Chemical Solubility Principles and Electrolysis Reactions
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Solubility Fundamentals
Definition of Solubility
Solubility: The maximum concentration of a solute in a saturated solution at a given temperature in a given solvent.
Factors Affecting Salt Solubility
- Temperature (Temp): Generally, higher temperatures increase solubility.
- Entropy: Highly soluble substances often exhibit high entropy (disorder); the reverse is true for less soluble substances.
- Ion Size: A greater size difference between ions typically leads to higher solubility.
- Charge Density: This is the ratio between the ion's charge and its size. Lower charge density results in higher solubility because the solvent can more easily overcome the electrostatic attraction between the ions of the compound.
Equilibrium Factors Affecting Solubility
- Common Ions: The presence of more common ions decreases solubility. The common ion effect reduces the effective concentration of ions that form the precipitate.
- pH and Solubility: Solution pH affects the solubility of an insoluble salt if the salt's anion is $\text{OH}^-$ (a base) or the conjugate base of a weak acid.
Electrolysis Processes
Electrolysis of Molten NaCl
Cathode: $2(\text{Na}^+ + 1e^- \rightarrow \text{Na} (s))$
Anode: $2\text{Cl}^- \rightarrow \text{Cl}_2(g) + 2e^-$
Electrolysis of Aqueous NaCl Solution (Kinetically Favorable)
Anode: $2\text{Cl}^- \rightarrow \text{Cl}_2(g) + 2e^-$
Cathode: $2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2(g) + 2\text{OH}^-$
Electrolysis of $\text{H}_2\text{SO}_4$ Solution
Anode: $2\text{H}_2\text{O} \rightarrow \text{O}_2(g) + 4\text{H}^+ + 4e^-$
Cathode: $2(2\text{H}^+ + 2e^- \rightarrow \text{H}_2(g))$
Electrolysis of $\text{Na}_2\text{SO}_4$ Solution
Anode: $2\text{H}_2\text{O} \rightarrow \text{O}_2(g) + 4\text{H}^+ + 4e^-$
Cathode: $2(2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2(g) + 2\text{OH}^-)$
Active Species in Electrolysis
Anode Species:
- $\text{H}_2\text{O} / \text{O}_2$
- $\text{Cl}^- / \text{Cl}_2$
- $\text{OH}^- / \text{O}_2$
Cathode Species:
- $\text{H}_2\text{O} / \text{H}_2$
- $\text{H}^+ / \text{H}_2$
- Metal ions (if reduction potential is favorable)
Inactive Species in Electrolysis
Oxidation (Anode):
- $\text{SO}_4^{2-}$
- $\text{NO}_3^-$
Reduction (Cathode):
- $\text{SO}_4^{2-}$
- $\text{NO}_3^-$
- Metal ions (if reduction potential is unfavorable)
Electroplating
Electroplating (EP): Obtaining metals (where $E^\circ = 0$; solids) by electrolysis of a solution containing their ions.
Solubility and Precipitation Chart
Cations and Anions: Solubility of Salts
| Cations | Anions | Solubility |
|---|---|---|
| All Alkalies, $\text{H}^+, \text{NH}_4^+$ | (General) | Soluble |
| (General) | $\text{NO}_3^-, \text{ClO}_3^-$ | Almost all Soluble |
| (General) | $\text{CH}_3\text{COO}^-$ | (Implied Soluble) |
| $\text{Ag}^+, \text{Cu}^+, \text{Hg}_2^{2+}, \text{Tl}^+, \text{Pb}^{2+}$ | $\text{Cl}^-, \text{Br}^-, \text{I}^-$ | Insoluble |
| (Other Ions) | $\text{Cl}^-, \text{Br}^-, \text{I}^-$ | Other Soluble |
| $\text{Ca}^{2+}, \text{Sr}^{2+}, \text{Ba}^{2+}, \text{Hg}_2^{2+}, \text{Pb}^{2+}$ | $\text{SO}_4^{2-}$ | Insoluble |
| (Other Ions) | $\text{SO}_4^{2-}$ | Other Soluble |
| Alkaline Metals, $\text{Ca}^{2+}, \text{Sr}^{2+}, \text{Ba}^{2+}, \text{NH}_4^+$ | $\text{S}^{2-}$ | Soluble |
| (Other Ions) | $\text{S}^{2-}$ | Insoluble |
| Alkaline Metals, $\text{Sr}^{2+}, \text{Ba}^{2+}, \text{NH}_4^+$ | $\text{OH}^-$ | Soluble |
| (Other Ions) | $\text{OH}^-$ | Insoluble |
| Alkali Metals, $\text{H}^+, \text{NH}_4^+$ | $\text{CO}_3^{2-}$ | Soluble |
| (Other Ions) | $\text{CO}_3^{2-}, \text{PO}_4^{3-}$ | Insoluble |