I.Criteria for predicting the direction of redox reactions. Electrode potential. Nernst equation.
Redox reactions have the general form:
A redox reaction progresses when two redox couples are in contact – one is oxidized, the other – reduced.
For example, oxidationofZn by Cu2+
The overall reactionisexpressedbytwohalf-reactionsofgeneralform:
a Ox + n.e- ⇄bRed
Every half-reaction generates characteristic electrical potential (voltage). The potential is determinedusingelectrochemicalcell,adevicethatproduces an electric current from energy releasedbyaspontaneousredoxreactionintwohalf-cells.
The potentials ofthesehalf-reactionscanbemeasured(thepotential difference) in electrochemical cells incomparisontothepotentialofstandardhydrogen electrode (SHE): H+/Ho (0 V).
The SHE consistsofinertplatinumelectrodeimmersedinto1MHCl solution (pH=0) that is in equilibriumwith1atmofhydrogengas,keptatstandardtemperature 25oC, T=298 K.
Electrochemicalcellshavetwoconductiveelectrodes,calledthe anode and the cathode. The anodeisdefinedastheelectrodewhereoxidationoccurs. The cathode is the electrode wherereductiontakesplace.Electrodescanbemadefrom any sufficiently conductive materials,suchasmetals,semiconductors,graphite.In between these electrodes is the electrolyte,whichcontainsionsthatcanfreelymove.
In a typical voltaiccell,theredoxpairsof copperand zincare involved in the following half-cell reactions:
Zinc electrode(anode): Zn→Zn²⁺ +2e⁻
It has standard electrodepotentialE°=-0.76V
Copper electrode(cathode):Cu²⁺ +2e→Cu°
This half-reaction hasstandardelectrodepotentialE°=+0.34V
E°cell = ∆Eo= 0.34 – (-0.76)= +1.10 V
Each half-reaction generates a definite electrical potential depending on conditions. The electrode potential of a redox couple at any particular conditions is determined by Nernst equation. The direction of a redox reaction depends on these potentials.
For a half-reaction a Ox + n e- ⇄ b Red the Nernst equation is:
where:
E - the redox potential at usedconcentrations in volts (V) Eo - the standard redox potential in volts (for the standard state conditions with 1mol/L solutions of ions, 1 atmof pressure for gases (1.105 Pa), 25 C temperature and pH= 0) n - number of moles of electronsstated in the half-reaction (redox capacity) R – ideal gas constant (8.314 J/K.mol) F - Faraday constant (96 500 C/mol); (C - Coulombs) T - temperature in Kelvin’s (T = 273 + 25oC = 298 K) a, b - stoichiometric coefficients.
For calculations at standard conditions, the Nernst equation can be simplified for by substituting R, F, T=298 K, and changing In into lg:
Organic reactions are often accompanied by proton transfer. For such reactions the concentration of hydronium ions (pH) isincluded in Nernst equation.
The potential E = Eo when the concentrations of oxidized and reduced forms are equal. In that case ln [Ox]/[Red] = 0 and E = Eo.
If the potentials of all half-reactions could be measured, then we could determine which oxidizing and reducing agents will react. Unfortunately, there is no way to measure individual electrode potentials, but the difference between two electrode potentials can be measured.
The electrode potential of the half-reaction
H+ + e- → ½ H2