What do oxidation and reduction mean, in terms of both oxygen and electrons?
Oxidation and reduction in terms of oxygen and electrons, redox reactions, oxidising and reducing agents, and writing half equations for the loss and gain of electrons.
A focused answer to OCR Gateway GCSE Chemistry A topic C3.3 on oxidation and reduction, covering the definitions in terms of oxygen and electrons, redox reactions, oxidising and reducing agents, and writing half equations for electron loss and gain.
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What this dot point is asking
OCR wants you to define oxidation and reduction in two ways (in terms of oxygen and in terms of electrons), recognise redox reactions, identify oxidising and reducing agents, and write half equations for the loss and gain of electrons (Higher). Redox links to the reactivity series, displacement and electrolysis.
Two definitions of oxidation and reduction
The oxygen definition is the simpler one and is used for reactions such as metals burning or metal oxides being reduced. The electron definition is more general and is used for ionic reactions, displacement and electrolysis.
Redox reactions
Oxidising and reducing agents
An oxidising agent is a substance that oxidises something else (so it gains electrons or gives oxygen, and is itself reduced). A reducing agent is a substance that reduces something else (so it loses electrons or removes oxygen, and is itself oxidised). For example, in the extraction of iron, carbon is the reducing agent because it removes oxygen from the iron oxide.
Half equations (Higher)
The number of electrons in a half equation equals the charge on the ion formed: a ion involves two electrons, a ion involves one electron, and so on. In electrolysis and displacement, the electrons lost by one species are gained by another.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksMagnesium reacts with copper oxide: Mg + CuO produces MgO + Cu. State which substance is oxidised and which is reduced, and explain your answers in terms of oxygen.Show worked answer →
A C3.3 redox question in terms of oxygen. Reward: magnesium is oxidised because it gains oxygen (it goes from Mg to MgO). Copper oxide is reduced because it loses oxygen (it goes from CuO to Cu). This is a redox reaction because oxidation and reduction happen together: as one substance gains oxygen, the other loses it. Markers credit magnesium oxidised (gains oxygen), copper oxide reduced (loses oxygen), and the reasoning in terms of gaining and losing oxygen. A common slip is to reverse the two, or to define them in terms of electrons when the question asks for oxygen.
OCR 20214 marksWhen magnesium reacts with dilute hydrochloric acid, magnesium atoms become magnesium ions. Write the half equation for this change, state whether the magnesium is oxidised or reduced, and explain your answer in terms of electrons.Show worked answer →
A Higher tier half-equation question. Reward: the half equation is (a magnesium atom loses two electrons to form a ion). The magnesium is oxidised, because oxidation is the loss of electrons (OIL RIG: Oxidation Is Loss). The magnesium atom has lost two electrons, so it has been oxidised. Markers credit the correct half equation with two electrons on the right-hand side, the answer oxidised, and the explanation that oxidation is loss of electrons. A common error is putting the electrons on the wrong side or losing the wrong number.
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