Question

In: Chemistry

How can the reaction’s direction in the cell be changed without changing the temperature? Be logical...

How can the reaction’s direction in the cell be changed without changing the temperature? Be logical and explicit.

Solutions

Expert Solution

When the cell is NOT under standard conditions, i.e. 1M of each reactants at T = 25°C and P = 1 atm; then we must use Nernst Equation.

The equation relates E°cell, number of electrons transferred, charge of 1 mol of electron to Faraday and finally, the Quotient retio between products/reactants

The Nernst Equation:

Ecell = E0cell - (RT/nF) x lnQ

In which:

Ecell = non-standard value

E° or E0cell or E°cell or EMF = Standard EMF: standard cell potential
R is the gas constant (8.3145 J/mol-K)
T is the absolute temperature = 298 K
n is the number of moles of electrons transferred by the cell's reaction
F is Faraday's constant = 96485.337 C/mol or typically 96500 C/mol
Q is the reaction quotient, where

Q = [C]^c * [D]^d / [A]^a*[B]^b

pure solids and pure liquids are not included. Also note that if we use partial pressure (for gases)

Q = P-A^a / (P-B)^b

substitute in Nernst Equation:

Ecell = E° - (RT/nF) x lnQ

E° cell is always fixed, as well as "n", therefore, only Q can be changed

we must favour Q < 1

if this is true, then (RT/nF) x lnQ, becmes negative

Ecell = E° - (RT/nF) x lnQ

increases

then

Q = [products]/[reactants]

increase reactants as much as possible

decrease product present


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