Notes

Outline

Ionic Equilibria :Acids and Bases

Arrenhius Acids and Bases Acid-Substance that produces Hydronium Ions in aqueous solution Examples--HCl, CH3COOH, HNO3 Base-Substance that produces Hydroxide ion in aqueous solution Examples—NH4OH, NaOH, Ca(OH)2 Restrictions Water must be the solvent Acids must be protic Bases must be Hydroxide compounds

Bronsted-Lowry Acids and Bases Acid-Substance that donates protons(H+) Examples—HBr, HCl, H2SO4 Base- Substance that accepts protons Examples—NH3, NaOH, anions Restrictions Acids must be protic

Conjugate Acid-Base Pairing Conjugate Acid-An acid produced by a base accepting a proton Conjugate Base- A base produced by an acid donating a proton HA   +   H2O = H3O +   +       A –   acid      base     conj.acid    conj base H3O + is conj acid of base H2O A – is conj base of acid HA

T-15.3 Bronsted Lowrey Acid Base Reaction

Relative Strengths of Conjugate Acid-Base Pairs The stronger the Acid the weaker the Conjugate Base The stronger the Base the weaker the Conjugate Acid HoAc  +  H2O =  H3O +  +  oAc –    Weak Acid                 Strong Conjugate Base

T-153 Conjugate Acid-Base Pairs and Their Relative Strengths

Determining The Predominate Direction Of An Acid Base Reaction Stronger Acids always lead to weaker  acid Strong Base always leads to a weaker base HoAc  +  H2O =  H3O +  +  oAc – Weaker acid         stronger acid                          Ü

Strong Acids HClO4    Perchloric Acid HClO3    Chloric Acid HI(aq)    Hydroiodic Acid HBr(aq)   Hydrobromic Acid HCl(aq)   Hydrochloric Acid H2SO4      Sulfuric Acid HNO3         Nitric Acid

Complete Ionization of Strong Acids Strong Acids completely ionize in Water HCl   +   H2O --à H3O +(aq)  +  Cl –(aq)     6M                        6M               6M HClO4  +  H2O -à H3O +(aq)  +  ClO4 –(aq)      2M                        2M                  2M

Slight Dissociation of Water 2H2O(l)  = H3O +(aq)   +   OH –(aq) According to the Equilibrium Law Kc = [H3O +] [OH -] / [H2O(l)]2 [H2O(l)]2 is a constant Therefore: Kc [H2O(l)]2 = Kw = [H3O+] [OH-] Kw = 1 X 10 –14 at 25 C

Hydronium Ion Hydronium Ion is a solvated proton In Water solvent it is a hydrated proton H+(aq) = H3O+ = Hydronium Ion It is the major acid species in an aqueous medium

T-15.2 Hydrated Proton

pH Scale pH = 7 Neutral solution (Hydronium ion and Hydroxide ion concentrations are equal) pH < 7   Acidic solution (Hydronium ion concentration is greater than Hydroxide Ion Concentration) pH > 7 Basic Solution (Hydronium Ion Concentration less than Hydroxide Concentration)

Relationship Between pH and Hydronium Ion Concentration pH = -log[H3O+] [H3O+] = Antilog (-pH)

Determining Hydronium and Hydroxide Concentrations using Kw Expression [H3O+] = Kw / [OH-] [OH-] = Kw / [H3O+] Kw = 1 X 10 –14

Determining pH From Hydronium Concentration pH = -log [H3O+]

Determining Hydronium Concentration From pH [H3O+] = Antilog (-pH)

Other “p” Scales pOH = - log [OH -] [OH-] = Antilog (-pOH) pKw = -log Kw = -log 1 X 10 –14 = 14 pKa = -log Ka pKb = -log Kb

Relationship Between pH and pOH Kw = [H3O+] [OH-] Log Kw = log{H3O+]  +  log[OH-] -logKw = -log[H3O+] + -log[OH-] pKw = pH  +  pOH

T-154 pH and pOH of Common Solutions

Two methods Of Determining pH Experimentally pH Meter Acid-Base Chemical Indicator

T-155 pH ranges and colors of Common Indicators

Determining the pH of a Solution of A Strong Acid pH = -log [H3O+] = -log [HA]

Strong Bases Hydroxides of Group 1 and 2 NaOH , Mg(OH)2 Oxides of Metals CaO, K2O Metal Hydrides KH, LiH Metal Nitrides Na3N

Complete Ionization Of Strong Bases NaOH + H2O  -à Na+(aq)  +  OH-(aq) 3M                         3M             3M

Determining the pH of A Strong Bases Solution pOH = -log [OH-] = -log[NaOH] pH = 14 – pOH

Weak Acids Organic carboxylic acids R-COOH HNO2, HC2H3O2, H2SO3, H2CO3, HCN, H2S, HF, H3PO4 Cations derived from weak bases like NH4+ Weak acids ionionize less than 10 % Ka’s less than 1

Ionization Constants of Monoprotic Weak Acids HNO2 = H+  +  NO2 –       Ka = [H+] [NO2-] / [HNO2] HF = H+   +   F-         Ka = [H+] [F-] / [HF]

