Electrolysis

• Ion Migration: When the power supply is activated, the electric field causes cations (positive ions) to migrate toward the negative cathode and anions (negative ions) to migrate toward the positive anode.

• Oxidation at the Anode: Negative ions lose electrons to the anode surface. For example, in molten lead(II) bromide, bromide ions are oxidized: $2Br^- \rightarrow Br_2 + 2e^-$

• Reduction at the Cathode: Positive ions gain electrons from the cathode surface. In the same molten system, lead ions are reduced: $Pb^{2+} + 2e^- \rightarrow Pb$

• Electron Flow: Electrons flow through the external circuit from the anode to the cathode, driven by the external DC power source, completing the electrical circuit.

• In aqueous solutions, water molecules dissociate into $H^+$ and $OH^-$ ions, meaning multiple types of ions compete for discharge at each electrode.

• Cathode Rules: The ion of the least reactive element is discharged. If the metal is more reactive than hydrogen (e.g., $Na, K, Mg$), hydrogen gas is produced. If the metal is less reactive (e.g., $Cu, Ag$), the metal is deposited.

• Anode Rules: If halide ions ($Cl^-, Br^-, I^-$) are present in high concentration, the halogen is produced. If no halides are present or the solution is very dilute, $OH^-$ ions are discharged to produce oxygen gas: $4OH^- \rightarrow O_2 + 2H_2O + 4e^-$

Summary Table of Discharge

• First Law: The mass ($m$) of a substance produced at an electrode is directly proportional to the quantity of electricity ($Q$) passed through the electrolyte. This is expressed as $Q = I \times t$, where $I$ is current in Amperes and $t$ is time in seconds.

• Second Law: The number of moles of substance discharged depends on the charge of the ion ($z$) and the total charge passed. One mole of electrons carries a charge of approximately $96500$ Coulombs, known as the Faraday Constant ($F$).

• The General Formula: The mass of a substance deposited can be calculated using: $m = \frac{I \cdot t \cdot M}{z \cdot F}$ where $M$ is the molar mass of the substance and $z$ is the number of electrons transferred per ion.

• Check the State: Always identify if the electrolyte is molten or aqueous. Molten electrolytes only have two types of ions, while aqueous electrolytes always include $H^+$ and $OH^-$ from water.

• Time Conversion: In quantitative problems, ensure time is converted to seconds before using $Q = It$. Forgetting to convert minutes or hours to seconds is a frequent source of error.

• Half-Equation Balance: When writing half-equations, ensure both the number of atoms and the total charge are balanced on both sides of the arrow.

• Reactivity Series: Memorize the position of Hydrogen relative to common metals (like Copper and Sodium) to correctly predict cathode products in aqueous solutions.