# Transport in electrolyte solutions Sähkökemian peruseet KE-31.4100 Tanja Kallio C213 CH 3.1 – 3.2.

## Presentation on theme: "Transport in electrolyte solutions Sähkökemian peruseet KE-31.4100 Tanja Kallio C213 CH 3.1 – 3.2."— Presentation transcript:

Transport in electrolyte solutions Sähkökemian peruseet KE-31.4100 Tanja Kallio tanja.kallio@aalto.fi C213 CH 3.1 – 3.2

Origin of transport potentiostat /load Bulk: conductivity Interphase (~  m): i)Electrode reactions ii)Polarization e-e- e-e-

Ion distribution in the bulk and near the surface J. Israelachivili, Intermolecular and surface forces Spatial distribution of ions obeys Boltzmann distribution - - - - - - - - - - + + + + + + + - - - - + counter ion Concentration profile (i)Stationary (ii)Function of time

Transport and reactions electrode i) mass transfer ii) adsorption iii) (electo)chemical reaction e-e- electrolyte Cu 2+ + e   Cu + iv) desorption v) mass transfer r = reaction rate J i = flux of I k = rate constant

Transport and mobility FfFf FcFc v charged particle in an electric field (F c ) Einstein eq: definition for mobility: F f = friction force q = charge of the particle E = electric field f = friction coefficient v = velocity D = diffusion coefficient u = mobility q

Mobility, molar conductivity and diffusion coefficient Ohm’s lawReaction rate at the electrode: using v from the previous slide Stokes’ law can be applied to determine friction coefficeint for ions   = viscosity a = ion radius

Walden rule K + (●) Cs + (■)

Measuring conductivity johto- kyky- mittari Exchange current Pt-electrodes calibration

Electronic vs. ionic conductivity http://wiki.answers.com/Q/Wh ich_metals_are_the_most_co nductive#ixzz25OZd6XAR Electronic conductivity: current is transported by electrons i) conductors ii) semi conductors iii) insulators Ionic conductivity: current is transported by ions i) strong electrolytes ii) weak electrolytes iii) non electrolytes MaterialCond / S cm -1 Ag1.59×10 5 Cu1.68×10 5 Au2.44×10 5 Pt1.06×10 4 C (amorphous)5-8×10 1 C (graphite)*2.5-5.0×10 3 / 3.0×10 1 Ge4.6×10 −2 Si10 -1 - 10 -5 Water10 -9 Glass10 -16 - 10 -12 PTFE (Teflon ® )10 -16 -10 -12

Structure, mobility Ca 2+ i ii iii Proton transport via Grothus or hopping mechanism

Strong electrolytes - Kohlrausch’s law (1/2) Dependency of diffusion coefficient on concentration Debye-Hückel limiting law for 1:1 electrolytes Using above diffusion coefficient can be written as B = 2.303 A insert D-H law

Strong electrolytes - Kohlrausch’s law (2/2) Kohlrausch’s law

Weak electrolytes – Ostwald dilution law (1/2)  is small   ± ~ 1. So for 1:1 electrolytes

Weak electrolytes – Ostwald dilution law (2/2) Combining this with rearranged equation for equilibrium constant for 1:1 electrolytes Ostwald dilution law c + and c - are low  + ~ +,  and - ~ -, 

Comparison of a weak and a strong electrolyte

Electrolyte dissociation in an organic solvent Dissociation is incomplete → Ostwald dilution law Because of interactions  ± must be included For 1:1 electrolytes It has been noted experimentally that tetrabutyyliammoniumtetrakis (4-klorofenyyli)borate in 1,2- diklooriethane

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