In an electrolytic cell, the voltage placed across the electrodes causes an otherwise nonspontaneous reaction to occur. A galvanic cell, however, contains the components of a spontaneous chemical reaction, with the oxidation and reduction half-reactions occurring in separate compartments of the cell. The free energy of reaction is liberated to drive an electron flow outside the cell. Whether the cell is electrolytic or galvanic, the electrode where oxidation occurs is called the anode, and the electrode where reduction occurs is called the cathode. In an electrolytic cell, though, oxidation at the anode must be forced to occur by the application of positive potential, and likewise the cathode in an electrolytic cell is negative. In a galvanic cell, on the contrary, oxidation is occurring spontaneously, feeding electrons to the circuit, so the anode is negative, and the cathode is positive, where cations are waiting to be reduced. The voltage difference, or emf, between the poles of a galvanic cell is called its cell potential, Ecell.
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