Physical Science / Unit 5: Chemical Reactions
5.3 Balancing Equations
Notes
To obey conservation of mass, an equation must have the same number of each atom on both sides.
- Count atoms on each side. Change only coefficients, never subscripts.
- Balance elements that appear in one compound first; balance $\text{O}$ and $\text{H}$ last.
- Example: $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ becomes $\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O}$ (4 H and 2 O on each side).
- Example: $\text{N}_2 + \text{3H}_2 \rightarrow \text{2NH}_3$.
Practice
What coefficient is needed in front of $\text{H}_2\text{O}$ to balance $\text{CH}_4 + \text{2O}_2 \rightarrow \text{CO}_2 + \_\text{H}_2\text{O}$?
The 4 H atoms in methane need $2 \times 2 = 4$ H in the water.
Which equation is balanced?
Only the third has 2 Mg and 2 O on each side.
A student balances $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ by changing the product to $\text{H}_2\text{O}_2$. What is wrong?
Balancing only uses coefficients; $\text{H}_2\text{O}_2$ is hydrogen peroxide, not water.
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