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Quiz 3:
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Electricity And Chemical Change: Electrochemistry

1 Conductors and Insulators

Testing Conductivity

We can test whether a substance conducts electricity by inserting it into a simple electrical circuit consisting of a battery, wires, and a light bulb. If the bulb lights up, the substance is a conductor. If the bulb does not light up, the substance is an insulator.

Testing Results

• Metals and graphite are solid conductors:
  • Example: Tin conducts electricity when tested.
  • Reason: The presence of free electrons that can move.
• Molecular substances do not conduct electricity:
  • Example: Ethanol does not conduct electricity when tested.
  • Reason: There are no charged particles that can move freely.
• Ionic substances have special properties:
  • Do not conduct electricity when solid.
  • Conduct electricity when molten or dissolved in water.
  • Example: Lead bromide does not conduct electricity when solid, but conducts when melted.

Important Terms

• Electrolysis: The decomposition of a substance by electric current.
• Electrolyte: A liquid that can conduct electricity because it contains ions.
• Non-electrolyte: A liquid that cannot conduct electricity.

2 The Principles of Electrolysis

Definition of Electrolysis

Electrolysis is the process of decomposing a compound using electric current. This process occurs when a solution or molten substance containing ions is subjected to electricity.

Main Components

• Electrode: Carbon/graphite rods that conduct electricity.
• Anode: The positive electrode.
• Cathode: The negative electrode.
• Electrolyte: A solution/melt containing ions.

Electrolysis Process

In Molten Salt

• Positive ions move to the cathode and gain electrons.
• Negative ions move to the anode and lose electrons.
• The result is metal at the cathode and non-metal at the anode.

In Solution

• Water decomposes to produce H⁺ and OH^- ions.
• At the cathode: metal or hydrogen gas is formed.
• At the anode: halogen gas (concentrated solution) or oxygen gas (dilute solution) is formed.

Electrolysis Product Rules

At the Cathode (-)

• If the metal is more reactive than hydrogen: hydrogen gas (H₂) is formed.
• If the metal is less reactive than hydrogen: metal is deposited.

At the Anode (+)

• Concentrated halide solution: halogen gas (Cl₂, Br₂, I₂) is formed.
• Dilute/non-halide solution: oxygen gas is formed.

Simple Example: Electrolysis of Concentrated NaCl Solution

• Cathode: hydrogen gas.
• Anode: chlorine gas.
• Remaining: NaOH solution.

3 The Reactions at the Electrodes

Electrolysis Reaction in Molten Lead Bromide

Ion Movement

• Positive lead ions (Pb²⁺) move to the cathode (negative electrode).
• Negative bromide ions (Br^-) move to the anode (positive electrode).
• This movement of ions generates electric current.

At the Cathode (Negative Electrode)

• Pb²⁺ ions gain 2 electrons.
• Lead metal is deposited.
• Reaction: Pb²⁺ + 2e^- → Pb.

At the Anode (Positive Electrode)

• Br^- ions lose electrons.
• Bromine gas is formed.
• Reaction: 2Br^- → Br₂ + 2e^-.

Electrolysis of NaCl Solution

Concentrated Solution

• Produces hydrogen gas at the cathode.
• Produces chlorine gas at the anode.
• Remaining solution contains NaOH.

Dilute Solution

• At the cathode: hydrogen gas is produced.
• At the anode: oxygen gas is produced.
• NaCl solution remains.

Writing Half-Reaction Equations

1. Identify the ions present.
2. Write the charge of the ions correctly.
3. Balance the number of electrons.
4. Add state symbols (l, g, aq).

Example: Electrolysis of MgCl₂ Molten

• Cathode: Mg²⁺ + 2e^- → Mg.
• Anode: 2Cl^- → Cl₂ + 2e^-.

4 The Electrolysis of Brine

Definition of Brine

Brine is a concentrated salt (NaCl) solution that can be obtained in two ways:

• Pumping water into salt mines.
• Evaporating seawater.

Electrolysis Process of Brine

Overall Reaction:

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Process in a Diaphragm Cell:

• At the cathode: hydrogen gas is produced.
• At the anode: chlorine gas is produced.
• The diaphragm separates the produced gases.
• The remaining solution contains NaOH.

Products and Their Uses

• Chlorine Gas:
  • Production of PVC.
  • Production of pharmaceuticals.
  • Water disinfectants.
  • Pesticide production.
  • Cleaning agents.
• Sodium Hydroxide:
  • Soap and detergent production.
  • Textile industry.
  • Paper production.
  • Ceramics production.
• Hydrogen Gas:
  • Nylon production.
  • Hydrogen peroxide production.
  • Margarine production.
  • Fuel cell materials.

Importance of This Process

The electrolysis of brine is crucial because:

• It produces three major industrial chemicals.
• The products are used in thousands of everyday items.
• Chlorine is the most important product, with an annual production of 50 million tons.

5 Two More Uses of Electrolysis

1. Copper Purification

• Anode: impure copper.
• Cathode: pure copper.
• Electrolyte: dilute CuSO₄ solution.
• Process:
  • Impure copper dissolves at the anode.
  • Pure copper deposits at the cathode.
  • Impurities settle as sludge.
• Result: Copper with 99.9% purity.
• Sludge contains precious metals (gold, silver, platinum).

2. Electroplating

• Purpose: to enhance appearance or prevent corrosion.
• Examples:
  • Car bumpers plated with chromium.
  • Cans plated with tin.
  • Cheap jewelry plated with silver.
• Electroplating Process:
  • Cathode: object to be plated.
  • Anode: plating metal.
  • Electrolyte: solution of plating metal salt.
  • Process:
    • Metal from the anode dissolves into ions.
    • These ions move to the cathode.
    • They deposit as a metal layer on the cathode.

Differences in Copper Sulfate Electrolysis Results

• Using Inert Electrodes:
  • At the cathode: copper deposits.
  • At the anode: oxygen gas is produced.
  • The blue solution fades.
• Using Copper Electrodes:
  • At the cathode: copper deposits.
  • At the anode: copper dissolves.
  • The color of the solution remains blue.