Why is the equilibrium constant K related only to $E^{\circ}_{cell}$ and not to $E_{cell}$?

Because at equilibrium E_cell itself is zero; K is a constant at a given temperature, so it can relate only to the constant E°_cell.

CBSE Class 12 · 2003 · Delhi · Redox Reactions and ElectrochemistryVery Short Answer

Why is the equilibrium constant K related only to

E_{ce l l}^{\circ}

and not to

E_{ce l l}

E_{ce l l}

changes continuously as the reaction proceeds and becomes zero at equilibrium — so it cannot characterise the equilibrium.

K

and

E_{ce l l}^{\circ}

are both constants at a given temperature; setting

E_{ce l l} = 0

in the Nernst equation leaves the fixed relation

E_{ce l l}^{\circ} = \frac{0.059}{n} lo g K

CBSE Class 12 Board · 2003 · Delhi · Subjective