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Oxidation vs reduction: what's the difference?

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University Course Reader · STEM

In the cell, zinc undergoes oxidation at the anode while copper ions are reduced at the cathode.

The reader highlighted one word in a textbook passage. Clicked broke down the chemistry term “oxidation” into plain English:

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Overview

Oxidation is an atom losing electrons and reduction is an atom gaining them, and they always happen together, because an electron that leaves one atom lands on another. The pair is called a redox reaction. When iron rusts, iron loses electrons to oxygen: the iron is oxidised and the oxygen is reduced. The name is the trap. The atom being reduced is the one gaining electrons.
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Overview

Oxidation means an atom has lost electrons and reduction means an atom has gained them, and they come as a pair, because a lost electron has to land somewhere. Together they are a redox reaction. Rust, a cut apple going brown and a battery powering a torch are all one substance letting go of electrons and another catching them. Reduced means you got more, and you can be oxidised with no oxygen anywhere near. 😎

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Detail

Oxidation is an atom losing electrons and reduction is an atom gaining them. Neither happens alone, because an electron that leaves one atom lands on another. Chemists call the pair a redox reaction. Rust is one: iron gives up electrons to oxygen, so the iron is oxidised and the oxygen is reduced. A zinc-copper battery is another: zinc gives up electrons at one terminal, they travel through the wire, and copper takes them at the other. Oxygen is not required. Oxidation is named after the first electron-taker chemists noticed, but any atom that pulls an electron away counts. Reduction points the wrong way too: the atom being reduced is the one gaining. The mnemonic is OIL RIG, Oxidation Is Loss, Reduction Is Gain. If the two atoms stick together afterwards, that is an ionic bond; the oxidation and reduction were the electron moving across, and they happen just as fully in the battery, where no bond forms. Salt dissolving in water pulls the ions apart, but no atom loses or gains an electron, so it is not redox.
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Detail

Oxidation means an atom has lost electrons and reduction means an atom has gained them, and one never happens without the other, because an electron leaving one atom has to arrive at another. The pair is a redox reaction. A zinc-copper battery is the clearest case: zinc gives up electrons, they travel through the wire to the copper, and the copper takes them, so zinc is oxidised, copper is reduced, and the moving electrons are the current. Wood burning is carbon giving up electrons to oxygen. Your cells passing electrons from food to the oxygen you breathe is how you stay warm. The names trip everyone up. Oxidation is named after oxygen, but oxygen is only the best-known electron-taker, so you can be oxidised with no oxygen present. Reduction sounds like losing, and the reduced atom is the one that got more electrons. Hold on to the electron and the names sort themselves out: lose it, oxidised; gain it, reduced. And if no electron moves at all, it is not redox. Stir salt into water and the ions separate, but no atom hands one over, so nothing is oxidised or reduced. 😎

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Analogy

Oxidation and reduction are the two ends of a relay hand-off. One runner loses the baton and the next runner gains it, in one movement. Losing electrons is the runner letting go: that atom is oxidised. Gaining them is the next runner taking hold: that atom is reduced. A runner cannot let go into empty air, and an atom cannot lose an electron unless another atom takes it, so a reaction always has both halves. Rust is iron letting go and oxygen taking hold, and both halves together are what chemists mean by redox.
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Analogy

Oxidation and reduction are a hand-me-down jacket changing hands. The older kid loses the jacket and the younger kid gains it, in one act, and there is no version where the jacket leaves one and lands on nobody. The kid who let go is oxidised, the kid who took it is reduced, and yes, the one who is reduced is the one who ended up with more. That is the whole naming problem in one jacket. 😎
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Formal definition — The same term, explained the usual way

Oxidation is the loss of electrons by an atom, ion or molecule; reduction is the corresponding gain of electrons. Because electrons lost by one species must be gained by another, the two processes occur together in what is termed a redox (reduction–oxidation) reaction. Historically, oxidation referred to combination with oxygen and reduction to the removal of oxygen or the recovery of a metal from its ore; the modern definitions in terms of electron transfer generalise these, so that oxidation and reduction may occur in the absence of oxygen. Formation of an ionic bond by electron transfer is one instance of a redox process; corrosion, combustion, respiration and the operation of electrochemical cells are others.

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