Dr. Fred Omega Garces Chemistry 201 Miramar College 16.3 Acids - Bases i. Characteristics of Acid-Base ii. Salt Solution Acids-Base in solution and structural features of acids Dr. Fred Omega Garces Chemistry 201 Miramar College

Acid-Base Strength Factors Affecting Acid Strength Polarity: A molecule will transfer H+ only if the H-X bond is polarized towards the X atom. Bond Strength: Weak H-X bonds will lead to greater dissociation; stronger acid. Stability of conjugate base X-: Greater stability of X-, the stronger the acid. Solvation of ions: The extent in which the ions are stabilized by water, leads to a stronger acid. Acid Strength involves the breaking of H-A and the formation of H-OH2. Factors that affect the ease with which bonds are broken and formed will influence the strength of the acid. A molecule will transfer H+ only if the H-X bond is polarized towards the X atom. Weak H-X bonds will lead to greater dissociation; stronger acid. Greater stability of X-, the stronger the acid. The extent in which the ions are stabilized by water, leads to a stronger acid.

I. Acid-Base Strength: Binary Hydrides Bond Strength; Stronger the bond strength the weaker the acid character. 1. Size increase of X, leads to longer and weaker H-X bond, so H+ is release more easily. (predominantly down PT) 2. EN of X: As X become more electronegative, the H-X becomes more polar, and H+ is more easily release. (predominantly across PT) H-F H-Cl H-Br H-I 569 kJ/mol 431 kJ/mol 368 kJ/mol 297 kJ/mol Bond Strength from atomic radius (orbital overlap) Na-H CH4 NH3 HF 1.0 2.5 3.1 4.1 Electro-negativity

I. Acid-Base Strength: Binary Hydrides Bond Strength; Stronger the bond strength the weaker the acid character. 1. Size increase of X, leads to longer and weaker H-X bond, so H+ is release more easily. (predominantly down PT) 2. EN of X: As X become more electronegative, the H-X becomes more polar, and H+ is more easily release. (predominantly across PT) Bond Strength from atomic radius (orbital overlap) H-F H-Cl H-Br H-I 569 kJ/mol 431 kJ/mol 368 kJ/mol 297 kJ/mol Increasing atomic size, Increasing acid strength Electro-negativity Na-H CH4 NH3 HF 1.0 2.5 3.1 4.1 Increasing EN, Increasing Acid Strength

Acid-Base Strength: Binary Hydrides Trend of Acidity for Binary compounds Bond Strength: Atomic size (Down PT) Bond polarity: Electronegative (across PT)

II. Acid-Base Strength: Oxyacids Oxyacids HOXOn: HNO3 , H2SO4 , H3PO4 Compounds with hydrogen, oxygen and third element Hypo-halous acid, HOCl, HOBr, HOI 1. Size: (No effect since H is bonded to O H-O-I H-O-Br H-O-Cl 2. Electro negativity H-O-I H-O-Br H-O-Cl 3. Number of oxygen atom H-O-Cl H-O-ClO H-O-ClO2 H-O-ClO3

II. Acid-Base Strength: Oxyacids Oxyacids HOXOn- HNO3 , H2SO4 , H3PO4 Compounds with hydrogen, oxygen and third element Hypo-halous acid, HOCl, HOBr, HOI 1. Size: (No effect since H is bonded to O Increasing Acid Strength H-O-I H-O-Br H-O-Cl Ka 2.3•10-11 2•10-9 3•10-8 Increasing Acid Strength 2. Electro negativity H-O-I H-O-Br H-O-Cl Increasing Acid Strength 3. Number of oxygen atom H-O-Cl H-O-ClO H-O-ClO2 H-O-ClO3

2. Acid-Base Strength: Oxyacids and EN Variation of the third element: Weakening of H-O bond is a result of the electronegativity of the third atom. (Large Atoms are less likely to draw electron density on to itself)

2. Acid-Base Strength: Oxyacids and EN (Mechanism) Variation of the third element Mechanism: The greater the electro-negativity (EN) of the X atom, the more the O-atom has to compensate for the e- density loss by removing e- density from H-atom thereby weakening the H-O bond. In general, acidity increases as electronegativity of the third element increases.

2. Acid-Base Strength: Oxyacids and Electronegativity (Example) Variation on the identity of third atom Electronegativity Which is the stronger acid ? H2SO4 because EN (S) > EN (Se).

3. Acid-Base Strength: Oxyacids and Variation of Oxygen Atoms Variation on the number of oxygen Polarity Which is a stronger acid in terms of polarity ? e- density is attracted to oxygen to a greater extent in H2SO4 compared to H2SO3. This weakens the H-O bond to a greater extent.

3. Acid-Base Strength: Oxyacids Variation on the number of oxygen Stability of conjugate (resonance energy) Hypochlorous acid HOCl (OH)Cl n=0 pKa= 7.5 chlorous acid HClO2 (OH)ClO n=1 pKa= 2.0 chloric acid HClO3 (OH)ClO2 n=2 pKa= S.A. perchloric acid HClO4 (OH)ClO3 n=3 pKa= -10

Acid-Base Strength: Organic Acids Carboxylic acids are the largest category of organic acids. These acids contain the carboxyl group (blue). Carboxylic group contains the acidic proton on the O-H group and additional O withdrawing electron density on the carbon Examples: Acetic Acid Formic Acid Benzoic acid pka = 4.76 3.74 4.20

Acid-Base Strength: Organic Acids Factors affecting acid behavior: 1. Additional oxygen on C result in polarizing O-H bond. This also stabilize the conjugate base. Acetic acid vs methanol 2. Strength of these acids result from the stability of their anions through resonance. In addition, increasing the number of electronegative atoms in the acid increases acid character, trifluoric acetic acid Ka = 5.0 •10-1 Super acids have negative pH.

