Electrochemistry

1. Fundamentals of Electrolysis

Definitions

  • Electrolysis: Decomposition of an ionic compound (molten or aqueous) by an electric current.
  • Electrolyte: An ionic compound that conducts electricity when molten or in aqueous solution.
  • Electrodes:
    • Anode: Positive electrode (+).
    • Cathode: Negative electrode (-).

Charge Transfer and Ion Movement

  • External Circuit: Electrons flow from anode to cathode.
  • Electrodes:
    • Anode: Oxidation occurs (loss of electrons).
    • Cathode: Reduction occurs (gain of electrons).
  • Electrolyte: Cations move to the cathode; anions move to the anode.

Electrolysis diagram

Basic Principles

  • Cathode Reactions: Metals or hydrogen gas are formed.
  • Anode Reactions: Non-metals (except hydrogen) are formed.
  • Molten Binary Compounds: Predict products based on the ions present (e.g., $\text{NaCl(l)} \rightarrow \text{Na}$ at cathode, $\text{Cl}_2$ at anode).

2. Specific Electrolysis Examples

Molten Lead(II) Bromide ($\text{PbBr}_2$)

  • Cathode: Lead metal deposits ($\text{Pb}$), grey metallic sheen.
  • Anode: Bromine gas evolves ($\text{Br}_2$), reddish-brown fumes.

Concentrated Aqueous Sodium Chloride ($\text{NaCl}$)

NaCl Electrolysis diagram

  • Cathode: Hydrogen gas ($\text{H}_2$), bubbles.
  • Anode: Chlorine gas ($\text{Cl}_2$), pale green gas, bleaches litmus.

3. Electroplating

Electroplating

  • Purpose: Improve appearance or provide corrosion resistance.
  • Method:
    • Object to be plated is the cathode.
    • Metal to be deposited is the anode.
    • Electrolyte is a soluble salt of the plating metal.

4. Advanced Concepts

Aqueous Copper(II) Sulfate ($\text{CuSO}_4$)

  • Inert Electrodes (e.g., Graphite):
    • Cathode: $\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$
    • Anode: $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4\text{e}^-$
  • Copper Electrodes:
    • Cathode: $\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$ (Copper deposited)
    • Anode: $\text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^-$ (Copper dissolved)
    • Result: Net transfer of copper from anode to cathode. Copper Electrolysis

Halide Compound Electrolysis (Aqueous)

  • Concentrated Solution: Halide ion ($\text{Cl}^-$, $\text{Br}^-$, $\text{I}^-$) is discharged at the anode.
  • Dilute Solution: Hydroxide ion ($\text{OH}^-$) is discharged at the anode, producing oxygen.

Ionic Half-Equations (Supplement)

  • Anode (Oxidation): $\text{X}^- \rightarrow \text{X} + \text{e}^-$
  • Cathode (Reduction): $\text{Y}^+ + \text{e}^- \rightarrow \text{Y}$

Example

Full Equation: $$\text{CuO} + \text{Mg} \rightarrow \text{MgO} + \text{Cu}$$

Showing Ions Present: $$(\text{Cu}^{2+} + \text{O}^{2-}) + \text{Mg} \rightarrow (\text{Mg}^{2+}+\text{O}^{2-}) + Cu$$

Oxidation: $$\text{Mg} \rightarrow \text{Mg}^{2+} + 2\text{e}^-$$

Reduction: $$\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$$

5. Hydrogen-Oxygen Fuel Cells

Core Requirements

  • Produces electricity.
  • Only product is water ($\text{H}_2\text{O}$).

Hydrogen Fuel Cell

Supplement Comparison (Fuel Cell vs Gasoline Engine)

Feature $\text{H}_2\text{-O}_2$ Fuel Cell Gasoline Engine
Pollutants None (only $\text{H}_2\text{O}$) $\text{CO}_2, \text{NO}_x$, particulates
Efficiency Higher energy conversion Lower (much heat lost)
Refueling Fast (like gasoline) Fast
Infrastructure Limited $\text{H}_2$ stations Extensive gas stations
Waste Pure water Greenhouse gases and toxins