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MCAT Chemistry Question — Spontaneous Reaction

What species is oxidized and what is the oxidizing agent in the following spontaneous reaction?

2Au3+(aq) + 3Cu (s) 2Au (s) +3Cu2+ (aq)

A) Au3+, Au

B) Cu, Cu2+

C) Au3+, Cu

D) Cu, Au3+


For the MCAT, remember: oxidation is loss of electrons (OIL) and reduction is gain of electrons (RIG). The compound containing the element that is reduced is the oxidizing agent, and the compound containing the element that is oxidized is the reducing agent.

In this reaction, Cu (s) is oxidized because it loses 2 e. Au3+ is the oxidizing agent because, although monatomic, it is the element in the compound that is reduced.

A) Au3+(aq), Au (s), incorrect, Au3+ (aq) is reduced to Au (s), and Au (s) is a product.

B) Cu (s), Cu2+ (aq), incorrect, Cu(s) is oxidized, and therefore the compound containing Cu (s) is the reducing agent. Note that although Cu (s) is the reducing agent in this reaction, the entire compound would be the reducing agent if the Cu were in a polyatomic molecule.

C) Au3+ (aq), Cu (s), incorrect, Au3+ (aq) is reduced, and Cu (s) is oxidized.

D) Cu (s), Au3+ (aq), correct.

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