Chemical equations
Balanced equations
- A chemical equation must be balanced: the same number of each atom on both sides.
- State symbols show the state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
$$\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$$
Practice
In H2 + O2 → H2O, what number must go in front of H2O to balance the oxygen and hydrogen? (2H2 + O2 → ?H2O)
2H2 + O2 → 2H2O balances 4 H and 2 O on each side.
Practice
The state symbol (aq) means the substance is:
(aq) = aqueous = dissolved in water; (s), (l), (g) are solid, liquid, gas.
Ionic equations
- An ionic equation shows only the ions that actually change.
- Ions that are the same on both sides are spectator ions and are left out.
- For an acid + alkali:
$$\text{H}^{+}(aq) + \text{OH}^{-}(aq) \rightarrow \text{H}_2\text{O}(l)$$
Practice
An ionic equation leaves out the spectator ions that do not change.
Only the ions that take part are shown; unchanged spectator ions are omitted.
You've got it
Key idea
- a balanced equation has equal atoms on both sides; state symbols are (s) (l) (g) (aq)
- an ionic equation drops the spectator ions that don't change
- neutralisation: $\text{H}^{+} + \text{OH}^{-} \rightarrow \text{H}_2\text{O}$