Electrolysis

Electrolysis Content Electrolysis of a molten binary ionic compound At the anode At the cathode - Anions are preferentially discharged, they lose electrons and form a gas Ionic half equation 𝑀𝑥− → 𝑥𝑒− + 𝑀 - Cations are preferentially discharged, gain electrons (oxidized) to form a liquid metal Ionic half equation 𝑀𝑥+ + 𝑥𝑒− → 𝑀 - The ionic half equation can be obtained by obtaining the ionic half equation of the cathode and the anode before balancing the number of electrons lost at the anode and the number of electrons gain at the cation Electrolysis of dilute solutions with inert electrodes At the anode At the cathode - Hydroxide ions from water are preferentially discharged due to its low position in the electrochemical series Ionic half equation 4𝑂𝐻−(𝑎𝑞) → 4𝑒− + 2𝐻2𝑂(𝑎𝑞) + 𝑂2(𝑔) - Cations are selectively discharged due to the low position in the electrochemical series Ionic half equation 𝑀𝑥+ + 𝑥𝑒− → 𝑀 Electrolysis of concentrated metal halide solutions with inert electrodes At the anode At the cathode - Concentration effect results in the halide ion being preferentially discharged over the hydroxide ions Ionic half equation 𝑀𝑥− → 𝑥𝑒− + 𝑀 - Cations are selectively discharged due to the low position in the electrochemical series Ionic half equation 𝑀𝑥+ + 𝑥𝑒− → 𝑀

Electroplating using reactive electrodes Simple Electric Cells - Electrodes used are to be of two different metal electrodes; The more reactive metal will undergo oxidation as it will lose its valence electrons more readily than the less reactive metals - The less reactive electrode is positive electrode; the more reactive electrode is the negative electrode - The further apart the 2 metal electrodes are in their reactivity, the greater the potential difference, greater electromotive force produced. Electrolysis vs Electrochemical Cell Electrolysis Electrochemical cell Electricity brings about chemical reaction (chemical energy is converted to chemical energy) Chemical reaction generates electricity (chemical energy is converted to electrical energy) Electrolyte should have cation of the metal used for electroplating At anode, oxidation occurs; Cu electrode dissolves into electrolyte 𝐶𝑢(𝑠) → 𝐶𝑢2+(𝑎𝑞) + 2𝑒− At cathode, Copper (II) Ions are preferentially discharged and deposited 𝐶𝑢2+ + 2𝑒− → 𝐶𝑢(𝑠)

Test yourself 1. Define electrolysis? 2. Suggest what a binary ionic compound is, with reference to an example? 3. What is the difference between electrolysis of a substance and a simple cell? 4. A concentrated and dilute solution of Sodium Chloride is an electrolyte, compare the products formed at the i. Anode ii. Cathode, including the ionic half equation iii. Include the overall equation for both scenarios 5. What is the relationship between difference of reactivity and the electromotive force produced? 6. A solution of concentrated Sodium Chloride and Potassium Iodide is an electrolyte, suggest which anion is selectively discharged, explain your answer. Glossary of Terms Electrolysis The conduction of electricity by an electrolyte leading to the decomposition of the electrolyte Electrolyte A molten ionic compound or an aqueous solution that conducts electricity through the positive and negative ions which are free to move in molten state or in aqueous solution Electrode Solid electrical contacts immersed in the electrolyte which conduct electricity through electron flow Inert electrode Electrodes that do not react during electrolysis (e.g. Pt or Graphite electrodes)