Summary of Acid Strength Acid Type Trend Binary 1. The more polar the H-X bond, the stronger the acid. This effect is dominant for acids of the same period. 2. The weaker the H-X bond, the stronger the acid. This effect is dominant for acids of the same group. Oxyacid 1. The greater the number of O atoms attracted to the central atom (the greater the oxidation number of the central atom), the stronger the acid. 2. For the same number of O atoms attached to the central atom, then the greater the electronegativity of the central atom, the stronger the acid. Carboxylic 1. The greater the electronegativity of the groups attached (Organic) to the carboxyl group, the stronger the acid.

Acid-Base and Salt solutions Conjugates: Strong Acid - Base CA  C+ + A - (100%) Weak Acids - Base CA  C+ + A - < (100%) A- is a base (proton acceptor) A- + H2O  AH + OH- C+ is an acid (proton donor) C+ + H2O  *C + H3O+ C = NH4+ or polyatomic with H in formula i.e., H2PO3-2, HSO4-, HCO3- (i) Salts from Strong base and strong acid: Consider NaOH and HCl  the salt NaCl no pH = 7 (ii) Salts from strong base and weak acid: Consider NaOH and HClO  the salt NaClO pH > 7 (iii) Salts from weak base and a strong acid: Consider NH4Cland HNO3  the salt NH4NO3 pH < 7 (iv) Salts from a weak base and a weak acid: Consider NH4Cl and HClO  the salt NH4ClO pH = ?

Acid-Base Properties of Salt Solution • Neutral: No affect of pH Salts form from strong acids and Bases Na from NaOH and Cl from HCl form NaCl Ca from Ca(OH)2 and NO3 from HNO3 for Ca(NO3)2 ______________________________________ • Basic: pH above 7 Salts are from strong base and weak acid Na from NaOH and ClO- form HClO form NaClO Ba from from Ba(OH)2 and C2H3O from HC2H3O2 form Ba(C2H3O2)2 • Acidic: pH below 7 Salts from a weak base and strong acid NH4+ from NH3 and Cl- from HCl form NH4Cl Al+3 from AlCl3 and NO3- from HNO3 form Al(NO3)3 • Acidic or Basic: Hydrolysis Calculation Salts from weak base and weak acids NH4C2H3O2 from NH4+ and HC2H3O2 NH4CN from NH4+ and HCN FeCO3 from Fe+2 and H2CO3

Hydrolysis: Salts from weak base and/or weak acids What is the pH of a salt solution when 37 g of NH4F is dissolved in 0.50L of water. 37 g of NH4F is dissolved in 0.50L is 2.0M solution of NH4F

Hydrolysis of Amphoteric substance What is the [H3O+],[OH-],[NH3],[HCO3-] and pH of a salt solution when 3.95 g of NH4HCO3 is dissolved in 1.00L of water. Strategy: Calculate the H3O+ from NH4+, H3O+ from HCO3- and the OH- from HCO3-. These must then be added to determine which is excess in order to determine the pH of the solution. Solve for H3O+ based on dissociation of NH4+. Solve for H3O+ based dissociation of HCO3- Solve for OH- based association of HCO3-

Generalizing Bronsted-Lowry Concept: Leveling Effect When dealing with Bronsted-Lowry Acid-Base systems, all reactions taking place in water with acid systems yield H3O+, where as base systems yield OH-. The question that comes to mind is- “Why are strong acids and strong bases equally strong in water?” Consider putting HCl or HNO3 in water, as soon as it goes into water, H3O+ forms. In fact all strong acid dissociate completely and react with water to form H3O+, The strong acid, i.e., HCl and HNO3 no longer exist. Any strong acid considered will simply donate it’s proton to H2O and form H3O+. The same is true for strong base, any strong base will accept a proton from H2O to form OH-. In water, the strongest acid is H3O+ and in base the strongest base is OH-. This phenomena is called the Leveling effect. Water equalizes (level) the strength of all strong acids by reacting with the acid to form products of water’s auto-ionization. To rank strong acids in terms of relative strength require using a different solvent system. i.e., dissolve HCl, HNO3 in acetic acid.

Lewis Acids-Bases Acid: Electron pair acceptor / Base: Electron pair donor Lewis Base, specie with lone pair electron Lewis Acid - Specie with empty valence orbital i) Reaction between ammonia and boron trihydride Lewis Base Lewis Acid Coordinate covalent bond ii) Metal ion and ligand; Cu(NH3)42+ : a) Cu+2 ion is electron acceptor: acid / NH3 ion is electron donor: base 2+ b) H2O + CO2  H2CO3: CO2 is the Lewis Acid, H2O is the Lewis base

Lewis Acid/Base: Metal Hydrolysis iii) Metal ion hydrolysis: High positive small cations attracts polar water. The ion polarizes the O-H bond which weakens the O-H bond in water. (Stronger Lewis acid) Fe(H2O)63+ (aq)  Fe(H2O)5(OH)2+ (aq) + H+ (aq) Be(H2O)42+ (aq)  Be(H2O)3(OH)2+ (aq) + H+ (aq) Al(H2O)63+ (aq)  Al(H2O)5(OH)2+ (aq) + H+ (aq)