January 2019 NY Regents Chemisty Exam
45 Which half-reaction equation represents reduction? (1) Cu → Cu^2+ + 2e-
(2) Cu^2+ + 2e- → Cu
(3) Ag + e- →Ag+
(4) Ag+ →Ag + e-
Solution: We can remember the mnemonic OIL RIG. OIL (oxidation is loss of electrons). RIG (reduction is gain of electrons). For this question, we are looking for reduction; gain of electrons. Gaining electrons means we would see them on the reactant side, eliminating choices 1 and 4. Next, we can see if the charges on both side of the equation are equal. For choice 2, the total charge on the left of the arrow is 0 (+2-2=0). The charge on the right of the equation is zero as well. For choice 3, the charge on the left is -1 while the charge on the right is +1, eliminating choice 3.
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