Unit 4: Electrochemistry

Electrochemistry studies the interconversion of chemical and electrical energy through reactions involving ion transport and electron transfer at electrode–electrolyte interfaces. This unit builds from how ionic solutions conduct current up to how cells generate and quantify voltage.

I. Foundations: Electrolytes, Cells and Cell Conductance

An electrolyte is a substance that furnishes mobile ions when molten or dissolved, allowing a solution to carry current; the flow is ionic in the bulk and electronic in the external wire.

• Strong vs weak electrolytes: Strong (NaCl, HCl, KOH) ionise almost completely, so conductance is high and rises modestly on dilution; weak (CH₃COOH, NH₄OH) ionise partially, so conductance rises steeply on dilution as the degree of ionisation α increases.
• Electrochemical cell: A two-electrode device coupling a redox reaction to an electric circuit; oxidation always occurs at the anode, reduction at the cathode.
• Cell conductance basis: Ohm's law applies, but for solutions we track conductance G = 1/R, where R is resistance in ohms and G is in siemens (S). Conductance depends on ion concentration, ionic mobility, temperature and the geometry of the measuring cell.
• Conventions used later: electrode potentials are reduction potentials by IUPAC convention; the standard hydrogen electrode is the zero reference; EMF is measured under conditions of negligible current draw.

II. Conductance and the Conductivity Cell

Conductance measurements convert an easily measured resistance into intrinsic solution properties, but only after correcting for the shape of the measuring cell.

A. Cell constant determination

The cell constant accounts for the fixed geometry (plate area and separation) of the conductivity cell so that a raw resistance reading can yield conductivity.

• Definition: the ratio of electrode separation to area.

G* = l / A        (units: cm⁻¹ or m⁻¹)

• Symbols: l = distance between electrodes, A = cross-sectional area of each electrode.
• Practical determination: measure the resistance R of a standard KCl solution of known conductivity κ (tabulated), then

G* = κ × R

• Worked value: 0.1 M KCl has κ = 1.29 S m⁻¹ (0.0129 S cm⁻¹) at 298 K. If it gives R = 220 Ω, then G* = 0.0129 × 220 = 2.84 cm⁻¹. This constant is then reused for any test solution in the same cell.

B. Specific and molar conductance (numericals)

Specific and molar conductance rescale raw conductance into concentration-independent quantities suitable for comparison.

• Specific conductance (conductivity) κ: conductance of a solution held between elec