1. Introduction to Electrolysis
- Definition: Electrolysis is the process of chemical decomposition of an electrolyte by passing a direct electric current through it. It involves the conversion of electrical energy into chemical energy.
- Electrolyte: A substance that, in a molten state or in an aqueous solution, conducts electricity and undergoes chemical decomposition. Examples include molten sodium chloride (NaCl) and copper sulfate solution (CuSO4).
- Non-electrolyte: A substance that does not conduct electricity. Examples include sugar solution and pure water.
- Electrolytic Cell: The apparatus used for electrolysis. It consists of two electrodes (anode and cathode) dipped in an electrolyte.
- Electrodes:
- Anode: The positive electrode connected to the positive terminal of the battery. Oxidation (loss of electrons) occurs here.
- Cathode: The negative electrode connected to the negative terminal of the battery. Reduction (gain of electrons) occurs here.
2. Mechanism of Electrolysis
- Ionization: When an electrolyte is melted or dissolved in water, it dissociates into mobile positive ions (cations) and negative ions (anions).
- Migration of Ions: When an electric current is passed through the electrolyte, cations move towards the cathode (negative electrode), and anions move towards the anode (positive electrode).
- Discharge of Ions:
- At the cathode, cations accept electrons and are discharged as neutral atoms or molecules (reduction).
- At the anode, anions donate electrons and are discharged as neutral atoms or molecules (oxidation).
3. Selective Discharge of Ions
When an electrolyte contains more than one type of cation or anion, the ion that is discharged at an electrode is determined by three factors:
- Position in the Electrochemical Series: Ions lower down in the series are preferentially discharged.
- Concentration: A higher concentration of an ion can lead to its preferential discharge, even if it is higher in the series.
- Nature of the Electrode: For example, a reactive anode (like copper) can dissolve and be discharged preferentially over anions from the electrolyte.
4. Electrolysis of Specific Substances
- Molten Lead Bromide (PbBr2):
- Anode: Bromide ions (Br−) are discharged. 2Br−−2e−→Br2 (Reddish-brown bromine gas is evolved).
- Cathode: Lead ions (Pb2+) are discharged. Pb2++2e−→Pb (Molten lead is deposited).
- Acidified Water (H2O):
- Anode: Hydroxide ions (OH−) from water are discharged. 4OH−−4e−→2H2O+O2 (Oxygen gas is evolved).
- Cathode: Hydrogen ions (H+) are discharged. 2H++2e−→H2 (Hydrogen gas is evolved).
- The volume ratio of hydrogen to oxygen is 2:1.
- Copper Sulfate Solution (CuSO4) using Inert Electrodes (Platinum):
- Anode: Hydroxide ions (OH−) from water are discharged preferentially over sulfate ions (SO42−). 4OH−−4e−→2H2O+O2 (Oxygen gas is evolved).
- Cathode: Copper ions (Cu2+) are discharged preferentially over hydrogen ions (H+). Cu2++2e−→Cu (A reddish-brown deposit of copper is formed).
- Copper Sulfate Solution (CuSO4) using Copper Electrodes:
- Anode: The copper anode itself dissolves. Cu−2e−→Cu2+ (The anode loses mass).
- Cathode: Copper ions from the solution are discharged. Cu2++2e−→Cu (Pure copper is deposited, and the cathode gains mass). The concentration of the copper sulfate solution remains constant.
5. Applications of Electrolysis
- Electroplating: The process of coating a base metal with a thin layer of a more expensive or corrosion-resistant metal. This is used to improve appearance and prevent corrosion. The article to be plated is made the cathode, and a block of the plating metal is made the anode. The electrolyte is a solution containing ions of the plating metal.
- Electro-refining: The process of purifying metals like copper. Impure copper is made the anode, pure copper is the cathode, and a copper sulfate solution is the electrolyte. The copper from the anode dissolves and is deposited as pure copper on the cathode.
- Electrometallurgy: The extraction of highly reactive metals like sodium, potassium, and aluminum from their molten salts. This process is necessary because these metals cannot be extracted by chemical reduction.
6. Electrorefining of Copper
Electrorefining is a process used to purify an impure metal using electrolysis. For copper, the setup consists of:
- Anode: A thick block of impure copper.
- Cathode: A thin strip of pure copper.
- Electrolyte: An acidified copper sulfate solution (CuSO4).
When a direct current is passed through the cell, the impure copper anode dissolves, and pure copper is deposited on the cathode.
- Anode reaction (Oxidation): Copper from the impure anode loses electrons and enters the solution as copper ions. Cu→Cu2++2e−
- Cathode reaction (Reduction): Copper ions from the electrolyte gain electrons and get deposited on the pure copper cathode. Cu2++2e−→Cu
- Anode Mud: Impurities like gold and silver, which are less reactive than copper, do not dissolve and fall to the bottom of the cell as “anode mud.” More reactive impurities, like iron and zinc, dissolve in the electrolyte but are not deposited on the cathode.
7. Electroplating
Electroplating is the process of coating a less expensive or less durable metal with a thin layer of a more valuable or corrosion-resistant metal using electrolysis. The key objectives are to enhance appearance and prevent corrosion.
General Setup:
- Anode: A block of the plating metal (e.g., copper or silver).
- Cathode: The object to be plated.
- Electrolyte: A solution containing ions of the plating metal.
8. Factors Affecting Electroplating
The quality of electroplating depends on several factors:
- Current Density: A low current density results in a smooth, uniform, and adherent coating. A high current density leads to a rough, uneven, and powdery deposit.
- Concentration of the Electrolyte: A low concentration of metal ions in the electrolyte also promotes a smooth, lustrous finish.
- Duration of Electrolysis: A longer duration results in a thicker plating layer.
- Pre-treatment: The surface of the article to be plated must be thoroughly cleaned and degreased to ensure proper adhesion of the coating.
9. Electroplating of Copper
To electroplate an iron spoon with copper:
- Anode: A strip of pure copper.
- Cathode: The iron spoon (which is cleaned and degreased).
- Electrolyte: A solution of copper sulfate (CuSO4).
- Reactions:
- Anode: Copper from the anode dissolves into the electrolyte as Cu2+ ions. Cu→Cu2++2e−
- Cathode: Copper ions from the electrolyte are deposited on the iron spoon. Cu2++2e−→Cu
- The copper anode continuously replenishes the copper ions in the solution, keeping the concentration of the electrolyte constant.
10. Electroplating of Silver
To electroplate a metal article with silver, such as a brass spoon:
- Anode: A strip of pure silver.
- Cathode: The brass spoon.
- Electrolyte: A solution of a soluble silver salt, typically potassium argentocyanide (K[Ag(CN)2]). Using a simple silver nitrate solution would result in a rough, non-adherent deposit. The complex ion ensures a slow, steady supply of Ag+ ions, leading to a smooth finish.
- Reactions:
- Anode: Silver from the anode dissolves to form silver ions. Ag→Ag++e−
- Cathode: The complex ion in the electrolyte dissociates to provide a controlled amount of silver ions, which are then deposited on the cathode. [Ag(CN)2]−⇌Ag++2CN− Ag++e−→Ag
- This results in a thin, uniform, and shiny layer of silver on the spoon
