The phrase “complete each ionization equation” commonly refers to writing the Brønsted–Lowry proton-transfer reaction in water, with hydronium (H3O+) formed by acids and hydroxide (OH−) formed by bases. The aqueous phase is assumed, and water appears explicitly as the reacting solvent.
Acid ionization pattern: \( \mathrm{HA + H_2O \rightleftharpoons H_3O^+ + A^-} \)
Base ionization pattern: \( \mathrm{B + H_2O \rightleftharpoons BH^+ + OH^-} \)
Ionization equations in water
Strong acids are represented with a one-way arrow because ionization is essentially complete in dilute aqueous solution. Weak acids and weak bases are represented with an equilibrium arrow because ionization is partial.
| Substance | Classification in water | Products formed in water | Arrow convention |
|---|---|---|---|
| HCl | Strong acid | H3O+ and Cl− | \(\rightarrow\) |
| HNO3 | Strong acid | H3O+ and NO3− | \(\rightarrow\) |
| H2SO4 (first ionization) | Strong acid (first proton) | H3O+ and HSO4− | \(\rightarrow\) |
| HSO4− (second ionization) | Weak acid | H3O+ and SO4^2− | \(\rightleftharpoons\) |
| HF | Weak acid | H3O+ and F− | \(\rightleftharpoons\) |
| CH3COOH | Weak acid | H3O+ and CH3COO− | \(\rightleftharpoons\) |
| H2CO3 (first ionization) | Weak acid | H3O+ and HCO3− | \(\rightleftharpoons\) |
| H3PO4 (first ionization) | Weak acid | H3O+ and H2PO4− | \(\rightleftharpoons\) |
| NH3 | Weak base | NH4+ and OH− | \(\rightleftharpoons\) |
| CH3NH2 | Weak base | CH3NH3+ and OH− | \(\rightleftharpoons\) |
Completed equations written explicitly
Strong acids (essentially complete ionization):
Weak acids (equilibrium ionization):
Weak bases (equilibrium ionization):
Ionization versus dissociation language
Molecular acids and bases form ions by reaction with water and are described by ionization equations. Ionic compounds separate into ions already present in the solid and are described by dissociation equations, represented without water as a reactant (for example, \( \mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)} \)).
Consistency checks for a correct equation
- Atom balance: identical element counts on both sides, including hydrogen and oxygen from water.
- Charge balance: total charge on the reactant side equals total charge on the product side.
- Conjugate pairs: acid and conjugate base differ by exactly one proton; base and conjugate acid differ by exactly one proton.
- Arrow meaning: \(\rightarrow\) for essentially complete ionization in water; \(\rightleftharpoons\) for equilibrium ionization.
Common pitfalls
- Hydrogen placement: the conjugate base keeps the remaining atoms from the acid after loss of one H+.
- Charge accounting: the minus sign on a conjugate base and the plus sign on hydronium or a conjugate acid reflect charge conservation.
- Polyprotic behavior: successive ionizations proceed one proton at a time, with distinct conjugate bases at each stage.