11/14/10. Properties of Acids! CHAPTER 15 Acids and Bases. Table 18.1

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1 11/14/10 CHAPTER 15 Acids and Bases 15-1 Properties of Acids! Sour taste React with active metals i.e., Al, Zn, Fe, but not Cu, Ag, or Au 2 Al + 6 HCl 2 AlCl3 + 3 H2 corrosive React with carbonates, producing CO2 marble, baking soda, chalk, limestone CaCO3 + 2 HCl CaCl2 + CO2 + H2O Change color of vegetable dyes blue litmus turns red React with bases to form ionic salts 15-2 Table

2 Structures of Acids Binary acids have acid hydrogens attached to a nonmetal atom HCl, HF 15-4 Tro, Chemistry: A Molecular Approach, 2/e 4 Structure of Acids Oxy acids have acid hydrogens attached to an oxygen atom H 2 SO 4, HNO Tro, Chemistry: A Molecular Approach, 2/e 5 Carboxylic acids have COOH group HC 2 H 3 O 2, H 3 C 6 H 5 O 7 Only the first H in the formula is acidic the H is on the COOH Structure of Acids 15-6 Tro, Chemistry: A Molecular Approach, 2/e 6 2

3 Properties of Bases Also known as alkalis Taste bitter alkaloids = plant product that is alkaline often poisonous Solutions feel slippery Change color of vegetable dyes different color than acid red litmus turns blue React with acids to form ionic salts neutralization 15-7 Tro, Chemistry: A Molecular Approach, 2/e 7 Common Bases 15-8 Tro, Chemistry: A Molecular Approach, 2/e 8 Structure of Bases Most ionic bases contain OH ions NaOH, Ca(OH) 2 Some contain CO 3 2 ions CaCO 3 NaHCO 3 Molecular bases contain structures that react with H + mostly amine groups 15-9 Tro, Chemistry: A Molecular Approach, 2/e 9 3

4 Arrhenius Theory Bases dissociate in water to produce OH ions and cations ionic substances dissociate in water NaOH(aq) Na + (aq) + OH (aq) Acids ionize in water to produce H + ions and anions because molecular acids are not made of ions, they cannot dissociate they must be pulled apart, or ionized, by the water HCl(aq) H + (aq) + Cl (aq) in formula, ionizable H written in front HC 2 H 3 O 2 (aq) H + (aq) + C 2 H 3 O 2 (aq) Tro, Chemistry: A Molecular Approach, 2/e 10 Arrhenius Theory HCl ionizes in water, producing H + and Cl ions Tro, Chemistry: A Molecular Approach, 2/e NaOH dissociates in water, producing Na + and OH ions 11 Hydronium Ion The H + ions produced by the acid are so reactive they cannot exist in water H + ions are protons!! Instead, they react with water molecules to produce complex ions, mainly hydronium ion, H 3 O + H + + H 2 O H 3 O + there are also minor amounts of H + with multiple water molecules, H(H 2 O) + n Tro, Chemistry: A Molecular Approach, 2/e 12 4

5 Arrhenius Acid Base Reactions The H + from the acid combines with the OH from the base to make a molecule of H 2 O it is often helpful to think of H 2 O as H-OH The cation from the base combines with the anion from the acid to make a salt acid + base salt + water HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) Tro, Chemistry: A Molecular Approach, 2/e 13 Typical Arrhenius Acids, Bases An acid is a substance that has H in its formula and dissociates in water to yield H + (H 3 O + ). HX + H 2 O H 3 O + + X - A base is a substance that has OH in its formula and dissociates in water to yield OH -. YOH OH - + Y Acid-Base Reactions: Neutralization When an acid and a base react, the H + ion from the acid reacts with the OH - ion from the base to form water and a salt. HCl + KOH H 2 O + KCl When the acid and base are strong, the reaction is complete and fully go to the products; thus, all strong a/b reactions are similar

6 Figure 18.1 Strong Acids Strong acid: HA(g or l) + H 2 O(l) H 3 O + (aq) + A - (aq) 100% dissociation: [H 3 O + ] ~ [HA] initial HNO 3 is a Strong Acid Figure 18.2 Weak Acids. Weak acid: HA(aq) + H 2 O(l) K a = [ H 3O + ][A ] [ HA] H 3 O + (aq) + A - (aq) K a <<

7 Acetic Acid is A Weak Acid K a and Acids Stronger acid higher [H 3 O + ] larger K a K a % Dissoc. Acid (1M) [H 3 O + ] ~10-2 ~10% HClO ~10-5 ~0.3% CH 3 COOH ~10-10 ~0.0025% HCN ACID STRENGTH

