Chemistry / Unit 7: Chemical Reactions
7.5 Double Replacement and Solubility
Notes
In a double replacement reaction, two ionic compounds swap partners: $\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$.
- It happens when a product is a precipitate (insoluble solid), a gas, or water.
- Solubility rules (key ones): compounds with $\text{Na}^+$, $\text{K}^+$, $\text{NH}_4^+$ or $\text{NO}_3^-$ are always soluble. Chlorides are soluble except with Ag, Pb and Hg. Most sulfates are soluble except with Ba, Pb and Ca. Most carbonates, phosphates and hydroxides are insoluble (except with group 1 and $\text{NH}_4^+$).
- Example: $\text{AgNO}_3\text{(aq)} + \text{NaCl(aq)} \rightarrow \text{AgCl(s)} + \text{NaNO}_3\text{(aq)}$.
- A net ionic equation shows only the ions that react: $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl(s)}$. Ions that do not change are spectator ions ($\text{Na}^+$ and $\text{NO}_3^-$).
Practice
Which compound is insoluble in water?
Barium sulfate is an exception to the sulfate rule. The others contain always-soluble ions.
When solutions of $\text{BaCl}_2$ and $\text{Na}_2\text{SO}_4$ are mixed, what precipitate forms?
The ions swap to make NaCl (soluble) and $\text{BaSO}_4$ (insoluble).
What is the net ionic equation for $\text{AgNO}_3\text{(aq)} + \text{KCl(aq)} \rightarrow \text{AgCl(s)} + \text{KNO}_3\text{(aq)}$?
$\text{K}^+$ and $\text{NO}_3^-$ stay dissolved, so they are spectator ions and are left out.
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