Recall the mnemonic Red Cat An Ox: oxidation occurs at the anode and reduction occurs at the cathode. In an electrolytic cell, a power source is used to make electrons flow in the direction opposite of how they would flow spontaneously. Since they are negatively charged, electrons will spontaneously move toward a positive charge (as they do in a galvanic cell), so in an electrolytic cell, they will move toward the negative electrode instead. The flow of electrons in an electrochemical cell is always from anode to cathode, so in an electrolytic cell, the electrons flow from the positively-charged anode to the negatively-charged cathode. The correct answer is (D).
Expert Approach:
I'm prompted to consider the flow of electrons in an electrolytic cell, and the answer choices indicate that I need to focus on the charge at the cathode and anode and the direction of electron flow. I begin by recalling that in all types of electrochemical cells, the anode is the site of oxidation, while reduction occurs at the cathode (An Ox, Red Cat); the electrons lost during oxidation at the anode flow toward the cathode to be used for reduction. I still need to identify the charge at the anode and cathode in an electrolytic cell, though. Electrolytic cells use an electric current to drive a non-spontaneous chemical reaction, which means that the flow of electrons from the anode to the cathode must be unfavorable. This could only be the case if the negatively charged electrons were being driven toward a negatively charged cathode (if the cathode were positively charged, the flow of electrons would be in a favorable direction and the reaction would be spontaneous). Putting it all together, I predict that electrons flow from a positive anode to a negative cathode in an electrolytic cell, which is what (D) says.I was careful not to mix up electrolytic and galvanic cells here. Although electrons flow from anode to cathode in both, the charges of the electrodes differ between the two cell types. If I had mixed them up, I would have incorrectly chosen (A), which says that electrons flow from the negative anode to the positive cathode.Finally, had I not known the difference between electrolytic and galvanic cells, I still could have narrowed the correct choice down to (A) and (D) if I were familiar with electrochemical cells generally. The mnemonic "An Ox, Red Cat" has the flow of electrons built into it: Since electrons are lost by the species at the anode during oxidation, they must flow to the species at the cathode to be used in a reduction reaction; this is true for all electrochemical cells.
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