Chemistry - Electrochemistry

Electrolysis is the process of using electricity to break down ionic compounds into their constituent elements. The substance undergoing electrolysis must be in a molten or aqueous state, known as an $electrolyte$, so that ions are free to move.

Electrodes: The positive electrode is the $anode$, and the negative electrode is the $cathode$. A common mnemonic is $PANIC$: Positive Anode, Negative Is Cathode.

Oxidation and Reduction: Oxidation occurs at the anode (loss of electrons), and reduction occurs at the cathode (gain of electrons). This is summarized by $OIL RIG$: Oxidation Is Loss, Reduction Is Gain.

Electrolysis of molten compounds: For example, in molten $PbBr_2$, lead ions ($Pb^{2+}$) are reduced to lead metal at the cathode, and bromide ions ($Br^-$) are oxidized to bromine gas at the anode.

Electrolysis of aqueous solutions: At the cathode, either the metal ion or $H^+$ is discharged. The less reactive species is discharged (usually $H^+$ unless the metal is below hydrogen in the reactivity series, like $Cu^{2+}$ or $Ag^+$).

At the anode in aqueous solutions: Hydroxide ions ($OH^-$) are discharged to form $O_2$ and $H_2O$ unless a high concentration of halide ions ($Cl^-$, $Br^-$, $I^-$) is present.

Electroplating: A process used to coat a metal object with a thin layer of another metal. The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.

Hydrogen Fuel Cells: These produce electricity from the reaction between hydrogen and oxygen. The overall reaction is $2H_2(g) + O_2(g) ightarrow 2H_2O(l)$, which is efficient and produces only water as a byproduct.