Electrolysis

Electrolysis is a process that uses a direct electric current to drive an otherwise non-spontaneous chemical reaction. The word literally means 'splitting by electricity'. For electrolysis to occur, a substance must be able to conduct electricity. Materials are classified as either electrical conductors (allow electricity to pass through, e.g., metals, graphite) or insulators (do not allow electricity to pass through, e.g., plastic, rubber). The substance being broken down is called the electrolyte. An electrolyte is an ionic compound that is either molten (melted) or dissolved in water. In these states, its ions are free to move and carry charge. A solid ionic compound cannot be electrolysed because its ions are held in a fixed crystal lattice and are not mobile. The electricity enters and leaves the electrolyte via electrodes. The positive electrode is the anode, and the negative electrode is the cathode.

When a simple ionic compound containing two elements is melted, it separates into its positive ions (cations) and negative ions (anions). During electrolysis, these mobile ions are attracted to the electrode with the opposite charge. Cations (+) move to the cathode (-). Anions (-) move to the anode (+). At the cathode, the positive ions gain electrons, which is a process called reduction. This typically forms a metal. At the anode, the negative ions lose electrons, which is a process called oxidation. This typically forms a non-metal. A good way to remember this is 'PANIC' (Positive Anode, Negative Is Cathode) and 'OILRIG' (Oxidation Is Loss, Reduction Is Gain of electrons).

Electrolysing an aqueous solution is more complex than a molten salt because water itself can partially ionise to form hydrogen ions (H⁺) and hydroxide ions (OH⁻). This means there are now four ions in the solution: the cation and anion from the salt, plus H⁺ and OH⁻ from the water. There is a 'competition' at each electrode to see which ion will be discharged (lose or gain electrons). The rules are: At the Cathode (-): The less reactive positive ion is discharged. If the metal ion from the salt is more reactive than hydrogen (e.g., Na⁺, K⁺, Ca²⁺), then hydrogen gas (H₂) will be produced from H⁺ ions. If the metal ion is less reactive than hydrogen (e.g., Cu²⁺, Ag⁺), the metal will be deposited. At the Anode (+): If the solution contains a high concentration of halide ions (Cl⁻, Br⁻, I⁻), the halogen (Cl₂, Br₂, I₂) will be produced. If there are no halides, or they are very dilute, then oxygen (O₂) is produced from the OH⁻ ions.

Brine is a concentrated aqueous solution of sodium chloride (NaCl). Its electrolysis is a major industrial process called the chlor-alkali process. The ions present are Na⁺, Cl⁻, H⁺, and OH⁻. At the cathode (-): Sodium is much more reactive than hydrogen, so hydrogen gas is produced. The half-equation is 2H⁺(aq) + 2e⁻ → H₂(g). At the anode (+): Because the solution is concentrated with chloride ions (Cl⁻), chlorine gas is produced instead of oxygen. The half-equation is 2Cl⁻(aq) → Cl₂(g) + 2e⁻. As H⁺ and Cl⁻ ions are removed from the solution, Na⁺ and OH⁻ ions are left behind, forming a solution of sodium hydroxide (NaOH). The three products - chlorine, hydrogen, and sodium hydroxide - all have important industrial uses.

So far, we have used inert electrodes (like graphite or platinum) that do not react. However, if we use active electrodes, they can participate in the reaction. This is the principle behind refining impure copper. An electrolytic cell is set up with a large block of impure copper as the anode (+) and a thin sheet of pure copper as the cathode (-). The electrolyte is an aqueous solution of copper(II) sulfate. At the anode (+): The copper atoms from the impure anode lose electrons and dissolve into the solution as copper ions. The more reactive metal impurities also dissolve, but the less reactive ones (like gold and silver) do not and fall to the bottom as 'anode sludge'. Anode reaction: Cu(s) → Cu²⁺(aq) + 2e⁻. At the cathode (-): The copper ions from the solution are attracted to the cathode, where they gain electrons and are deposited as a layer of very pure copper. Cathode reaction: Cu²⁺(aq) + 2e⁻ → Cu(s). Overall, copper is transferred from the impure anode to the pure cathode, leaving the impurities behind. This process is vital as even small amounts of impurity drastically reduce copper's electrical conductivity.

Electroplating is a process that uses electrolysis to coat one metal object with a thin, decorative, or protective layer of another metal. For example, silver plating cheap metal cutlery or chromium plating steel car parts to prevent rust. The setup is crucial:

1. The Cathode (-) is the object to be plated (e.g., a steel spoon). It must be the negative electrode to attract the positive metal ions.
2. The Anode (+) is a bar of the pure plating metal (e.g., pure silver). It dissolves to replenish the metal ions in the electrolyte.
3. The Electrolyte must be a solution containing ions of the plating metal (e.g., silver nitrate solution for silver plating).

During the process, metal ions from the electrolyte are reduced and deposited onto the cathode object. Simultaneously, the metal anode is oxidised, dissolving into the electrolyte to replace the ions that were deposited. This keeps the electrolyte concentration constant and allows the process to continue.