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Chapter 4

CH 320M Chapter 4 Textbook Notes

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

4.1 ArrheniusAcids and Bases Thursday,September 05, 2013 12:01 AM Arrhenius definition: + • An acid is a substance that dissolves in water to produce H ions • A base is a substance that dissolves in water to produce OH ions - This was established back then, but we now know that these definitions aren't accurateto refer to an acid and base. Today's definition: An H ion immediately combines with a water molecule to give a hydronium ion H O + 3 • Hydration of the hydronium ion itself gives the ion H O 3 2+ + + • The monohydrated and dihydrated forms (H O and H O 3 are the M3JO2 hydrated forms present in aqueous solution ○ Present in equal concentrations • Arrhenius is modified to take into account these interactions of H with water molecules. Arrhenius definitions are still valid as long as it's about AQUEOUS SOLUTIONS 4.2 Bonsted-Lowry Acids and Bases Thursday, September26, 2013 9:14 AM 1923, Danish chemist Johannes Bronsted and English chemist Thomas Lowry independently proposed the followingdefinitions Bronsted-Lowry definition: • An acid is a proton donor • A base is a proton acceptor • And acid-base reaction is a proton-transfer reaction Conjugate Acid-Base Pairs Differ by a Proton Thursday, September26, 2013 9:16 AM Conjugateacid-base pair - any pair of moleculesor ions that can be interconverted by transfer of a proton • Conjugatebase - when an acid transfers a proton to a base, the acid is converted to this • Conjugateacid - when a base accepts a proton, it's converted to this Curvedarrows can also be used to show the transfer of a proton: **the arrows can't point backwards! The Bronsted-Lowry definitions DON'T REQUIREWATER as a reactant Ex: Example 4.1 Thursday, September26, 2013 9:20 AM Example 4.2 Thursday, September26, 2013 9:20 AM Bronsted-Lowry Bases with Two or More Receptor Sites Thursday, September 26, 2013 9:21 AM Many organic compounds have two or more receptorsites! The morestablecharged speciesis the one in whichthe chargeis more delocalized---relativechargedelocalizationcan be understand by resonance. Ex: • Wenow examineeach cationand determinewhichis more stable(lowerin energy) ○ Writethecontributingstructures   A-1and A-3makethe greatercontributions ○ Two resonancecontributingstructureforprotonation on the hydroxyl oxygengivingcationB   B-2at best makesa minorcontributionbecause of the adjacent positivecharges Chapter 4 Page 6 Example 4.3 Thursday, September26, 2013 9:27 AM π Electrons as Bronsted-Lowry Bases Thursday, September 26, 2013 9:28 AM So far,we consideredproton transferto atomshavinga nonbondingpair of electrons -----also occur with compoundshaving π electrons Ex: Carbocation- a species in which oneof its carbonshasonly 6 electrons in its valence shell andcarries a charge of +1 Chapter 4 Page 8 Example 4.4 Thursday, September26, 2013 9:36 AM 4.3 AcidDissociationConstants,pK , and the RelativeStrengths of a Acidsand Bases Thursday,September26, 2013 9:37 AM Any quantitative measure of the acidity of organic acids or bases involves measuring the equilibrium concentrations of the various components in an acid-base equilibrium. • Strength of an acid is expressed by an equilibrium constant • Ex: ○ ○ ○ Water is the solvent for this reaction Acid dissociation constanta- K • The larger the pKa, the weakerthe acid • The smaller the pKa, the stronger the acid • The weaker the acid, the stronger its conjugatebase • The stronger the acid, the weakerits conjugatebase Chapter 4 Page 10 Example 4.5 Thursday, September26, 2013 9:43 AM 4.4 The Position of Equilibrium in Acid-Base Reactions Thursday, September26, 2013 9:44 AM Negative pKa value ---- majorityof moleculesof the acid are dissociated in water • Ex: HCl has pKa = -7 ○ Positive pKa value ----most acid moleculesremainundissociated in water • Ex: acetic acid has pKa of 4.76 ○ Example of the acid-base reaction of acetic acid and ammonia to from acetate ion and ammonium ion: THE MOST IMPORTANT CONCEPTIS AS FOLLOWS: in an acid-base reaction, the position of equilibrium alwaysfavors reaction of the stronger acid and stronger base to form the weakeracid and weaker base. How to Calculate the Equilibrium Constants for Acid-Base Reactions Thursday, September26, 2013 9:56 AM Example 4.6 Thursday, September26, 2013 9:58 AM 4.5 Thermochemistry and Mechanisms of Acid-Base Reactions Thursday, September26, 2013 9:58 AM Reaction mechanism - step-by-step description of how a chemical reaction occurs • Describes which bonds are broken and which bonds are formed Thermochemistry - the study of the energy of the entire system at everyinstant of a reaction Thermal Reactions and Transition States Thursday, September26, 2013 10:00 AM Most chemical reactions occur via COLLISIONS! Thermal reactions - reactions that result by virtueof the kineticenergy put into a reaction vessel due to temperature • During collisions,kinetic energyof the reactants is converted into potential energy ○ The potential energyis stored in the chemical structures in the form of the structural strains  Energy is releasedas the moleculesagainadopt their optimal geometries Transition state - commonly calledactivated complex; the highest energypoint on a reaction coordinate diagram Reaction Coordinate Diagrams and Thermochemistry Thursday, September 26, 2013 10:07 AM Reaction coordinate diagrams - a graph showing the energy charges that occur during a chemical reaction • Vertic
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