T-16.1 Ionization Constant Values of Weak Acids at 25 C

Relationship Between Ka and Acid strength The value of the ionization constant rises with acid strength The value of pKa decreases in value with increasing acid strength(See Table of ionization constants)

Determining the Ka from the pH and initial concentration of a weak acid 1.Determine the [H+]eq from the pH [H+]eq = Antilog (-pH) 2. Subtract the [H+] from the given initial concentration of weak acid to get eq conc of weak acid [HA]eq Initial – [H+] = [HA] 3. [H+] = [A-] 4. Ka = [H+] [A-] / [HA]=[H+]2 / Initial – [H+]

Determining the Percent Ionization of a weak acid from the initial concentration and the pH 1. Determine [H+] from pH [H+] = Antilog(-pH) 2. Determine % ionization %ionization = ([H+] / initial conc) x 100 Example

Determination of the pH of a weak acid when given the initial conc and the Ka 1. Let x = [H+] = [A-] 2. Initial conc – x = [HA] 3. Ka = [H+][A-]/ [HA] = (x)(x) / initial conc– x 4. If x<< initial conc Then initial – x = initial conc 5. Ka = (x)(x) / initial conc 6. Solve for x 7. Determine pH pH = -log[H+]= -log(x) Example

Relationship Between Acid Concentration and % ionization As acid concentration decreases %ionization increases More solvent molecules in diluted acid to tear polar acid molecules apart into ions

Poly Protic Acids Each Hydrogen ion  comes off separately Diprotic Acids H2Y--à H+  +  HY- Ka1 = [H+] [HY-] / [H2Y] HY- --à H+   +   Y-2 Ka2 = [H+] [Y-2] / [HY-] Examples H2S, H2CO3, H2SO3

PolyProtic Acids (cont) Triprotic Acids Examples H3PO4, H3AsO4 In a polyprotic acid Ka1>Ka2>Ka3

Weak Bases and Kb NH3  +  H2O = NH4 +   +  OH –   Kb = [NH4+] [OH -] / [NH3] Other examples Anions(conjugate bases of weak acids CN-  +  H2O = HCN   +   OH-    Kb = [HCN] [OH-] / [CN-] Organic Amines CH3-NH2  +  HA = CH3NH3 +  + A-    Kb =[CH3NH3+] [A-] / [CH3NH2] [HA]

Determination of Hydroxide ion concentration when given initial concentration of a weak base and the Kb Example

Determination of the initial concentration of salt when given the volume of solution and the pH of salt solution Example

Relationship Between Ka, Kb and Kw for Acid/conjugate base Pairs Ka Kb = Kw Where Ka =  ionization constant of the weak acid Kb = the ionization constant of the conjugate base Kw = ionization constant Ka = Kw / Kb Kb = Kw / Ka

Determination of Kb for a base Example

pH Of A  A Neutral Salt Solution Salt containing the Cation of a strong base and an anion of a strong acid will form a neutral solution, pH=7 NaCl  +  H2O -à Na+(aq)  +  Cl-(aq)

pH of A Basic Salt Solution Salt containing a Cation of a strong base and an anion of a weak acid will give a basic pH pH>7 NaoAc  +  H2O -à HoAc  +  OH-  +  Na+

pH of An Acidic Salt Solution Salt containing a Cation of a weak base and the anion of a strong acid will form an acidic pH pH<7 NH4Cl  +  H2O -à NH3(aq)  +  H3O+(aq)   +  Cl-(aq)

pH of An Acid/Basic Salt Solution Salt containing a Cation of a weak base and an anion of a weak acid will form a pH depending upon size of Ka for the acid cation and Kb for the base anion NH4CN  +  H2O --à NH3(aq)  +  H3O+(aq)  + HCN(aq)   +  OH-(aq)

Factors Affecting Acid Strength Electronegativity differences Dissociation Energy of H-Y bond Stability of Conjugate Base Inductive Effect of Oxy Acids Resonance Effect of Organic Carboxylic Acids

Electronegativity Effect Electronegativity Effect for Central atoms in the same period HF>H2O>NH3>CH4 HCl>H2S>PH3>SiH4

Dissociation Energy of H-Y Bond Dissociation energy of H-Y bond CHºC-H > CH2=CH-H>CH3-CH2-H

Stability Of The Conjugate Base Stability of Conjugate Base The larger conjugate bases are more stable and originate from stronger acids Relative Stability of conj base I- > Br->Cl->F- Relative Acid Strength HI>HBr>HCl>>HF

Inductive Effect In Oxy Acids Inductive effect occurs through sigma bonding Due to high  electronegativity atoms In oxy acids the more Oxygen atoms in the molecule the greater the inductive effect Results in stronger acids as Protons are more isolated HClO4>HClO3>HClO2>HClO H2SO4>H2SO3 HNO3>HNO2 ROH>RH

Resonance Effect in Organic Carboxylic Acids CH3COOH   = CH3COO-  +  H+ Organic Carboxylic Acids are more acidic compared to alcohols because of resonance of Conjugate Base Carboxylate ion

Lewis Definition of Acid and Base Acid-Electron acceptor Examples-Transition Metal Cations, Boron compounds, Aluminum compounds Base-Electron donor Organic Amines Alcohols