Which equation represents a reduction half- reaction?

June 2018 NY Regents Chemistry Exam

47 Which equation represents a reduction half- reaction?

(1) Fe→Fe^3+ + 3e–

(2) Fe+3e- →Fe^3+
(3) Fe^3+ →Fe+3e–

(4) Fe^3+ + 3e- →Fe

Solution: We can remember the mnemonic OIL RIG. OIL (oxidation is loss of electrons). RIG (reduction is gain of electrons). For the reduction half-reaction, we expect to see electrons on the left side of the equation, eliminating choices 1 and 3. Furthermore, the charge on both sides of the equation must be equal. For 2, the charge on the left (reactant side) is 3- and the charge on the right (product side) is 3+. In choice 4, the charge on both sides is 0.

Answer: 4

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