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UA CHEM 241A - Acids/Bases
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Chem 241a 1st Edition Lecture2 Outline of Last Lecture I. BondingII. Lewis StructuresIII. VSEPR TheoryIV. Orbital HybridizationOutline of Current Lecture I. Bronsted-LowryII. Factors affecting acids/basesIII. Lewis acids/basesCurrent LectureI. Bronsted-Lowry: came up with a definition of protein acids and basesa. This is a narrow definition that states:b. All reactions (rxns) are proton (H+) transfers. For example:c. When talking about acids and bases, the following sentences all mean the same thing (and their opposites):i. The equilibrium lies to the right (left)ii. Hx (the acid in the reactants) is a stronger (weaker) acid than Hy (the acid in the products)iii. Y (the base in the reactants) is a weaker (stronger) base than X (the base in the products)d. All acid/base rxns proceed towards the weaker acid/base pairi. This relates to thermodynamics and stabilitye. This is expressed by the equilibrium constant, or more specifically, Ka:These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.i. Keq = [A-][H3O+]/[H2O][HA]; thereforeii. Ka = Keq[H2O] = [A-][H3O+]/[HA]iii. However what we are concerned with is the pKa, which equals -logKaiv. When the pKa is large, acid dissociates more  bigger pKa = weaker acidf. The acidic element – atom directly bound to the H+ of an acidi. Ex: H3C-O∷-H  The oxygen is the acidic elementii. If the acidic element contains a positive charge, is becomes more acidic1. H3O+ is more acidic than H2O, etc.g. General trend: when the same acidic element becomes more positive (either from – to 0 or 0 to +), it becomes more acidic as wellh. We can use pKa’s to predict acidities of unknown compounds (cmpds) and directions of reactivity1i. There are four factors that that make a cmpd more or less acidic:II. Factors:a. Element effects (with different acidic elements)- same column or row onlyi. Left to right, acidity increases1. Ex: NH3 is stronger than H2Oii. Conversely, their conjugate bases produce opposite effects1. Ex: OH- is a more stable (stronger) base than NH2iii. Top to bottom acidity increases due to localization of the e- and its bigger radii1. Ex: HI is a stronger acid than HClb. Resonance: always stabilizingi. The acidity of HA is greater when its conjugate base (A-) is resonance stabilizedii. When cmpds have more resonance structures, the charge is delocalized over other nearby atoms and share the negative charge1. Ex:c. Inductive effects: dipoles creates a partially positive or fully positive charge in the molecule that pulls the negative charge away from the acidic element, thus making the acid strongeri. Ex: Cl-CH2-OH vs. H3C-OH1 Refer to Dr. Dollinger’s “Some Important pKa values” to memorize and aid in acids/bases: http://bit.ly/1nBzreeii. The dipole towards the chlorine pulls the negative charge away from the O- element in its conjugate baseiii. When two polar molecules are compared, the more polar molecule (more electronegative) is more acidiciv. Ex: FCH2-OH vs. ClCH2-OHv. F is more electronegative, thus creating a greater dipole and a more acidic elementd. Hybridization: the greater s character creates a more acidic element2i. Ex: CH4 with sp3 (25% s) < H2C=CH2 with sp2 (33% s) < HC≡CH with sp (50% s)III. Lewis Acids/Bases – e- transfer rxnsa. Acid: e- pair acceptorb. Base: e- pair donorc. Lewis base: a molecule that has an available e- pair to donatei. Ex: OH- has three different e- pairs it could donateii. Ex2: CH2CH2 has a double bond it could donated. Lewis Acid: accepts e- pairsi. Ex: BF3 does not have a full octet, so can easily accept e- pairsRemember! There are review sessions Mondays at 3:00pm in Koffler 218 and Dr. Dollinger has office hours Tuesdays and Thursdays from 1:00-2:30 pm or by appointment in Koffler 335! 32 Refer to the Ch. 1 lecture notes for notes on s characters3 Pictures use of


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UA CHEM 241A - Acids/Bases

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