Electrochemistry supports a wide range of fields in modern industry, including metal smelting, electroplating, brine electrolysis, and sensors.
Basic Knowledge
① Brine electrolysis (ion-exchange membrane method): NaOH, Cl2, and H2 are produced by electrolyzing an aqueous NaCl solution. At the anode, 2Cl-→Cl2+2e-, and at the cathode, 2H2O+2e-→H2+2OH-。
② Aluminum smelting (Hall–Héroult process): Al2O3, refined from bauxite, is melted together with cryolite Na3AlF6 and electrolyzed.
③ Electroplating: zinc plating (corrosion prevention), chromium plating (decoration), gold plating (electronic components)
④ Anodizing: alumite (an aluminum oxide film)
⑤ Fuel-cell vehicles and EVs: comprehensive applications of electrochemistry
Brine electrolysis（NaCl→NaOH+Cl2+H2）
Hall–Héroult process（Al2O3 molten-salt electrolysis）
Electroplating（Zn, Cr, Ni, Au, Ag）
Alumite（Al anodized oxide film）
Electrorefining（purification of Cu, Ni, and Zn）
Electrochemical sensors（pH electrodes, blood glucose sensors）
Further Study (Principles and Applications)
The ion-exchange membrane used in brine electrolysis is a cation-selective membrane that allows only cations to pass through. It prevents the NaOH produced in the cathode chamber from reacting with Cl2 in the anode chamber. As a result, the purity of NaOH improved greatly (the process changed from the former mercury and diaphragm methods in the 1970s).
Aluminum smelting requires an enormous amount of electricity (about 14,000 kWh to produce 1 t), and is often called “canned electricity.” In Japan, domestic smelting was withdrawn because of high energy costs, and recycling (recycled Al requires 1/30 of the energy used for new aluminum) is now central. This is the environmental significance of aluminum-can recycling.
- Brine electrolysis = simultaneous production of NaOH/Cl2/H2
- Ion-exchange membrane method = the modern standard
- Aluminum smelting = enormous electricity consumption
- Aluminum recycling = 1/30 of the energy for new aluminum
- Electroplating = corrosion prevention, decoration, and electronic components
- Alumite = hard film on the aluminum surface
- Electrochemical sensors = analytical instruments
Points to Note (Common Confusions and Frequent Mistakes)
① In aqueous NaCl electrolysis (brine electrolysis), hydrogen is produced (Na does not precipitate because its tendency to ionize is too great). ② In molten NaCl electrolysis, sodium metal precipitates. ③ Al does not precipitate as Al3+ during aqueous electrolysis, so molten-salt electrolysis is essential. ④ Alumite is an oxide film of Al2O3.
Practice
- When saltwater (an aqueous NaCl solution) is electrolyzed, what gases are produced at the anode and cathode?
- What substance is produced at the cathode when molten NaCl is electrolyzed? Explain the difference from aqueous electrolysis.
- Explain why molten-salt electrolysis is used to smelt Al.