Home / General Chemistry I / 2. Aqueous Reactions

2. Aqueous Reactions

Many reactions happen in water, marked $(aq)$. Whether dissolved ions react, neutralize, or do nothing at all depends on solubility — a precipitate forming is often the whole point of the reaction.

Precipitation Reactions and Solubility Rules

Key rules of thumb:

Nitrates ($\text{NO}_3^-$), acetates, and Group 1 / ammonium salts are always soluble.
Halides are soluble except $\text{Ag}^+$, $\text{Pb}^{2+}$, and $\text{Hg}_2^{2+}$.
Sulfates are soluble except $\text{Ba}^{2+}$, $\text{Pb}^{2+}$, $\text{Sr}^{2+}$, and $\text{Ca}^{2+}$ (slightly).
Carbonates, phosphates, and hydroxides are insoluble except with Group 1 metals or $\text{NH}_4^+$.

Example: Mixing $\text{AgNO}_3(aq)$ and $\text{NaCl}(aq)$ gives solid $\text{AgCl}$ (silver halide = insoluble): $$\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$$

Molecular, Ionic, and Net Ionic Equations

The full ionic equation splits every $(aq)$ compound into ions; spectator ions ($\text{Na}^+$, $\text{NO}_3^-$ above) cancel out, leaving the net ionic equation:

$$\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)$$

The net ionic equation shows the actual chemical change.

Acid–Base Neutralization

An acid plus a base produces a salt and water. For $2\,\text{HCl} + \text{Ca}(\text{OH})_2 \rightarrow \text{CaCl}_2 + 2\,\text{H}_2\text{O}$, the net ionic equation is always $$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$$

Gas-Forming and Redox Reactions in Solution

Carbonates plus acid release $\text{CO}_2$ gas: $2\,\text{HCl} + \text{Na}_2\text{CO}_3 \rightarrow 2\,\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2$. Redox reactions track electron transfer with oxidation numbers — e.g., in $\text{H}_2\text{SO}_4$, H is +1 and O is −2, so S must be +6.

Practice Problems

Write the net ionic equation for Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq).
Does a precipitate form when BaCl₂(aq) mixes with Na₂SO₄(aq)? If so, identify it.
Balance and write the net ionic equation for H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O.
Assign oxidation numbers to every element in K₂Cr₂O₇.