Reducing agent

A reducing agent is the substance that causes reduction by donating electrons to another species. Because it gives electrons away, the reducing agent is itself oxidised in the same reaction. Malay: agen penurunan · Chinese: 还原剂.

As with its partner, the name describes what the agent does to the other species, not to itself. Follow the electrons: the reducing agent pushes electrons onto something else, so that something gains electrons and is reduced, while the agent, having lost electrons, has its own oxidation number rise. Common reducing agents in this chapter include reactive metals such as zinc and magnesium, carbon, hydrogen, and potassium iodide solution.

Example. When zinc powder is added to iron(III) chloride solution, zinc gives electrons to iron(III) ions (Fe₃⁺ plus e- to Fe²⁺). The iron(III) is reduced to iron(II), so zinc is the reducing agent, and the zinc itself is oxidised from 0 to +2. In extraction, carbon is the reducing agent that removes oxygen from a metal oxide.

Confusion to avoid. Do not confuse reducing agent with oxidising agent. The reducing agent loses electrons and is oxidised; the oxidising agent gains electrons and is reduced. The reliable clue is the substance whose oxidation number rises, that is the reducing agent. A metal high in the reactivity series is a strong reducing agent because it loses electrons readily, whereas a metal low in the series is a weak one. Students also forget that the same substance can act as a reducing agent in one reaction and an oxidising agent in another, so always judge it by what happens to the electrons in the reaction in front of you.

Recognising the reducing agent connects this term to the reactivity series, to displacement and to metal extraction later in the chapter, and identifying it correctly, together with the substance it reduces, is a routine mark in SPM Chemistry.