8 Some Guidelines Strong Acids: 1. Hydrohalic acids: HCl, HBr, HI (not HF) 2. Oxoacids: HNO 3, H 2 SO 4, HClO 4. Weak Acids: 1. HF 2. H is not bonded to O or a halogen : HCN, H 2 S. 3. Oxoacids where #O =, or exceeds by 1, the number of inoizable protons: H 3 PO 4, HNO 2, HClO. 4. Carboxylic acids (RCOOH) Strong Base 100% dissociation: [OH - ] ~ [YOH] initial Weak Base

9 Some Guidelines Strong Bases: 1. Group IA oxides or hydroxides (M 2 O, MOH). 2. Oxides (MO) or hydroxides (M(OH) 2 ) of Ca, Sr, Ba. Weak Bases: 1. NH 3 2. Amines in general (RNH 2, R 2 NH, R 3 N): CH 3 Ch 2 NH 2, (CH 3 ) 2 NH, (C 3 H 7 ) 3 N. 3. Note all the above has an N with a lone electron pair Autoionization of Water and the ph Scale + H 2 O(l) H 2 O(l) H 3 O + (aq) OH - (aq) Autoionization of Water and K w H 2 O(l) + H 2 O(l) K c = H 3 O + (aq) + OH - (aq) [H 3 O + ][OH - ] [H 2 O] 2 The Ion-Product Constant for Water K c [H 2 O] 2 = K [H 3 O + ][OH - w = ] = 1.0 x at 25 o C A change in [H 3 O + ] causes an inverse change in [OH - ] In an acidic solution, [H 3 O + ] > [OH - ] In a basic solution, [H 3 O + ] < [OH - ] In a neutral solution, [H 3 O + ] = [OH - ] 9

10 Figure 18.4 The relationship between [H 3 O + ] and [OH - ] and the relative acidity of solutions. [H 3 O + ] Divide into K [OH - w ] [H 3 O + ] > [OH - ] [H 3 O + ] = [OH - ] [H 3 O + ] < [OH - ] ACIDIC SOLUTION NEUTRAL SOLUTION BASIC SOLUTION Figure 18.5 The ph values of some familiar aqueous solutions. ph = -log [H 3 O + ] Table 18.3 The Relationship Between K a and pk a Acid Name (Formula) K a at 25 o C pk a Hydrogen sulfate ion (HSO 4- ) 1.0x10-2 Nitrous acid (HNO 2 ) 7.1x Acetic acid (CH 3 COOH) Hypobromous acid (HBrO) Phenol (C 6 H 5 OH) 1.8x x x

11 Figure 18.6 The relations among [H 3 O + ], ph, [OH - ], and poh Figure 18.7 Methods for measuring the ph of an aqueous solution. ph (indicator) paper ph meter Figure 18.8 Proton transfer as the essential feature of a Brønsted-Lowry acid-base reaction. Lone pair binds H + (acid, H + donor) (base, H + acceptor) Lone pair binds H + (base, H + acceptor) (acid, H + donor)

12 Brønsted-Lowry Acid-Base Definition An acid is a proton donor, any species which donates a H +. A base is a proton acceptor, any species which accepts a H +. An acid-base reaction can now be viewed from the standpoint of the reactants AND the products. An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair Table 18.4 The Conjugate Pairs in Some Acid-Base Reactions Conjugate Pair Acid + Base Base + Acid Conjugate Pair Reaction 1 HF + H 2 O F - + H 3 O + Reaction 2 HCOOH + CN - HCOO - + HCN Reaction 3 NH CO 2-3 NH 3 + HCO - 3 Reaction 4 H 2 PO OH - HPO H 2 O Reaction 5 H 2 SO 4 + N 2 H + 5 Reaction 6 HPO SO 2-3 HSO N 2 H 2+ 6 PO HSO Sample Problem 18.5 PROBLEM: Predict the net direction and whether K c is greater or less than 1 for each of the following reactions (assume equal initial concentrations of all species): (a) H 2 PO 4- (aq) + NH 3 (aq) (b) H 2 O(l) + HS - (aq) Predicting the Net Direction of an Acid-Base Reaction HPO 4 2- (aq) + NH 4+ (aq) OH - (aq) + H 2 S(aq) PLAN: Identify the conjugate acid-base pairs and then consult Figure 18.9 (button) to determine the relative strength of each. The stronger the species, the more preponderant its conjugate. SOLUTION: (a) H 2 PO 4- (aq) + NH 3 (aq) HPO 2-4 (aq) + NH 4+ (aq) stronger acid stronger base weaker base weaker acid Net direction is to the right with K c > 1. (b) H 2 O(l) + HS - (aq) OH - (aq) + H 2 S(aq) weaker acid weaker base stronger base stronger acid Net direction is to the left with K c < 1. 12

