Electrolysis of Aqueous Solutions

Cation Competition: At the cathode, the cation of the solute and water molecules compete for electrons to undergo reduction.

Reactivity Series Rule: Cations of very reactive metals (e.g., $K^+$, $Na^+$, $Ca^{2+}$, $Mg^{2+}$, $Al^{3+}$) are much harder to reduce than water; therefore, water is reduced to hydrogen gas ($H_2$) and hydroxide ions ($OH^-$).

Reduction of Water Equation: The half-reaction is $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$, which typically occurs at $E^\circ = -0.83\text{ V}$ (at pH 7).

Less Reactive Metals: Cations of metals lower in the reactivity series than hydrogen (e.g., $Cu^{2+}$, $Ag^+$, $Au^{3+}$) are more easily reduced than water and will plate out as solid metal on the cathode.

Anion Competition: At the anode, the anion of the solute and water molecules compete to lose electrons and undergo oxidation.

Oxidation of Water Equation: The half-reaction is $2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$, occurring at $E^\circ = +1.23\text{ V}$.

Halide Rule: In concentrated solutions, halide ions ($Cl^-$, $Br^-$, $I^-$) are oxidized to their respective halogens ($Cl_2$, $Br_2$, $I_2$) instead of water, despite water having a lower theoretical oxidation potential.

Polyatomic Anions: Stable oxoanions like $SO_4^{2-}$, $NO_3^-$, and $PO_4^{3-}$ are extremely difficult to oxidize; in their presence, water is almost always oxidized to produce oxygen gas.

Standard Reduction Potential ($E^\circ$): Species with more positive reduction potentials are more easily reduced at the cathode, while species with more negative reduction potentials (or lower oxidation potentials) are more easily oxidized at the anode.

Concentration Effect: Increasing the concentration of an ion can shift the equilibrium and favor its discharge over water, a phenomenon most notable with chloride ions in brine electrolysis.

Overpotential (Overvoltage): An additional voltage beyond the theoretical value required to overcome kinetic barriers, such as the slow formation of gas bubbles ($O_2$ or $H_2$) on certain electrode surfaces.

Electrode Material: Inert electrodes (graphite, platinum) do not participate in the reaction, whereas active electrodes (e.g., copper anode in copper refining) may oxidize themselves into the solution.

The 'Water Check': Always write down the water half-reactions alongside the ion half-reactions before deciding on the product; never assume the salt ions will react.

Halide Concentration: If a question mentions 'concentrated' sodium chloride, the product is $Cl_2$; if it says 'dilute', the product is $O_2$ from water oxidation.

Predicting pH: If $H_2$ gas is produced at the cathode, the solution becomes more basic ($OH^-$ remains); if $O_2$ gas is produced at the anode, it becomes more acidic ($H^+$ remains).

Mass Balance: Remember that if water is electrolyzed, the overall concentration of the remaining solute increases as the solvent volume decreases.