8.3 Chemical Process by Electricity

8.3.1 Electrochemical Cells

• Chemical energy from burning fuels can be converted into heat energy, which can then be transformed into electrical energy.

• Electrochemical cells convert chemical energy directly into electrical energy or use electrical energy to carry out chemical reactions.

• An electrochemical cell consists of two metal or graphite electrodes submerged in an electrolyte solution. These electrodes are connected by a wire or a battery.

• Types of Electrochemical Cells:

• Electrolytic Cells: Cells where electricity from an external source is used to drive a chemical reaction (electricity is used to promote a chemical reaction).

• Galvanic Cells: Cells where chemical reactions occur spontaneously to produce electrical energy.

Conductor:

• Materials that allow electricity to pass through are called conductors (e.g., metals, graphite, molten salts, salt solutions, acids, and bases).

• Types of Conductors:

1. Electronic Conductors: Materials that conduct electricity through free electrons. Metals like iron (Fe), copper (Cu), and nickel (Ni) are electronic conductors.

2. Electrolytes: Substances that conduct electricity in their molten or dissolved state, but not in solid form. Electrolytes dissociate into ions to conduct electricity (e.g., sodium chloride (NaCl), copper sulfate (CuSO4), sulfuric acid (H2SO4), and water (H2O)).

Electrolytes Types:

• Strong Electrolytes: Completely ionized in molten or solution state (e.g., sodium chloride, copper sulfate).

• Weak Electrolytes: Only partially ionized in molten or solution state (e.g., water, ethanoic acid).

Electrode:

• An electrode is a conductor that facilitates the flow of electrons in an electrochemical cell.

• There are two types of electrodes:

• Anode: Where oxidation occurs (electrons are lost).

• Cathode: Where reduction occurs (electrons are gained).

8.3.2 Electrolytic Cell, Electrolysis and Mechanism of Electrolysis

• Electrolytic Cell: Uses external electricity to conduct chemical reactions in electrolytes. The process is called electrolysis.

• Example of Electrolysis (Sodium Chloride):

• Molten sodium chloride (NaCl) undergoes electrolysis to produce chlorine gas (Cl2) at the anode and sodium metal (Na) at the cathode.

• Anode Reaction (Oxidation): Cl⁻ → Cl2 + e⁻

• Cathode Reaction (Reduction): Na⁺ + e⁻ → Na

Electrolysis of Concentrated Sodium Chloride Solution:

• In solution, NaCl dissociates into Na⁺ and Cl⁻ ions, along with H⁺ and OH⁻ ions from water.

• At the Cathode: H⁺ ions are reduced first, forming hydrogen gas (H2).

• H⁺ + e⁻ → H

• 2H → H2

• At the Anode: Cl⁻ ions are oxidized first, forming chlorine gas (Cl2).

• Cl⁻ → Cl + e⁻

• 2Cl → Cl2

• The remaining Na⁺ and OH⁻ combine to form sodium hydroxide (NaOH).

Factors Influencing Discharge at Electrodes:

1. Tendency to Discharge (Reactivity Series): More reactive ions discharge first.

2. Concentration of Ions: More concentrated ions will discharge first despite their position in the reactivity series.

3. Nature of the Electrode: The material of the electrode can affect which ions discharge first.

Electrolysis of Pure Water:

• Pure water undergoes electrolysis when a small amount of sulfuric acid is added to increase ionization.

• Cathode Reaction (Reduction): 4H⁺ + 4e⁻ → 2H2 (Hydrogen gas produced).

• Anode Reaction (Oxidation): 4OH⁻ → O2 + 2H2O + 4e⁻ (Oxygen gas produced).

8.3.3 Uses of Electrolysis

• Extraction of Metals:

• Active metals such as alkali metals, alkaline earth metals, and aluminum are extracted via electrolysis.

• Example: Electrolysis of sodium chloride (NaCl) produces sodium metal (Na) at the cathode and chlorine gas (Cl2) at the anode.

• Purification of Metals:

• Electrolysis is used to purify metals like copper, zinc, and aluminum. The impure metal is placed at the anode, and pure metal is deposited at the cathode.

• Electroplating:

• Electroplating is the process of coating one metal with another to enhance its appearance or prevent corrosion.

• Example: Silver electroplating involves passing electricity through a solution of silver nitrate (AgNO3), where silver ions from the anode are reduced and deposited on the cathode (e.g., on an iron spoon).

Commercial Use of Electrolysis:

• Electrolysis is used to extract metals such as sodium, aluminum, and zinc from ores.

• It is also used for electroplating, producing electrical wires (e.g., copper), aluminum for aircraft, and other materials.

• Chlorine Gas: Produced from electrolysis of sea water, used as an anti-parasite agent.

• Sodium Hydroxide: Produced from electrolysis and used in various industrial applications.