Meaning of strong electrolytes
Strong electrolytes are substances that produce ions in water to an extent that is effectively complete at typical laboratory concentrations. Because the solution contains many mobile ions, it conducts electricity strongly.
In practice, “strong electrolyte” means the dissolved solute exists primarily as separated ions, so ion concentrations can be obtained directly from the formula and the solution molarity.
Dissociation vs ionization
Two related processes lead to ions in aqueous solution:
| Process | Typical solute type | What happens in water | Example equation |
|---|---|---|---|
| Dissociation | Ionic compounds (salts) | Pre-existing ions separate and become hydrated | \(\mathrm{NaCl(s) \rightarrow Na^{+}(aq) + Cl^{-}(aq)}\) |
| Ionization | Molecular acids/bases | Molecules react with water to form ions | \(\mathrm{HCl(g) \rightarrow H^{+}(aq) + Cl^{-}(aq)}\) |
Which substances are strong electrolytes
Strong electrolytes in general chemistry fall into three common categories.
| Category | Criterion | Common examples | Notes |
|---|---|---|---|
| Soluble ionic compounds | Salt that dissolves significantly in water | \(\mathrm{NaCl}\), \(\mathrm{KNO_3}\), \(\mathrm{CaCl_2}\) | Strong once dissolved; insoluble salts do not create many ions in solution. |
| Strong acids | Essentially complete proton donation in water | \(\mathrm{HCl}\), \(\mathrm{HBr}\), \(\mathrm{HI}\), \(\mathrm{HNO_3}\), \(\mathrm{HClO_4}\) | \(\mathrm{H_2SO_4}\) is strong for the first ionization step. |
| Strong bases | Essentially complete hydroxide release in water | \(\mathrm{NaOH}\), \(\mathrm{KOH}\), \(\mathrm{Ba(OH)_2}\), \(\mathrm{Ca(OH)_2}\) | Solubility can limit how much base dissolves, but the dissolved portion dissociates fully. |
Worked example: ion concentrations for a strong electrolyte
Consider a \(0.100\ \mathrm{M}\) aqueous solution of calcium chloride, \(\mathrm{CaCl_2(aq)}\). As a strong electrolyte, it is treated as completely dissociated:
- Relate ions to one formula unit. One \(\mathrm{CaCl_2}\) produces 1 \(\mathrm{Ca^{2+}}\) and 2 \(\mathrm{Cl^{-}}\).
- Multiply the solution molarity by stoichiometric coefficients. \[ [\mathrm{Ca^{2+}}] = 1 \times 0.100 = 0.100\ \mathrm{M} \] \[ [\mathrm{Cl^{-}}] = 2 \times 0.100 = 0.200\ \mathrm{M} \]
- Total dissolved-ion concentration (sum of ion molarities). \[ [\text{ions}]_{\text{total}} = [\mathrm{Ca^{2+}}] + [\mathrm{Cl^{-}}] = 0.100 + 0.200 = 0.300\ \mathrm{M} \]
Related concept: van’t Hoff factor for strong electrolytes
For colligative properties, the ideal van’t Hoff factor \(i\) is the number of ions produced per formula unit for a strong electrolyte (assuming complete dissociation and ideal behavior). For calcium chloride:
Real solutions may deviate slightly from the ideal due to ion pairing, especially at higher concentrations.
Visualization: complete dissociation of a strong electrolyte
Final result for the example
For a \(0.100\ \mathrm{M}\) \(\mathrm{CaCl_2}\) solution treated as a strong electrolyte: \( [\mathrm{Ca^{2+}}] = 0.100\ \mathrm{M} \), \( [\mathrm{Cl^{-}}] = 0.200\ \mathrm{M} \), and the total dissolved-ion concentration is \(0.300\ \mathrm{M}\).