13 Figure 18.9 Strengths of conjugate acid-base pairs Calculating [H 3 O + ] and [OH - ]: ICE Approach Using or Finding Ka of Weak Acids and Bases Given Ka, find equilibrium concentration Given equilibrium concentration, find Ka Note: Ka is for the following reaction: AH + H 2 O H 3 O + + A - If you are given Ka for a base, then you must convert to Kb before using it for the reaction B + H 2 O OH - + BH

14 15-40 K a = 1.8 x 10-5 = [H 3 O+ ][OAc - ] = [HOAc] x x Approximating K a = 1.8 x 10-5 = x x = [H 3 O + ] = [K a 1.00] 1/

15 15-43 Example of Non-Compliant Condition K b = 1.8 x 10-5 = [NH 4 + ][OH - ] [NH 3 ] = x x

16 Strong Acid and Strong Base Mixing equal molar quantities of a strong acid and strong base produces a neutral solution Strong Acid and Weak Base Mixing equal molar quantities of a strong acid and weak base produces the bases s conjugate acid. The solution is acid. Strong Base and Weak Acid Mixing equal molar quantities of a weak acid and strong base produces the acid s conjugate base. The solution is basic. 16

17 Weak Acid and Weak Base Summary of Acid and Base Reactions Percent HA dissociation = [HA] dissociated [HA] initial x 100 H 3 PO 4 (aq) + H 2 O(l) H 2 PO 4- (aq) + H 2 O(l) HPO 4 2- (aq) + H 2 O(l) Polyprotic acids acids with more than one ionizable proton H 2 PO 4- (aq) + H 3 O + (aq) HPO 4 2- (aq) + H 3 O + (aq) PO 4 3- (aq) + H 3 O + (aq) K a1 > K a2 > K a3 K a1 = [H 3O + ][H 2 PO 4- ] [H 3 PO 4 ] = 7.2x10-3 K a2 = [H 3O + ][HPO 4 2- ] [H 2 PO 4- ] = 6.3x10-8 K a3 = [H 3O + ][PO 3-4 ] [HPO 2-4 ] = 4.2x

18 ACID STRENGTH K b = [BH + ][OH - ] [B] BASE STRENGTH Relationship Between Ka and Kb Acid HA + H 2 O H 3 O + + A - K a = [ H 3O + ][A ] [ HA] Conjugate Base A - + H 2 O OH - + HA K b = [ HA][OH ] [A ] K a K b = K w

19 B + H 2 O BH + + OH - K b = (BH+ )(OH ) (B) Since Kw = KaKb or Ka = Kw/Kb or Ka = Kw/Kb Conjugate acid to B BH + + H 2 O K a = (H + )(B) (BH + ) H 3 O + + B Figure The effect of atomic and molecular properties on nonmetal hydride acidity. Bond strength decreases, acidity increases 6A(16) H 2 O H 2 S H 2 Se 7A(17) HF HCl HBr Electronegativity increases, acidity increases H 2 Te HI Figure The relative strengths of oxoacids. H O I < H O Br < H O Cl δ + δ δ + δ δ + δ O H O Cl << H O Cl δ + δ δ + δ O O

20 Table 18.7 K a Values of Some Hydrated Metal Ions at 25 o C Free Ion Hydrated Ion K a Fe 3+ Fe(H 2 O) 3+ 6 (aq) 6 x 10-3 Sn 2+ Sn(H 2 O) 2+ 6 (aq) 4 x 10-4 Cr 3+ Cr(H 2 O) 3+ 6 (aq) 1 x 10-4 Al 3+ Al(H 2 O) 3+ 6 (aq) 1 x 10-5 Cu 2+ Cu(H 2 O) 2+ 6 (aq) 3 x 10-8 Pb 2+ Pb(H 2 O) 2+ 6 (aq) 3 x 10-8 Zn 2+ Zn(H 2 O) 2+ 6 (aq) 1 x 10-9 Co 2+ Co(H 2 O) 2+ 6 (aq) 2 x ACID STRENGTH Ni 2+ Ni(H 2 O) 6 2+ (aq) 1 x Figure The acidic behavior of the hydrated Al 3+ ion. Electron density drawn toward Al 3+ Nearby H 2 O acts as base H 2 O H 3 O + Al(H 2 O) 6 3+ Al(H 2 O) 5 OH

21 Molecules as Lewis Acids An acid is an electron-pair acceptor. A base is an electron-pair donor. F F B F + N H H H F F B F N H H H acid base adduct Figure The Mg 2+ ion as a Lewis acid in the chlorophyll molecule