The overpotential of an electrochemical reaction is the difference between the measured potential and the standard potential. In this case, the standard potential is 0.8V, and the measured potential is 1.2V.Overpotential = Measured potential - Standard potentialOverpotential = 1.2V - 0.8VOverpotential = 0.4VThe overpotential of the electrochemical reaction is 0.4V.