Electrochemistry

The ionic-electronic method (also called half-reaction method) separates the overall redox reaction into two half-reactions that are balanced independently:

• Write the oxidation and reduction half-reactions
• Balance atoms (except H, O) by stoichiometric coefficients
• Balance O using $\mathrm{H_2O}$, balance H using $\mathrm{H^+}$ (acidic) or $\mathrm{OH^-}$ (basic)
• Balance charge with electrons $e^-$
• Multiply each half-reaction so electrons cancel when summed

A galvanic (voltaic) cell converts chemical energy into electrical energy. Two half-cells: anode (oxidation, −) and cathode (reduction, +) connected by a salt bridge.

The standard cell potential: $$E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode}$$ $$\Delta G^\circ = -nFE^\circ_{cell}$$ The Nernst equation relates the potential to concentrations: $$E = E^\circ - \dfrac{RT}{nF}\ln Q = E^\circ - \dfrac{0.0592}{n}\log Q\;(\text{at }25°\mathrm{C})$$

Common types of electrochemical cells:

• Concentration cell: same electrodes but different concentrations; $E = -(0.0592/n)\log(Q)$
• Fuel cell: continuous supply of fuel (e.g. $\mathrm{H_2/O_2}$) → electrical energy + water
• Lead-acid battery (car battery): Pb anode, PbO₂ cathode, H₂SO₄ electrolyte. Rechargeable.
• Common dry cells: Zn-carbon, alkaline, lithium-ion (rechargeable, high energy density)

Corrosion is the spontaneous oxidation of a metal (usually Fe to rust $\mathrm{Fe_2O_3\cdot xH_2O}$) in the presence of O₂ and water.

The electrochemical mechanism: anodic areas (oxidation: $\mathrm{Fe \to Fe^{2+} + 2e^-}$) and cathodic areas (reduction: $\mathrm{O_2 + 2H_2O + 4e^- \to 4OH^-}$) form galvanic cells on the metal surface.

Prevention: coating (paint, galvanizing with Zn), cathodic protection (sacrificial anode like Mg), alloying (stainless steel with Cr).

Electrolysis uses electrical energy to drive a non-spontaneous redox reaction. The Faraday constant $F = 96485\,\mathrm{C/mol}$ is the charge of one mole of electrons.

Faraday's laws:

• The mass of a substance liberated is proportional to the charge passed
• $m = \dfrac{Q\cdot M}{n\cdot F}$ where $Q = I\cdot t$