Chemistry · Ch 1 — Basic Concepts of Chemistry and Chemical Calculations
Redox Reactions
Redox Reactions
A cut apple turning brown, LPG burning, iron rusting -- all of these everyday changes are oxidation reactions. The classical definition, based on oxygen and hydrogen, states that oxidation is the addition of oxygen (or removal of hydrogen), and reduction is the reverse (removal of oxygen or addition of hydrogen). Every oxidation is always accompanied by a reduction happening at the same time (one substance's oxygen/hydrogen has to go somewhere), which is why these are together called redox reactions.
Two classical-oxidation examples: 4Fe + 3O₂ → 2Fe₂O₃ (oxygen adds onto iron -- this is rusting), and H₂S + Cl₂ → 2HCl + S (hydrogen is removed from H₂S). Two classical-reduction examples: CuO + C → Cu + CO (oxygen is removed from cupric oxide) and S + H₂ → H₂S (hydrogen is added to sulphur).
The classical definition breaks down once you consider redox reactions that involve neither oxygen nor hydrogen -- so chemistry uses a more general, electron-transfer definition instead: oxidation is the loss of an electron, and reduction is the gain of an electron. For example, Fe²⁺ → Fe³⁺ + e⁻ is oxidation (an electron is lost), while Cu²⁺ + 2e⁻ → Cu is reduction (electrons are gained). This electron-based view is more powerful because it works for every redox reaction, and it leads directly to the idea of oxidation number (Section 1.8.1), which gives a systematic bookkeeping method for tracking electron transfer even in complicated molecules. …
What this figure shows. A set of photographs illustrating everyday oxidation reactions: burning of LPG gas and the rusting of iron are named as two examples in the accompanying text, alongside the oxidation of carbohydrates and lipids to CO₂ and H₂O that produces energy in living or …
Worked out. A real-life aside explaining why the iron in haemoglobin resists oxidation even though iron rusts readily in air. Haemoglobin has four subunits, each with a porphyrin ring (heme) attached to the globin protein; the Fe²⁺ ion sits in an octahedral coordination complex, with four positions occupied by the porphyrin ring, a fifth by an imidazole ring of a histidine residue, and the sixth used to bind oxygen. The surrounding globin protein chain provides a hydrophobic environment that makes oxidation of Fe²⁺ difficult, though about 3% of haemoglobin is still oxidised daily to methemoglobin (where the iron is Fe³⁺ and cannot bind oxygen); the enzyme methemoglobin reductase converts it back. A companion note explains that cyanide, unlike oxygen, binds irreversibly to haemoglobin and blocks oxygen transport from t …