Which half-cell reaction would have the greatest tendency to occur at the cathode?

The correct answer is the second.

Explanation:

Oxidation occurs when an element loses electrons, acquiring a positive charge. Reduction occurs when an element gains electrons and goes from a positively charged state to a neutral one.

Aluminum metal (Al) oxidizes, so we say that Al is the reducing species, which acts by reducing . Copper () is reduced, so it is the oxidizing agent, it acts oxidizing to Cu.

oxidizes to Cu, and Al reduces to .

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