19_Lecture Electrochemistry

Chapter 19: Electrochemistry

Overview

  • Electrochemistry: The study of redox reactions that produce or require electric currents.

  • Redox Reactions: Reactions where electrons are transferred between atoms.

    • Oxidation: Loss of electrons; occurs at the anode.

    • Reduction: Gain of electrons; occurs at the cathode.

Chapter Outline

  • Redox Reactions

  • Galvanic Cells

  • Standard Reduction Potentials

  • Batteries

  • Corrosion

  • Electrolysis

  • Electrometallurgy

Redox Reactions

Definition and Examples

  • Oxidation-Reduction (Redox) Reactions:

    • Electrons transfer example: 2 Na(s) + Cl2(g) → 2 Na^+ + 2 Cl^- (s).

    • Na → Na^+ + 1 e^- (oxidation)

    • Cl2 + 2 e^- → 2 Cl^- (reduction)

Characteristics of Oxidation and Reduction

  • Oxidation:

    • Oxidation number increases.

    • Electrons lost or oxygen gained.

  • Reduction:

    • Oxidation number decreases.

    • Electrons gained or oxygen lost.

Electrochemistry

Electrochemical Cells

  • Voltaic (Galvanic) Cells:

    • Spontaneous redox reactions produce electrical current.

  • Electrolytic Cells:

    • Non-spontaneous redox reactions facilitated by electrical energy.

Components of Electrochemical Cells

  • Half-Cells: Oxidation and reduction occur in separate compartments.

  • Electron Flow: Through an external circuit via conductive electrodes.

Electrodes in Electrochemical Cells

  • Anode:

    • Site of oxidation (e.g., Zn → Zn^2+ + 2 e^-).

    • Negative charge in galvanic cells.

  • Cathode:

    • Site of reduction (e.g., Cu^2+ + 2 e^- → Cu).

    • Positive charge in galvanic cells.

Cell Notation

  • Standard notation:

    • Electrode | Electrolyte || Electolyte | Electrode.

    • Example: Zn(s) | Zn^2+(aq) || Cu^2+(aq) | Cu(s).

Batteries

Battery Types

  • Leclanché Acidic Dry Cell:

    • Electrolyte: ZnCl2 + NH4Cl.

  • Alkaline Dry Cell:

    • Electrolyte: KOH; better longevity than acidic.

  • Lead Storage Battery:

    • Rechargeable, electrolyte: 30% H2SO4.

  • Nickel-Cadmium (NiCad) Battery:

    • Electrolyte: KOH solution; rechargeable but sensitive to misuse.

  • Lithium-Ion Battery:

    • High energy density, environmentally friendly.

Electrolysis

Definition and Process

  • Electrolysis: Use of electrical energy to induce a non-spontaneous reaction.

  • Applications:

    • Metal extraction, H2 production, metal plating.

Electrolytic Cells

  • Power source: Usually a DC supply.

  • Reaction: Cations to cathode (gain electrons) and anions to anode (lose electrons).

Corrosion

Mechanism of Rusting

  • Oxidation of Fe: Iron oxidizes to Fe2+ at anodic regions, and O2 is reduced at cathodic regions. Rust forms under moisture presence.

Prevention Techniques

  • Galvanization: Coating iron with zinc (sacrificial anode).

  • Cathodic protection: Electrical contact with a more active metal to prevent oxidation of iron.

  • Passivation: Formation of a protective oxide layer.

  • Use of alloys or coatings: Stainless steel, tin cans.

Summary

  • Understanding how redox reactions underlie electrochemical processes is fundamental to applications like batteries and corrosion prevention. Regular practices such as galvanization are key to protecting metals from oxidative damage.