Discussion
A cell's EMF must be measured at essentially zero current because
Answer: D
Correct answer: only then is it the reversible thermodynamic value
Drawing current makes the reaction proceed irreversibly: the electrodes polarise, the internal resistance produces a voltage drop, and the terminal voltage falls below the true EMF. Only at essentially zero current is the measured potential the reversible value related to the free energy by \( \Delta G = -nFE \). This is why EMF is measured by a potentiometer, which balances the cell against a known voltage and draws no current at balance.
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