MODULE 5: CHEMICAL EFFECTS OF ELECTRIC CURRENT AND CELLS
[MAXIMUM VOLUME EXPANDED EXAM AND TUTORIAL GUIDE]
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SECTION 5.1: PRINCIPLES OF ELECTROLYSIS AND CHEMICAL EFFECTS
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* Introduction to Chemical Effects: When a Direct Current (DC) passes
through a conducting liquid or chemical solution, it causes chemical
decomposition and breaks the solution into its constituent parts. This
entire phenomenon is scientifically termed as the Chemical Effect of Current.
* Core Process Elements:
- Electrolyte: A chemical compound solution in a liquid state that conducts
electricity and undergoes decomposition (e.g., Dilute Sulfuric Acid
H2SO4, Copper Sulfate CuSO4, Silver Nitrate AgNO3).
- Electrodes: Solid metallic plates or rods immersed inside the electrolyte
to pass electric current into and out of the solution.
* Anode: The electrode connected to the POSITIVE (+) terminal of the DC source.
* Cathode: The electrode connected to the NEGATIVE (-) terminal of the DC source.
- Voltameter: The vessel, container, or tank made of glass, rubber, or wood
in which the complete process of electrolysis takes place.
- Ions: Electric charge carriers formed in the electrolyte. Positive ions
(Cations) move toward the Cathode; negative ions (Anions) move toward the Anode.
SECTION 5.2: FARADAY'S MANDATORY LAWS OF ELECTROLYSIS
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The quantitative aspects of chemical decomposition were formulated by Michael
Faraday through two fundamental laws:
* Faraday's First Law of Electrolysis:
- Statement: The mass (m) of a chemical substance deposited or liberated at
any electrode during the process of electrolysis is directly proportional
to the total quantity of electricity (Q) passed through the electrolyte.
- Mathematical Expression:
m∝Q
Since Q = I x t (Current x Time), we get:
m=ZxIxt
- Parameters:
* m = Mass of substance deposited (measured in grams or milligrams).
* I = Current flowing through the electrolyte (measured in Amperes).
* t = Time duration of current flow (measured in seconds).
* Z = Electrochemical Equivalent (ECE) constant of the specific substance.
- SI Unit of ECE (Z): Grams per Coulomb (g/C) or Milligrams per Coulomb (mg/C).
, * Faraday's Second Law of Electrolysis:
- Statement: When the exact same quantity of electricity is passed through
several different electrolytes connected in series, the masses of the
individual substances deposited at the electrodes are directly proportional
to their respective Chemical Equivalent (E) weights.
- Mathematical Expression:
m1 / m2 = E1 / E2
- Chemical Equivalent Weight (E): Calculated as the Atomic Weight of an
element divided by its Valency. (E = Atomic Weight / Valency).
SECTION 5.3: STRUCTURAL TAXONOMY OF CELLS: PRIMARY VS SECONDARY
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* Primary Cells (Non-Rechargeable Systems):
- Chemical Actions: The internal chemical reactions are completely irreversible.
Once the active chemical materials are consumed, the cell is discharged.
- Operating Logic: Converts chemical energy directly into electrical energy.
It cannot be recharged by an external current and must be discarded.
- Key Variants and Specifications:
* Voltaic Cell: E.M.F = 1.08 Volts. Suffers from Polarization and Local Action faults.
* Daniel Cell: E.M.F = 1.11 Volts. Uses copper and zinc plates with a porous pot.
* Leclanche Cell: E.M.F = 1.46 Volts. Ideal for intermittent work like bells.
* Standard Dry Cell: E.M.F = 1.50 Volts. Portable version containing Zinc container.
* Mercury Cell: E.M.F = 1.35V to 1.40V. Small button cell used in watches.
* Secondary Cells (Rechargeable Accumulators):
- Chemical Actions: The internal chemical reactions are completely reversible.
When electrical energy is forced in from outside, it is stored as chemical energy.
- Operating Logic: Acts as an electrical accumulator storage system. Can be
recharged and reused hundreds of times.
- Key Variants and Specifications:
* Lead-Acid Cell: Nominal E.M.F = 2.0V to 2.2V per cell. Heavy weight, high capacity.
* Nickel-Iron Cell (Edison Cell): Nominal E.M.F = 1.2V per cell. Robust, alkaline.
