Electrolysis is the process of using an external power source to force a redox reaction to occur. Faraday's laws define the relationship between the amount of electricity and the amount of product formed.
Basic Knowledge
In electrolysis:
・Cathode(cathode, reduction): connected to the negative terminal of the power source, where cations are reduced
・Anode(anode, oxidation): connected to the positive terminal of the power source, where anions or the electrode itself is oxidized
The behavior of the anode differs depending on the electrode material: with insoluble electrodes such as Pt and C(carbon), ions in the electrolyte are oxidized(Cl-→Cl2, 2H2O→O2+4H++4e-), while with soluble electrodes such as Cu, Ag, and Ni, the electrode itself dissolves(Cu→Cu2++2e-).
Faraday's laws: the amount deposited is proportional to the amount of electricity passed. Q=It=nF(F=Faraday constant=96500 C/mol). F is the amount of electricity carried by 1 mol of electrons.
Electric charge Q（Q=It [C]）
Faraday constant F（96500 C/mol）
Amount of substance n（n=It/(zF), z=number of transferred electrons）
Cathode（Reduction; on the negative-terminal side of the power source）
Anode（Oxidation; on the positive-terminal side of the power source）
Electrorefining（Impure copper→pure copper, 99.99% or higher）
Further Exploration (Principles and Applications)
In the electrorefining of copper, impure blister copper is used as the anode and pure copper as the cathode, and electrolysis is carried out in aqueous CuSO4. At the anode, Cu→Cu2++2e- dissolves, and at the cathode, Cu2++2e-→Cu is deposited. Ag and Au, which have a lower metal activity than Cu, settle as anode slime(and are recovered as precious-metal by-products), while Zn and Ni, which have a higher metal activity, remain in solution as ions.
Representative applications include aluminum refining by the Hall–Héroult process(electrolysis of molten Al2O3 with cryolite at 1,000℃), NaOH production by the ion-exchange membrane process, and electrodeposition of zinc, chromium, and nickel for plating.
- Power-source negative terminal=cathode(reduction); power-source positive terminal=anode(oxidation)
- Q=It [C]
- F=96500 C/mol
- n=It/(zF)
- Insoluble electrodes(Pt, C) and soluble electrodes(Cu, Ni)
- Over 99.99% purity in copper electrorefining
- Aluminum refining=Hall–Héroult process
Points to Note (Common Confusions and Frequent Mistakes)
① The terminology appears reversed in electrolysis and cells: in a cell, the negative electrode is oxidation, while in electrolysis, the cathode is reduction. The common rule is anode=oxidation and cathode=reduction. ② The anode reaction depends on the electrode material(Pt/C or Cu/Ag). ③ Pay attention to the number of transferred electrons z in electric-charge calculations(Cu2+ has z=2, and Al3+ has z=3). ④ 1 F = 96500 C = 1 mol of electrons.
Practice
- When aqueous CuSO4 is electrolyzed using Cu electrodes, write the reaction equations at the cathode and anode.
- When a current of 0.50 A is passed for 9650 seconds, how many C of electric charge pass through? How many mol of electrons is this?
- Find the ratio of the amounts of Cu, Ag, and Al deposited(molar ratio) for the same amount of electricity.