Answer
Recall that in a voltaic cell, the cell potential must be positive. Therefore, the half-cell with the more positive reduction potential is the one in which reduction occurs. In our case, the copper half-cell has the more positive reduction potential (+0.34 V); the aluminum half-cell has a reduction potential of -1.66 V, so we reverse the sign to get the oxidation potential and reverse the reaction, as well, because the one presented is the reduction half-reaction.
Work Step by Step
Recall that in a voltaic cell, the cell potential must be positive. Therefore, the half-cell with the more positive reduction potential is the one in which reduction occurs. In our case, the copper half-cell has the more positive reduction potential (+0.34 V); the aluminum half-cell has a reduction potential of -1.66 V, so we reverse the sign to get the oxidation potential and reverse the reaction, as well, because the one presented is the reduction half-reaction.
