Oxidising agent

An oxidising agent is the substance that causes oxidation by accepting electrons from another species. Because it takes in electrons, the oxidising agent is itself reduced in the same reaction. Malay: agen pengoksidaan · Chinese: 氧化剂.

The idea often trips students because the agent does the opposite of its name to itself: it oxidises the other species but is reduced itself. Reason it out by following the electrons. The oxidising agent pulls electrons away from something else, forcing that something to lose electrons and be oxidised; in gaining those electrons the agent’s own oxidation number falls. Common oxidising agents in this chapter include acidified potassium manganate(VII), acidified potassium dichromate(VI), chlorine water and oxygen.

Example. When chlorine is bubbled into potassium iodide solution, chlorine takes electrons from iodide ions (Cl₂ plus 2e- to 2Cl⁻). The iodide is oxidised to iodine, so chlorine is the oxidising agent, and chlorine is itself reduced from 0 to -1. A colour change from colourless to brown confirms the iodine formed.

Confusion to avoid. Do not swap oxidising agent with reducing agent. The oxidising agent gains electrons and is reduced; the reducing agent loses electrons and is oxidised. A quick rule: the agent is always changed in the direction opposite to what it does to the other species. Identify the agent by the substance whose oxidation number falls. It also helps to remember that the strength of an oxidising agent depends on how readily it accepts electrons, which is why elements low in the reactivity series and non-metals high in the electrochemical series tend to be the strongest.

Spotting the oxidising agent, especially in tests with acidified manganate(VII) which turns from purple to colourless, or dichromate(VI) which turns from orange to green, links this term to displacement and to the electrochemical series, and it is a standard identification asked in SPM Chemistry.