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Lecture 8

CHEM 1A03 Lecture 8: Acid Base Chemistry (Chapter 16).docx

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Jeff Landry

Acid-Base Chemistry - Early qualitative tests to distinguish acids and bases by taste - Focus on fundamental acid-base chemistry to describe their reactivity in the context of energy, health, and environment - Acidic: sour (coke, phosphoric acid), alkaline: bitter (cocoal leaf, cocaine) Electrolyte Theory Arrhenius Theory - Electrolyte dissolution behaviour + - - Acids ionize to generate proton  H (aq) whereas bases ionize to produce hydroxide  OH (aq) - Strong Acid HCl (aq)  H+ (aq) + Cl- (aq) - Strong Base NaOH (aq)  Na+ (aq) + OH- (aq) - Theory applicable to protic acid & metal hydroxides in water, however many exceptions  BF 3 and NH 3 - Neutralization reaction: Net Ionic Equation H (aq) + OH(aq)  H2O(l) Bronsted-Lowry Theory - Acid = H+ donor, base = H+ acceptor - Acid/base reactants & conjugate acid/base products: - - H2O is an amphoteric species (behaves as both an acid or a base) - HCl is not an acid until it dissociates; NH3 is not basic… until it dissociates to give you H+ or OH- - Not a base or acid until its dissociated to make those ions - F- + H2O  HF + OH- : BASIC because HF has not yet dissociated into H+ - Strong acid / base goes to completion; weak acid / base does not go to completion, equilibrium arrows used in equations that don’t go to completion - Single arrow indicates strong acid/base Lewis Theory - Acid = e- pair acceptor, base = e- pair donor; an electron pair is shared - Least restrictive definition that describes aqueous/non-aqueous solvents, as well as unusual non-protic acid systems - - Iclicker 1: bronsted lowry acid in reaction: C) HCO3- Electron Flow - Conjugate base always has one less proton, conjugate acid always has one more proton than its corresponding bases/acids - HCl – acid; H2O – base; Cl – conjugate base; H3O – conjugate acid Equilibrium: Weak Acid & Ka - Acetic acid partially dissociates, gives us some conjugate base and some conjugate acid; Ka is the proportional equilibrium constant with respect to an acid reaction - Ka is for an acidic reaction: HA + H2O  A- + H3O, defined for this equilibrium - Ka = [conjugate base][H3O+]/[reactant acid] - Ka<1 favour reactants over products - Strong acids favour products, weak acids favour reactants - Acidity of H3O+ > CH3COOH, basicity of CH3COO- > H2O Equilibrium: Weak Base & Kb - B + H2O  HB+ + OH- - Kb governs this basic reaction - The larger Kb, the more product there is; the more products, the more OH-, the more basic - Relatively stronger the base, the higher the Kb value - Equilibrium favours reverse process since Kb<1 - Basicity of OH- > NH3 - Acidity of NH4+ > H2O Auto-ionization of Water & pH Scale - Water is amphoteric and undergoes acid-base reaction with itself - Auto-ionization with defined equilibrium forms the basis of pH scale - - Aqueous solution – [H3O+][OH-] ALWAYS = 1.0 x 10^-14 - Wide concentration range of H3O+/OH-  use log10 scale - - -log(x) = p(x) - Iclicker 2: what is the [H3O+] in a solution with a pH of 3.25? d) 10^-pH Strong Acids and Bases - Strong acids to know (6): o HCl, HBr, HI, HClO4, HNO3, H2SO4 - Strong bases to know: o Group I & Group II hydroxides (many Group II partially soluble) - o Hydrides (H) o Oxides (O ) (if you put an oxide in water, O2- + H2O  2OH-) o CH3- is a very strong base b/c CH4 is a really weak acid and is stable? o NH3 – base and wants to accept a proton, easy to make NH2- Weak Acids / Bases - Weak acids / bases undergo partial reversible ionization in H2O - Extent of equilibrium ionization described by magnitude of Ka or Kb - For acid: smaller pKa = more acidic (pKa = -log(Ka) - For base: smaller pKb = more basic (pKb = -log(Kb) - Many organic molecuels have weakly acidic carboxylic acid groups - Many weak bases are centered around N eg. NH3, CH3NH2, (CH3CH2)2NH pKa & pKb Relationship for Conjugate Pairs - Take an acid and add it to water - Conjugate base from rxn 1 will only react with H3O (conjugate acid) to reverse equilibrium - Take salt of rxn 1 (conjugate base) and throw it in water will now act as a base, react with water – remove a proton, give you a conjugate acid and OH- - For any acid/base conjugate pair, Ka*Kb = Kw
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