MA

topic_6_Electrolysis

Electrolysis Overview

  • Definition: Electrolysis is a chemical change caused by passing an electric current through a compound that is either molten or in solution.

Conductivity of Substances

  • Conductivity: For substances to conduct electricity, there must be free charged particles (electrons or ions).

Ionic Compounds and Electricity

  • Ionic compounds consist of positive ions (cations) and negative ions (anions).

  • Solid State: Do not conduct electricity because ions are locked in place in a lattice structure.

  • Molten or Dissolved: Conduct electricity because ions are free to move.

Covalent Molecular Compounds and Electricity

  • Characteristics: Do not conduct electricity in any state or solution due to the lack of charged particles.

  • Example Compounds: Water (H2O), Ammonia (NH3), Chlorine (Cl2).

Properties of Ionic Compounds

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  1. Composed of cations and anions.

  2. Solid at room temperature.

  3. High melting and boiling points.

  4. Soluble in water.

  5. Cannot conduct electricity when solid.

  6. Conduct electricity when molten or dissolved in water.

Key Terms in Electrolysis

  • Electrolyte: A liquid or solution that conducts electricity due to the movement of ions.

  • Electrodes: Conducting materials (often carbon or platinum) through which electric current enters and exits the electrolyte.

    • Anode: Positive electrode.

    • Cathode: Negative electrode.

Electrolysis Process

  • Current Flow: Positive ions move towards the cathode (gain electrons, reduction).

  • Negative ions move towards the anode (lose electrons, oxidation).

Example: Electrolysis of Molten Lead(II) Bromide (PbBr2)

  • Melting Required: Nothing happens until PbBr2 melts.

  • After melting, electric current causes:

    • Brown bromine gas (Br2) at the anode.

    • Metallic lead (Pb) at the cathode.

  • Ionic Structure: Solid PbBr2 has tightly packed ions; melting allows free movement.

Oxidation and Reduction Principles

  • Oxidation: Loss of electrons.

  • Reduction: Gain of electrons.

  • Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

Redox Reactions in Electrolysis

  • Oxidation and reduction occur simultaneously, leading to the term Redox reaction.

Electrolysis of Aqueous Solutions

  • When electrolysing NaCl(aq):

    • At the Anode: Chlorine gas (Cl2) is formed.

    • At the Cathode: Hydrogen gas (H2) is produced instead of sodium due to hydrogen's higher reduction potential.

    • The solution becomes alkaline due to OH- ions remaining in solution.

Gas Testing:

  • At Anode: Bleaches moist litmus paper (Chlorine).

  • At Cathode: Produces a pop sound with a lit splint (Hydrogen).

Summary of Electrolysis

  1. Current Flow: Requires ions to conduct.

  2. Products: Vary based on solution reactivity and the type of electrodes used.

  3. Concentration Effects: More dilute solutions generally favor hydrogen production at the cathode.

Application of Concepts

  • Recognize half-equations for oxidation and reduction.

  • Be prepared to use concepts of electrolysis in different ionic solutions (e.g., NaCl, CuSO4, etc.).

  • To understand why certain ions discharge preferentially at electrodes based on competing reactions.

Important Equations:

  • For hydrogen production: 2H+ + 2e- → H2 (Reduction at Cathode).

  • For chlorine production: 2Cl- → Cl2 + 2e- (Oxidation at Anode).