[MAXIMUM VOLUME EXPANDED EXAM AND TUTORIAL GUIDE]
======================================================================
SECTION 5.1: PRINCIPLES OF ELECTROLYSIS AND CHEMICAL EFFECTS
----------------------------------------------------------------------
* Introduction to Chemical Effects: When a Direct Current (DC) passes
through a conducting liquid or chemical solution, it causes chemical
decomposition and breaks the solution into its constituent parts. This
entire phenomenon is scientifically termed as the Chemical Effect of Current.
* Core Process Elements:
- Electrolyte: A chemical compound solution in a liquid state that conducts
electricity and undergoes decomposition (e.g., Dilute Sulfuric Acid
H2SO4, Copper Sulfate CuSO4, Silver Nitrate AgNO3).
- Electrodes: Solid metallic plates or rods immersed inside the electrolyte
to pass electric current into and out of the solution.
* Anode: The electrode connected to the POSITIVE (+) terminal of the DC source.
* Cathode: The electrode connected to the NEGATIVE (-) terminal of the DC source.
- Voltameter: The vessel, container, or tank made of glass, rubber, or wood
in which the complete process of electrolysis takes place.
- Ions: Electric charge carriers formed in the electrolyte. Positive ions
(Cations) move toward the Cathode; negative ions (Anions) move toward the Anode.
SECTION 5.2: FARADAY'S MANDATORY LAWS OF ELECTROLYSIS
----------------------------------------------------------------------
The quantitative aspects of chemical decomposition were formulated by Michael
Faraday through two fundamental laws:
* Faraday's First Law of Electrolysis:
- Statement: The mass (m) of a chemical substance deposited or liberated at
any electrode during the process of electrolysis is directly proportional
to the total quantity of electricity (Q) passed through the electrolyte.
- Mathematical Expression:
m∝Q
Since Q = I x t (Current x Time), we get:
m=ZxIxt
- Parameters:
* m = Mass of substance deposited (measured in grams or milligrams).
* I = Current flowing through the electrolyte (measured in Amperes).
* t = Time duration of current flow (measured in seconds).
* Z = Electrochemical Equivalent (ECE) constant of the specific substance.
- SI Unit of ECE (Z): Grams per Coulomb (g/C) or Milligrams per Coulomb (mg/C).
, * Faraday's Second Law of Electrolysis:
- Statement: When the exact same quantity of electricity is passed through
several different electrolytes connected in series, the masses of the
individual substances deposited at the electrodes are directly proportional
to their respective Chemical Equivalent (E) weights.
- Mathematical Expression:
m1 / m2 = E1 / E2
- Chemical Equivalent Weight (E): Calculated as the Atomic Weight of an
element divided by its Valency. (E = Atomic Weight / Valency).
SECTION 5.3: STRUCTURAL TAXONOMY OF CELLS: PRIMARY VS SECONDARY
----------------------------------------------------------------------
* Primary Cells (Non-Rechargeable Systems):
- Chemical Actions: The internal chemical reactions are completely irreversible.
Once the active chemical materials are consumed, the cell is discharged.
- Operating Logic: Converts chemical energy directly into electrical energy.
It cannot be recharged by an external current and must be discarded.
- Key Variants and Specifications:
* Voltaic Cell: E.M.F = 1.08 Volts. Suffers from Polarization and Local Action faults.
* Daniel Cell: E.M.F = 1.11 Volts. Uses copper and zinc plates with a porous pot.
* Leclanche Cell: E.M.F = 1.46 Volts. Ideal for intermittent work like bells.
* Standard Dry Cell: E.M.F = 1.50 Volts. Portable version containing Zinc container.
* Mercury Cell: E.M.F = 1.35V to 1.40V. Small button cell used in watches.
* Secondary Cells (Rechargeable Accumulators):
- Chemical Actions: The internal chemical reactions are completely reversible.
When electrical energy is forced in from outside, it is stored as chemical energy.
- Operating Logic: Acts as an electrical accumulator storage system. Can be
recharged and reused hundreds of times.
- Key Variants and Specifications:
* Lead-Acid Cell: Nominal E.M.F = 2.0V to 2.2V per cell. Heavy weight, high capacity.
* Nickel-Iron Cell (Edison Cell): Nominal E.M.F = 1.2V per cell. Robust, alkaline.
