When an electric current is passed through an electrolyte solution or a molten salt, electrolysis occurs and substances are decomposed. We will study the relationship between electric current and chemical changes.
Basic Knowledge
Electrolysis is the process of causing a chemical change by passing an electric current from an external source through an electrolyte solution or a molten electrolyte. The electrode into which current flows is called the cathode(− electrode), and the electrode from which it flows out is called the anode(+ electrode).
In the electrolysis of copper chloride solution, copper (Cu) is deposited at the cathode, and chlorine (Cl2) is produced at the anode. The equations are:
Cathode: Cu2+ + 2e- → Cu
Anode: 2Cl- → Cl2 + 2e-
In the electrolysis of hydrochloric acid (HCl), hydrogen is produced at the cathode and chlorine at the anode.
Electrolysis (the process of decomposing an electrolyte using an external electric current)
Cathode (− electrode) (the electrode into which electrons flow. Cations are attracted to it and reduced)
Anode (+ electrode) (the electrode from which electrons flow out. Anions are attracted to it and oxidized)
Deposition (the process in which ions in a solution emerge as a solid metal)
Electroplating (a technology that uses electrolysis to attach a thin layer of another metal to a metal surface)
Refining (obtaining a pure metal by removing impurities through electrolysis. Copper refining)
Further Study
Electrolysis is also important industrially. It is applied to aluminum smelting (refining alumina from bauxite), copper refining, and the chlor-alkali process, which produces chlorine and sodium hydroxide from salt water.
In the electrolysis of water (H2O), hydrogen (H2) is produced at the cathode and oxygen (O2) at the anode in a volume ratio of 2:1. 2H2O → 2H2 + O2 is also attracting attention in the context of a hydrogen-energy society.
- Electrolysis = forcibly causing a chemical change with an external electric current
- Cathode(−) → cations gather and are reduced (gain electrons)
- Anode(+) → anions gather and are oxidized (lose electrons)
- Copper chloride solution → Cu at the cathode, Cl2 at the anode
- Electrolysis of water → cathode H2: anode O2 = 2:1 (volume ratio)
- Copper refining, electroplating, and aluminum smelting are applications of electrolysis
- Electrolysis of sodium chloride solution produces NaOH・Cl2・H2
Points to Note
① Cations gather at the cathode(−), and anions gather at the anode(+) (remember this using the law of attraction). ② The electrode names “anode” and “cathode” are also used for cells, but the direction of current is reversed, so be careful. ③ The volume ratio of gases produced by electrolysis of water is H2:O2 = 2:1 (there is twice as much hydrogen).
Practice
- What is produced at the cathode and anode, respectively, during the electrolysis of copper chloride solution?
- What is the volume ratio of the gases produced by electrolysis of water (cathode:anode)?
- Using the term electrolysis, explain what kind of technology electroplating is.