Chapter 11: Chemical Effects of Electric Current

1. Introduction to Chemical Effects of Electric Current

Chemical Effects of Electric Current,When electric current passes through certain liquids, it can cause chemical reactions. These liquids are called electrolytes, and the process is known as electrolysis. This phenomenon is called the chemical effects of electric current.

Most liquids that conduct electricity are solutions of acids, bases, or salts. Pure water is a poor conductor, but when salt or acid is added, it conducts electricity.

2. Do Liquids Conduct Electricity?

• To test if a liquid conducts electricity, we use a simple circuit with a battery, bulb (or LED), and two electrodes dipped in the liquid.
• If the bulb glows → the liquid conducts electricity → it is a conductor.
• If the bulb does not glow → the liquid is a non-conductor or insulator.

Examples:

3. Conductors vs Poor Conductors

• Good conductors: Salt solutions, acids, bases (have many free ions).
• Poor conductors: Pure water, rainwater (very few ions).
• Adding even a small amount of salt or acid makes water a good conductor.

4. Electrolytes and Non-Electrolytes

• Electrolytes: Liquids that allow electric current to pass and get decomposed (e.g., salt water, dilute H₂SO₄, NaOH solution).
• Non-electrolytes: Liquids that do not conduct electricity (e.g., distilled water, sugar solution, alcohol).

5. Electrolysis – The Chemical Reaction

When electricity passes through an electrolyte:

• The electrolyte splits into its ions.
• Positive ions (cations) move towards the cathode (negative electrode).
• Negative ions (anions) move towards the anode (positive electrode).

This decomposition of electrolyte by electric current is called electrolysis.

6. Electrolysis of Water (Important Activity)

• Take distilled water + few drops of dilute sulphuric acid (to make it conductive).
• Set up two electrodes connected to a battery.
• Observations:
  • Bubbles of gas at both electrodes.
  • Gas collected at cathode → Hydrogen (H₂) [volume twice that at anode]
  • Gas collected at anode → Oxygen (O₂)

Reaction: 2H₂O (l) → 2H₂ (g) + O₂ (g)

• Acid only helps in conduction; it is not consumed.
• This shows water contains hydrogen and oxygen in the ratio 2:1 by volume.

7. Electroplating

Definition: The process of depositing a layer of one metal on another metal using electricity.

Applications:

• To prevent corrosion (e.g., coating iron with chromium or nickel).
• For decoration (gold or silver plating on jewellery).
• To make objects shiny and attractive.

Example: Chromium plating on car parts, taps, bicycle handles.

Process of Electroplating (e.g., copper plating on an iron key):

1. Object to be plated (iron key) → made cathode.
2. Pure copper strip → made anode.
3. Electrolyte → copper sulphate solution.
4. When current is passed:
   • Cu²⁺ ions move to cathode → deposit as copper metal on the key.
   • Copper from anode dissolves to maintain Cu²⁺ concentration.

Reaction: At cathode: Cu²⁺ + 2e⁻ → Cu (deposition) At anode: Cu → Cu²⁺ + 2e⁻ (dissolution)

8. Electrodes

• Anode: Positive electrode (connected to + terminal of battery).
• Cathode: Negative electrode (connected to – terminal of battery).
• Inert electrodes: Do not take part in reaction (e.g., graphite, platinum).
• Active electrodes: Take part in reaction (e.g., copper anode in electroplating).

9. Some Important Chemical Effects

1. Electrolysis of water → produces H₂ and O₂.
2. Electroplating → metal coating.
3. Electro-refining → purification of metals (e.g., copper).
4. Production of chemicals → like NaOH, Cl₂, H₂ in industries.

10. LED (Light Emitting Diode) in Tester

• LED is better than bulb because it glows even with very small current.
• Has two legs: longer leg → connect to positive terminal, shorter leg → negative.
• LED glows only when connected in correct polarity.

Key Points to Remember

• Pure water does not conduct electricity; impurities (salts, acids) make it conductive.
• Strong electrolytes → acids, bases, salts.
• Gases evolved during electrolysis of acidified water: H₂ (cathode), O₂ (anode) in 2:1 ratio.
• Electroplating uses the object as cathode and the plating metal as anode.
• Chromium plating gives shiny, corrosion-resistant surface.
• Electrolysis is widely used in industries for metal extraction, purification, and plating.

Quick Revision Table

Important Examples

1. Why does the bulb glow brightly in salt water but dimly in lemon juice? → Salt water has more ions than lemon juice → better conduction.
2. Why is a small amount of acid added during electrolysis of water? → To increase the number of ions so that water conducts electricity.
3. Why is copper from anode dissolved during electroplating? → To replace the Cu²⁺ ions deposited on cathode → solution concentration remains constant.

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