E_cell and K_c

There is a mathematical relationship between E_cell and K_c, but you are not required to know this for your "A" level:

E_cell = RT/F lnK_c

Where T is the absolute temperature, R is the gas constant (8.314 J mol^-1 K^-1) and F is the Faraday constant (96500 C)

However, you are expected to have a rough idea of how they match up. It is useful to split up the position of equilibrium into five general areas:

1. Reaction complete	K_c > 10¹⁰	E_cell > + 0.60 V
2. More products than reactants	1 < K_c < 10¹⁰	0 < E_cell < + 0.60 V
3. Equilibrium	K_c = 1	E_cell = 0 V
4. More reactants than products	1 > K_c > 10^-10	0 > E_cell > - 0.60 V
5. Reaction does not go	K_c < 10^-10	E_cell < - 0.60 V

The value chosen for the "reaction going to completion" is completely arbitrary. Some books use a value of + 0.4 V. Bear in mind, even with a value of + 0.1 V, there is 49 times more product than reactant. You might not notice the 2% of reactant that is left unreacted at the end of the reaction.

Ecell and Kc

E_cell and K_c