1. The Law of Conservation of Mass
The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element on the reactant (left) side MUST equal the total number of atoms on the product (right) side.
2. The Golden Rule: Only Change Coefficients, Never Subscripts!
When balancing an equation, you may place big numbers (coefficients) in front of chemical formulas (e.g., \(2\text{H}_2\text{O}\)). NEVER alter the small subscript numbers inside a chemical formula (e.g., changing \(\text{H}_2\text{O}\) to \(\text{H}_2\text{O}_2\)), as this changes the chemical identity of the substance!
Worked Example 1 (Formation of Water):
Unbalanced Equation: \(\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}\)
Atom Inventory:
Reactants: H = 2, O = 2
Products: H = 2, O = 1 (Oxygen is unbalanced)
Step 1: Put coefficient 2 in front of \(\text{H}_2\text{O}\):
\(\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\) (Products now have H = 4, O = 2)
Step 2: Put coefficient 2 in front of \(\text{H}_2\):
\(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\) (Balanced: H = 4, O = 2 on both sides).
Unbalanced Equation: \(\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}\)
Atom Inventory:
Reactants: H = 2, O = 2
Products: H = 2, O = 1 (Oxygen is unbalanced)
Step 1: Put coefficient 2 in front of \(\text{H}_2\text{O}\):
\(\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\) (Products now have H = 4, O = 2)
Step 2: Put coefficient 2 in front of \(\text{H}_2\):
\(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\) (Balanced: H = 4, O = 2 on both sides).
3. Combustion of Methane
Worked Example 2:
Unbalanced: \(\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}\)
Balanced: \(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\).
Unbalanced: \(\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}\)
Balanced: \(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\).
4. Practice Questions
- Balance: \(\text{N}_2 + \text{H}_2 \rightarrow \text{NH}_3\)
- Balance: \(\text{Mg} + \text{O}_2 \rightarrow \text{MgO}\)