16. Electrolysis, electrode polarization, conductivity, overvoltage, corrosion

Faraday’s law, gas coulometer, breakdown voltage, ion mobility, overvoltage, active corrosion protection

Electrochemistry:

·       (1785) Martinus van Marum electrolyzed zinc and antimony by an electrostatic generator

·       (1799) Volta column → real electrolysis possibility

·       (1800) William Nicholson and Anthony Carlisle used the Volta column to decompose water

·       (1807) Humprey Davy made the discovery that instead of K, Na water electrolyzes

·       Davy’s student Michael Faraday discovered oxidation degree, anode, cathode, electrode and ions (scientists thought the atom is the smallest particle)

Electrode polarization

Electrolysis

·       when we are doing electrolysis, we are investing energy to get a chemical reaction E=UΔq U=voltage Δq=transfer of charged particles 

·       voltage is intensive state function

·       charge is extensive state function

·       driving force: voltage

·       Faraday’s laws

o   Faraday 1:

§  The amount of material converted in the electrolysis is proportional with the charge, 1F charge is required for the conversion of equimolar (M/z) amount of material. M=molar mass, z:ox number change, 1F=96485c, the charge of 1 mol of electron

§  Q=n*z*F

o   Faraday 2:

§  Charge (q) transferred in unit time (t): Current (I)

§  I=d[q]/dt → if I is constant → Q=I*t

§  Current density:

·       current at a unit area

·       Gas coulometer

o   an electrochemical device used to measure the amount of gas produced/consumed by an electrochemical reaction

o   it works by passing a current through a solution or electrolyte, causing a redox reaction that generates or consumes a gas (often hydrogen, oxygen, or chlorine)

o   the amount of gas produced is proportional to the amount of electrical charge passed through the system, according to Faraday's laws of electrolysis.

o   can be used to determine the efficiency of electrolysis

o   can be used to determine the amount of charge passed in the electrochemical process

·       Breakdown voltage

o   breakdown voltage (also known as the decomposition voltage) is the minimum voltage required to cause a particular electrochemical reaction to occur

o   the difference of the two potentials of the electrodes (according to Volta)

·       Overvoltage

o   the extra voltage required beyond the theoretical equilibrium potential so the electrochemical reaction happens at a desired rate (speed)

o   it is the difference between the actual potential at which an electrochemical reaction occurs and the equilibrium potential

o   Overvoltage can arise due to various factors such as activation energy barriers, and resistance within the electrochemical cell.

Conductivity

·       G=1/R=I/U (unit: siemens) (G=conductivity)

·       Resistance

o   R=U/I (Ohm’s law)

o   R=resistance

o   U=voltage

o   I=current

·       Specific conductivity

o   resistance is proportional with the length (l) and inversely proportional with cross section (q)

o   R=ρ*l/q

o   ρ=specific resistance (ohm*cm)

o   since conductivity is the reciprocal of resistance → specific conductivity is also the reciprocal of specific resistance (κ: S*cm^-1)

o   Definition:

§  specific conductivity (κ) is the reciprocal of the resistance of a 1 cm cube measured between two plates

§  specific conductivity is a property of a given solution

o   depends on the concentration (c_i: mol/l)

o   depends on the limiting molar conductivity (l_i)

o   if the solution is ideal (ions move independently) → κ=∑c_il_i

Ion mobility

·       the measure of the speed at which ions move through a solvent under the        influence of an electric field

·       depends on the ions’ charge, size and the viscosity of the solvent

·       affects conductivity

·       Grotthus-Zundel mechanism

Corrosion

·       A redox process where a metal turns into its ions due to outside “forces” (like rain or
a scratch)

·       Passive corrosion protection

o   we coat the metal with another metal

o   if the coating gets damaged a galvanic cell will be formed

o   when we use passive corrosion protection the coating only protects until the damage occurs

o   for passive protection we need to coat with a metal that has a higher electrode potential than the metal we want to protect

§  e.g. we coat iron with tin ε⁰(Sn)=-0,14V ε⁰(Fe)=-0,44V

§  this reaction should happen once iron start to corrode:
Sn +Fe²⁺ → Sn²⁺ + Fe

·       according to the reaction tin oxidizes

·       according to their standard potentials iron should oxidize

·       so this reaction doesn’t happen → tin only provides mechanical protection

·       Active corrosion protection

o   we coat the metal with another metal

o   if the coating gets damaged a galvanic cell will be formed

o   when we use active corrosion protection the coating protects even after thedamage occurs

o   for active protection we need to coat with a metal that has a lower electrode potential than the metal we want to protect

§  e.g. we coat iron with zinc ε⁰(Zn)=-0,76V ε⁰(Fe)=-0,44V

§  this reaction should happen once iron start to corrode:
Zn +Fe²⁺ → Zn²⁺ + Fe

·       according to the reaction zinc oxidizes

·       according to their standard potentials zinc should oxidize

·       so this reaction happens →  zinc protects even after the damage, since it corrodes before iron so until there is zinc left it protects iron