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Electrolysis Notes

Electrolysis

Introduction

  • Electrolysis uses electric current to drive non-spontaneous chemical reactions.

Electric Current

  • Electric current is any flow of electric charge.
  • Electrons are more easily removed from atoms than protons.
  • Most electric current is electron flow.
  • Electrons move freely through metals due to valence electrons in metallic bonds.
  • Movement of electrons through metal is called metallic conduction.

Batteries and Electric Potential

  • A battery is a two-terminal device creating a difference in electric potential energy.
  • Electrons at one terminal have higher electric potential energy than at the other terminal.
  • Electrons flow from the high potential energy terminal (negative) to the low potential energy terminal (positive) if a low-resistance path is available.
  • Metal wires offer low resistance paths for electrons.
  • Potential energy lost by electrons is converted into heat and light (e.g., in a light bulb filament).

Ionic Conduction

  • Electric current also exists when ions (positive or negative) move along a path.
  • Movement of ions through a solution is called ionic conduction.
  • All types of electric current flow due to differences in electric potential energy.

Battery Function

  • Batteries provide a difference in electric potential energy.
  • Chemical reactions inside the battery cause one terminal to have higher potential energy than the other.
  • The higher potential energy terminal is negative; the lower is positive.
  • Electrons flow from high to low potential energy when a low-resistance path is provided.
  • A battery connected to a light bulb forms a complete circuit.

Current Through an Electrolyte

  • Electrodes (metal pieces connected to a battery) are suspended in an ionic solution.
  • High potential energy electrons flow onto the negative electrode.
  • Electrons flow off the other electrode, making it positive.

Ion Migration and Redox Reactions

  • Cations (A+) are attracted to the negative electrode and migrate towards it.
  • When a cation contacts the negative electrode, it gains an electron and becomes a neutral atom (reduction).
  • Anions (B-) migrate toward the positive electrode.
  • When an anion touches the positive electrode, it gives up an electron and becomes a neutral atom (oxidation).
  • Electrons leaving the negative terminal and arriving at the positive terminal complete the circuit.
  • Anions donating electrons to the positive electrode are oxidized.
  • Processes at the electrodes occur simultaneously.

Electrolytic Solutions and Circuits

  • Ions in the solution are consumed during electrolysis.
  • Current flows only as long as ions are available to accept and donate electrons.
  • An electrolytic solution is the internal part of the circuit; wires and the battery are the external part.
  • Positive ions (cations) move toward the negative electrode.
  • Negative ions (anions) move toward the positive electrode.
  • Electrons flow in the external circuit from the negative electrode to the positive electrode.
  • Ion flow allows electrical conductivity.

Electrolysis of Liquid NaCl

  • Solid sodium chloride (NaCl) consists of alternating sodium ions and chloride ions in a crystal lattice.
  • Ionic solids do not conduct electric current because ions cannot migrate.
  • Liquid NaCl conducts electricity because ions can migrate.
  • Ionic substances in liquid form conduct electricity similarly to ions in solution.

Electrolysis Process in Liquid NaCl

  • Electrodes connected to a battery are placed in liquid NaCl.
  • Sodium ions migrate toward the negative electrode (reduction).
  • Chloride ions migrate toward the positive electrode (oxidation).
  • Half-equations represent processes at the electrodes:
    • At the positive electrode (reduction): Na^+ + e^- \rightarrow Na
    • At the negative electrode (oxidation): 2Cl^- \rightarrow Cl_2 + 2e^-
  • Oxidation and reduction must occur simultaneously.

Combining Half-Reactions

  • Half-reactions can be added to represent a complete reaction.
  • The number of electrons donated and accepted must be equal.
  • For NaCl electrolysis, the sodium half-reaction is doubled:
    • 2(Na^+ + e^-) \rightarrow 2Na^+ + 2e^- \rightarrow 2Na
    • 2Cl^- \rightarrow Cl_2 + 2e^-
    • 2Na^+ + 2e^- + 2Cl^- \rightarrow 2Na + Cl_2 + 2e^-
  • Canceling electrons gives the net reaction:
    • 2Na^+ + 2Cl^- \rightarrow 2Na + Cl_2

Spontaneity and Energy Input

  • Electrolysis forces current through a liquid or solution to cause a non-spontaneous chemical reaction.
  • Spontaneous reactions occur when products have less potential energy than reactants.
  • In electrolysis, products have more potential energy than reactants.
  • Electrolysis requires energy input (e.g., from a battery).

Electrolysis of Water

  • Pure water does not conduct current or undergo electrolysis because of low ion concentration.
  • Adding a substance that supplies ions (e.g., H2SO4 or Na2SO4) enables electrolysis.
  • Hydrogen gas is produced at the negative electrode, and oxygen gas is produced at the positive electrode.

Electrodes and Half-Reactions in Water Electrolysis

  • Negative electrode: cathode (reduction occurs).

  • Positive electrode: anode (oxidation occurs).

  • Half-reaction at the cathode: 4H2O + 4e^- \rightarrow 2H2(g) + 4OH^-

  • Half reaction at the anode: 2H2O \rightarrow O2(g) + 4H^+ + 4e^-

  • Net reaction (separate chambers): 6H2O \rightarrow 2H2(g) + O2(g) + 4H^+ + 4OH^- *Net reaction (same chamber): 2H2O \rightarrow 2H2(g) + O2(g)

Electroplating

  • Electroplating uses electrolysis to coat one material with a layer of metal.
  • Achieved by controlling electrode material and current level.
  • Commonly used to cover a cheap metal with a more expensive one.
  • Also used to create a surface with better electrical conductivity.
  • Example: Coating a cheap metal with gold for surface properties like attractiveness, corrosion resistance, and conductivity.

Silver Plating Example

  • To silver plate a spoon, place it as the cathode in an electrolysis setup with silver nitrate solution.
  • Silver ions migrate to the spoon (cathode) and adhere to it.
  • A silver anode provides a steady supply of silver ions through oxidation:
    • Half-reaction at the cathode: Ag^+ + e^- \rightarrow Ag
    • Half-reaction at the anode: Ag \rightarrow Ag^+ + e^-

Applications of Electroplating

  • Some gold and silver jewelry is electroplated.
  • Electrical switch connection points are often gold plated.
  • Chromium plating is used on automobile parts.