1422-Chapt-21-Redox-Electrochem

1422-Chapt-21-Redox- - REDOX 1 Chapter 21 REDOX Electrochemistry REDOX 2 There are two types of chemical reactions 1 acid-base reactions(Lewis 2

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REDOX 1 Chapter 21 REDOX & Electrochemistry
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REDOX 2 There are two types of chemical reactions: 1) acid-base reactions (Lewis) 2) electron-transfer reactions Oxidation/oxidize: when you remove electrons from a material Reduction/reduce: when you add electrons to a material You cannot have oxidation without reduction ; you cannot generally reduce a molecule without oxidizing another molecule (and the other way around). Oxidizing agent: a chemical that causes another material to be oxidized (the oxidizing agent is reduced !) Reducing agent: a chemical that causes another material to be reduced (the reducing agent is oxidized !) REDOX
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REDOX 3 Oxidation States In order to understand redox reactions, we first need to be able to figure out what the oxidation state of an element is. The oxidation state is a method to indicate how many electrons are "assigned" to a particular element. For this we use a +/- system: +n indicates that an atom has lost electrons and now has a positive charge n indicates that an atom has gained electrons and now has a negative charge 0 indicates that an atom has its elemental number of electrons assigned to it and, therefore, has no charge Oxidation state is a “formalism”, that is, is may or may not reflect the actual charge on an atom. Common reference atoms & their oxidations states: Alkali metals = +1, alkaline earths = +2 O = 2 (exception = peroxides, H 2 O 2 , 1) Halides = 1 (exception = oxyhalides)
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REDOX 4 The key to being able to figure out the oxidation state of an element in a molecule is to note its electronegativity : The higher the electronegativity the more the element likes to add electrons to its valence shell. The lower the electronegativity the more likely an element will lose electrons. Let's dissect an example reaction: Reaction: + 32 P H O P H + O P 25 2 + H OO P 2 + H Electronegativities: O = 3.5 P = 2.1 H = 2.1 -3 +1 0 +1 +5 -2 -2 Atoms that gain electrons usually gain enough to fill their valence shell (octet rule). Atoms that lose electrons only lose enough to get them down to the next lowest filled valence shell ( but not always all the way! ).
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REDOX 5 When two elements have the same electronegativity (as with phosphorus and hydrogen) the element that is furthest to the upper right hand side of the periodic table is the one that gets the electrons. H Sc Mn Li Na K Rb Cs Ba Sr Ca Mg Be Ti V Cr Fe Co Ni Cu Zn YZ r N b M o Tc Ru Rh Pd Ag Cd Au Pt Ir Os Re W Ta Hf La He Ne Ar Kr Xe Rn F Cl I At O S Se Te Po N P As Sb Bi C Si Ge Sn Pb B Al In Tl 1 2 34 567 89 1 0 13 14 15 16 17 18 11 12 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Periodic Table of the Elements Hydrogen Helium Neon Fluorine Oxygen Nitrogen Carbon Boron Lithium Berylium Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Galium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Pal adium Silver
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This note was uploaded on 11/12/2011 for the course CHEM 1422 taught by Professor Staff during the Fall '08 term at LSU.

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1422-Chapt-21-Redox- - REDOX 1 Chapter 21 REDOX Electrochemistry REDOX 2 There are two types of chemical reactions 1 acid-base reactions(Lewis 2

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