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l17_redox_handou

l17_redox_handou - MIT Department of Biology 7.014...

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Redox Chemistry Handout This handout is intended as a brief introduction to redox chemistry. For further reading, consult an introductory chemistry or microbiology textbook. Redox reactions involve the transfer of electrons (usually abbreviated e ) from one molecule to the other. Reduction is when a molecule gains electrons . Oxidation is when a molecule loses electrons . (One way to remember this is the mnemonic “LEO says GER,” which translates to “ L oss of E lectrons is O xidation; G ain of E lectrons is R eduction.”) Since electrons cannot exist free in solution, an oxidation must always be paired with a reduction; hence the term redox (reduction and oxidation) reaction. The terminology can be very confusing: oxidation loss of e reduction gain of e oxidizing agent gains e during reaction and is therefore reduced during reaction reducing agent loses e during reaction and is therefore oxidized during reaction oxidized form form of molecule lacking (it’s all relative) an e reduced form form of the molecule having an additional (again, relative) e There are three ways to represent a redox reaction; these are shown below with a representative biological redox reaction: (1) Overall reaction: acetaldehyde + NADH + H + ⎯ → ethanol + NAD + (2) Electron-transfer diagram: acetaldehyde NAD + e ethanol NADH + H + (3) Half-reactions Acetaldehyde + 2 H + + 2e Ethanol + NADH NAD + + H + + 2e Acetaldehyde + NADH + H + Ethanol + NAD + In the reaction shown above: NADH is oxidized to NAD + acetaldehyde is reduced to ethanol acetaldehyde is the oxidizing agent NADH is the reducing agent NADH and ethanol are the reduced forms NAD + and acetaldehyde are the oxidized forms 1/6 MIT Department of Biology 7.014 Introductory Biology, Spring 2005
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There are two main ways that redox chemistry is discussed this class: 1) Given a redox reaction and the direction it proceeds, what is the e flow (that is, which molecule is oxidized and which is reduced)? In order to solve this kind of problem, you will need to know how to tell which of the two forms of a given molecule is the reduced form and which is the oxidized form. The easiest way is to: LOOK IT UP. Use a table of standard oxidation or reduction potentials, like the one on page 6 of this handout. These show the two forms of many common molecules and the redox relationship between
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