 3 O
Step 2: Balance iron. Place 2 in front of Fe: 2Fe + O₂ → Fe₂O₃
Left: 2 Fe, 2 O | Right: 2 Fe, 3 O (oxygen still unbalanced)
Step 3: Balance oxygen. We need 3 O on the left. Try coefficient 3/2 for O₂, then multiply everything by 2 to get whole numbers:
4Fe + 3O₂ → 2Fe₂O₃
Check: Left: 4 Fe, 6 O | Right: 4 Fe, 6 O. Balanced!
Balance: C₃H₈ + O₂ → CO₂ + H₂O (combustion of propane)
Step 1: Count atoms (unbalanced).
Left: 3 C, 8 H, 2 O | Right: 1 C, 2 H, 3 O
Step 2: Balance carbon. Place 3 in front of CO₂: C₃H₈ + O₂ → 3CO₂ + H₂O
Step 3: Balance hydrogen. Place 4 in front of H₂O: C₃H₈ + O₂ → 3CO₂ + 4H₂O
Step 4: Balance oxygen. Right side: (3×2) + (4×1) = 10 O. So we need 10 O on left: 5O₂.
Final: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Check: Left: 3C, 8H, 10O | Right: 3C, 8H, 10O. Balanced!
Identify the reaction type: 2KClO₃ → 2KCl + 3O₂
Step 1: Count the reactants and products. One reactant (KClO₃) produces two products (KCl and O₂).
Step 2: A single compound breaking into simpler substances = decomposition.
Step 3: Verify balance. Left: 2K, 2Cl, 6O | Right: 2K, 2Cl, 6O. Balanced!
Answer: This is a decomposition reaction. Potassium chlorate decomposes into potassium chloride and oxygen gas when heated.
Knowledge Check
Select the correct answer for each question. Click “Check Answer” to see feedback.
Question 1
What is the correct balanced equation for the reaction of magnesium with oxygen?
Question 2
In the equation: N₂ + 3H₂ → 2NH₃, how many hydrogen atoms are on each side?
Question 3
What type of reaction is: Zn + 2HCl → ZnCl₂ + H₂?
Question 4
Balance this equation: _Al + _O₂ → _Al₂O₃. What are the coefficients?
Question 5
Why is it incorrect to balance an equation by changing the subscripts in a formula?
Key Concepts Summary
- •The law of conservation of mass requires equal atoms on both sides of an equation.
- •Balance equations by adjusting coefficients only — never change subscripts.
- •Five reaction types: synthesis, decomposition, single displacement, double displacement, combustion.
- •Coefficients represent mole ratios of reactants and products.
- •Always check your work by counting atoms on both sides after balancing.