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Chapter 4

CH 320M Chapter 4 Powerpoint Notes

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University of Texas at Austin
CH 320M
John A.Colapret

Chapter 4 Thursday, September 05, 2013 12:00 AM ARRHENIUS ACIDS AND BASES: 1884, Svante Arrhenius proposed these definitions:  Acid - a substance that produces H 3 ions aqueous solution  Base - a substance that produces OH- ions in aqueous solution This definition of an acid is a slight modification of the original definition, which was that an acid is a substance that produces H in aqueous solution BRONSTED-LOWRY DEFINITIONS: *Colapret said go by these definitions!!! Acid - proton donor Base - proton acceptor Example: CONJUGATE ACIDS & BASES: Conjugate base - species formed from an acid when it donates a proton to base Conjugate acid - species formed from a base when it accepts a proton from an acid Acid-base reaction - a proton-transfer reaction Conjugate acid-base pair - any pair of molecules or ions that can interconverted by transfer of a proton Example: Use curved arrows to show the flow of electrons in an acid-base reaction RESONANCE IN ACIDS: Many organic molecule have 2+ more sites that can act as proton acceptors:  Favored site of protonation is the one in which the charge is more DELOCALIZED π ELECTRONS AS BASIC SITES: Proton-transfer reactions occur with compounds having π electrons, as for example the π electrons of C- C double and triple bonds Example: the π electrons of 2-butene react with HBr by proton transfer to form a new C-H bond  Formation of carbocation --- a species in which one of its C has only 6 electrons in its valence shell and carries a charge of +1 ACIDS & BASE STRENGTHS: Strength of an acid is expressed by an equilibrium constant  The acid dissociation of acetic acid is given by the following equation  Equilibrium expression for the dissociation of any uncharged acid, HA: Water ---- solvent; concentration is a constant equal to approx. 55.5 mol/L MEMORIZEEEEEEE THISSSS: ACID-BASE EQUILIBRIA: Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base! THERMOCHEMISTRY & MECHANISMS: Reaction mechanism - a step-by-step description of how a chemical reaction occurs Thermochemistry - the study of the energy of an entire system at each and every instant of a reaction Reaction coordinate diagram - a graph showing the energy changes that occur during a chemical reaction  Energy is plotted o
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