electrolytic
Summary
An electrolytic cell uses an external power source to force a nonspontaneous redox reaction (ΔG > 0, E°cell < 0). Oxidation is still at the anode and reduction at the cathode, but the anode is positive, the opposite of a galvanic cell.
Detail
Meaning: An electrolytic cell uses an external power source, such as a battery, to drive a nonspontaneous redox reaction: ΔG > 0 and E°cell < 0. Reduction still occurs at the cathode and oxidation at the anode. Examples include electrolysis of molten NaCl into Na and Cl₂, electroplating, and recharging a battery.
Equation: n = It/F, where n is moles of electrons (mol), I is current (A), t is time (s), and F is the Faraday constant, about 96,500 C per mole of electrons.
On the MCAT: Passages describe electrolysis or electroplating and ask which electrode gains mass or what forms at each electrode. Calculations convert current and time to charge, then to moles of electrons, then to moles and mass of product using the half-reaction ratio.
Common confusion: Students memorize "anode negative, cathode positive." That is the galvanic convention. In an electrolytic cell the anode is positive and the cathode negative, set by the power source. What never changes is anode = oxidation, cathode = reduction, and electrons flow through the external wire from anode to cathode.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.