Chemistry / Unit 7: Chemical Reactions
7.2 Balancing Chemical Equations
Notes
A balanced equation has the same number of each kind of atom on both sides.
- Change only coefficients (numbers in front), never subscripts. Changing a subscript changes the substance.
- Steps: (1) count atoms on each side, (2) balance elements that appear in only one substance on each side first, (3) leave H and O (or lone elements) for last, (4) reduce coefficients to the lowest whole numbers.
- Treat a polyatomic ion that stays together as one unit.
- Example: $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$ has 2 N and 6 H on each side.
Practice
Balance: $\_\ \text{Al} + \_\ \text{O}_2 \rightarrow \_\ \text{Al}_2\text{O}_3$
$4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3$: 4 Al and 6 O on each side.
Balance: $\_\ \text{Fe} + \_\ \text{Cl}_2 \rightarrow \_\ \text{FeCl}_3$
$2\text{Fe} + 3\text{Cl}_2 \rightarrow 2\text{FeCl}_3$: 2 Fe and 6 Cl on each side.
A student balances $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$ by writing $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}_2$. What is wrong?
$\text{H}_2\text{O}_2$ is hydrogen peroxide, not water. The correct answer is $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$.
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