Chemistry

Electrochemical cells

Galvanic versus electrolytic, Nernst, Faraday, and a corrosion sketch.

Basics

Galvanic versus electrolytic

A galvanic (voltaic) cell does work from a spontaneous redox. Electrons leave the anode (oxidation) and enter the cathode (reduction) through the outer circuit. An electrolytic cell uses an external supply to drive a non-spontaneous reaction. Oxidation is still at the anode and reduction at the cathode, but the signed terminals can feel reversed versus galvanic use. A salt bridge or separator keeps charge balance with ions. Cell notation often puts oxidation on the left and reduction on the right.

Nernst and concentration

The standard potential E° is the reference at 1 M, 1 bar, and the stated temperature. The working potential is E = E° − (RT/nF) ln Q with reaction quotient Q. At 25°C one often writes E = E° − (0.059/n) log Q. When concentrations match, Q=1 and E→E°. A concentration cell makes voltage from unequal concentrations of the same half-cell. If H⁺ appears, put it in Q and watch pH.

Faraday’s laws

Charge passed Q = I t scales with moles reduced (or oxidized). In m = (M / n F) I t, F is Faraday’s constant (~96485 C/mol) and n is electrons per formula. Electroplating and electrowinning mass use this. Below 100% current efficiency, real mass is less than theory. Keep coulombs and seconds consistent.

Corrosion sketch

Rusting iron is a local cell: anode Fe → Fe²⁺ + 2e⁻; cathode O₂ and water take electrons and make hydroxide. Salt water, acid, and chloride boost the electrolyte path. A sacrificial anode (zinc) dissolves so the iron does not. Paint, plating, and passive films keep oxygen and water off. Stainless relies on a chromium-oxide film, yet chlorides can pit it.

Formulas

Nernst

E = E° − (RT/nF) ln Q ≈ E° − (0.059/n) log Q (25°C)

Larger Q (more products) lowers E.

Symbols

n moles of electrons
Q reaction quotient
F Faraday constant

Cell and free energy

ΔG = −n F E

E>0 is spontaneous (galvanic). Electrolysis pays ΔG>0 with external work.

Symbols

E cell potential

Deposited mass

m = (M I t) / (n F)

Ideal 100% efficiency. M is molar mass.

Symbols

I current
t time

Standard cell potential

E°_cell = E°_cathode − E°_anode

From reduction tables; subtract the anode’s reduction potential.

Key table

F	about 9.6485×10⁴ C per mole of e⁻
SHE	standard hydrogen electrode E°=0; reference for other halves
Sacrificial anode	Zn or Mg on ships and pipes; they oxidise first