Balancing Equations and Conservation of Mass
Conservation of Mass
The law of conservation of mass states that no atoms are created or destroyed during a chemical reaction. The total mass of the products always equals the total mass of the reactants.
This means:
- The same types and numbers of atoms exist before and after the reaction
- They are simply rearranged into different substances
- The total mass of a sealed system does not change
Apparent Mass Changes
Sometimes it appears that mass changes during a reaction. This happens when:
- Gas escapes: If a reaction produces a gas in an open container, the mass appears to decrease because the gas leaves (e.g. a metal reacting with acid — hydrogen escapes)
- Gas is taken in: If a substance reacts with a gas from the air, the mass appears to increase (e.g. magnesium burning in air — it gains oxygen)
In both cases, if you could weigh all the reactants and all the products (including gases), the total mass would be conserved.
Writing Word Equations
A word equation shows the names of reactants and products:
magnesium + oxygen → magnesium oxide
sodium hydroxide + hydrochloric acid → sodium chloride + water
Writing Symbol Equations
A symbol equation uses chemical formulae:
Mg + O₂ → MgO (unbalanced)
Balancing Equations
A balanced equation has the same number of each type of atom on both sides. You balance by putting numbers in front of formulae — never change the subscripts in a formula.
Method for Balancing
1. Write the correct formulae for all reactants and products
2. Count each type of atom on both sides
3. Adjust coefficients (big numbers in front) to balance one element at a time
4. Start with the most complex molecule or any element that appears in only one reactant and one product
5. Leave single elements (like O₂) until last
6. Check all atoms balance
Worked Example 1
Balance: Fe + O₂ → Fe₂O₃
Count: Fe: 1 on left, 2 on right. O: 2 on left, 3 on right.
Step 1: Balance Fe → put 2 in front of Fe on left: 2Fe + O₂ → Fe₂O₃
Step 2: Balance O — 3 on right is awkward. Try putting 2 in front of Fe₂O₃: 2Fe + O₂ → 2Fe₂O₃
Now: Fe = 2 left, 4 right. O = 2 left, 6 right.
Fix Fe: 4Fe + O₂ → 2Fe₂O₃
Fix O: 4Fe + 3O₂ → 2Fe₂O₃
Check: Fe = 4 each side. O = 6 each side. Balanced.
4Fe + 3O₂ → 2Fe₂O₃
Worked Example 2
Balance: CH₄ + O₂ → CO₂ + H₂O
C: 1 each side (balanced)
H: 4 left, 2 right → put 2 in front of H₂O
CH₄ + O₂ → CO₂ + 2H₂O
O: 2 left, 4 right (2 in CO₂ + 2 in 2H₂O) → put 2 in front of O₂
CH₄ + 2O₂ → CO₂ + 2H₂O
State Symbols
State symbols show the physical state of each substance:
| Symbol | State |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous (dissolved in water) |
Example: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
Using Balanced Equations for Mass Calculations
Balanced equations give the mole ratio. Combined with relative formula masses, you can calculate masses.
Example
What mass of carbon dioxide is produced when 10 g of calcium carbonate decomposes?
CaCO₃ → CaO + CO₂
Mr of CaCO₃ = 100, Mr of CO₂ = 44
Moles of CaCO₃ = 10 ÷ 100 = 0.1 mol
From equation: 1 mol CaCO₃ produces 1 mol CO₂
Moles of CO₂ = 0.1 mol
Mass of CO₂ = 0.1 × 44 = 4.4 g
Half Equations (Higher Tier)
Half equations show what happens at each electrode during electrolysis or in redox reactions. They must be balanced for atoms and charge.
Example — reduction of copper ions:
Cu²⁺ + 2e⁻ → Cu
Example — oxidation of chloride ions:
2Cl⁻ → Cl₂ + 2e⁻
Exam Tips
- Never change a chemical formula to balance an equation — only put numbers in front
- Always include state symbols when asked
- Show your atom count on both sides to demonstrate it is balanced
- Conservation of mass questions often ask you to explain apparent mass changes — always mention gas escaping or being absorbed