Electrochem and G
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During redox (reduction and oxidation) reactions electrons are exchanged between reactants. Chemical energy may be converted to electrical energy by using wires to connect reactants of redox reaction. The maximum voltage is related to the free energy of the redox reaction. The charge transferred times the potential E in volts is the maximum energy or free energy available if an electrical cell is set up properly. Each mole of electrons has a charge of 1 faraday (1 F = 96485 C), and n moles of electron have a charge of n F.

ΔG = -nFE

n= moles, F = faraday (a constant), E = voltage

The free energy and voltage are related in the same way at standard conditions which is simply a special case.

ΔGo = - nFEo

The previously derived dependence of free energy on the activity quotient relates the voltage to the activity quotient.

ΔG = ΔGo + RT ln Q

- nFE = - nFEo + RT ln Q

E = Eo - ( RT /nF) ln Q

This equation is called the Nernst equation, and it is used extensively in electrochemistry.

At equilibrium, the voltage is zero so analogously to the free energy equations at equilibrium then,

Eo = ( RT/nF) ln Q = (RT/nF) ln Keq

Electrolytic cells are a convenient way to measure equilibrium constants and free energies of redox reactions. Half cells are set up with high resistance between them to minimize current and the voltage is measured. Measurements need not be made at standard conditions because activities may be used to calculate the Q and determine both Eo and Keq.

Quiz 1C Quiz 2S
Gr
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