Which term best fits the definition: a molecule that causes the addition of electrons to a substrate?

• A. Oxidizing agent
• B. Catalyst
• C. Reducing agent
• D. Electron donor

Answer: (C)

In redox chemistry, electrons move from one species to another. The molecule that donates electrons to a substrate causes that substrate to gain electrons, so the substrate is reduced. The donor of those electrons is the reducing agent, and it itself is oxidized in the process. That’s why this term fits the definition best. An oxidizing agent would take electrons away from the donor, causing oxidation of the donor and reduction of itself. A catalyst simply speeds up the reaction without being consumed, and while an electron donor describes the same idea in plain language, the standard term used in chemistry for this role is reducing agent.