Chapter 14: Acids and Bases

Chapter 14: Acids and Bases 14.1 The Nature of Acids and Bases Bronsted-Lowry Acid-Base Systems Bronsted acid: proton donor Bronsted base: proton acceptor Bronsted acid base reaction: proton transfer from acid to base HA + B A - + BH + HF + H 2 O F - + H 3 O + this reaction is an equilibrium (HF is a weak acid, reacts less than 100%) HF is the acid, H 2 O is the base. Acid-Base Conjugate Pairs The reverse reaction is also a proton transfer, from H 3 O + (acid) to A -, (base). HF and F - make up a conjugate acid-base pair. F - is the conjugate base of HF. conjugate base: the species left behind when a Bronsted acid donates a proton. H 2 O and H 3 O + make up a conjugate acid-base pair. H 3 O + is the conjugate acid of H 2 O. conjugate acid: the species formed when a Bronsted base accepts a proton. H 3 O + + OH - H2 O + H 2 O H 3 O + and H 2 O are a conjugate acid-base pair; H 2 O and OH - are a conjugate acidbase pair In water, a substance is considered an acid if it undergoes this reaction HA + H 2 O A - + H 3 O + In water, a substance is considered a base if it undergoes this reaction B + H 2 O BH + + OH - 14-1

Neutralization In the Arrhenius concept of acids and bases, HA + BOH BA (a salt) + H 2 O In B-L, HA + B A - + BH +, where A - is the conjugate base of HA and BH + is the conjugate acid of B L-B reactions proceed in the direction of the weaker acid and base stronger acid + stronger base weaker conjugate base + weaker conjugate acid however, the solution is not necessarily neutral at the endpoint. 14.2 Acid Strength Strong acids dissociate 100% in water HA + H 2 O 100% A - + H 3 O + The common strong acids are HCl, HBr, HI, HNO 3, HClO 4, H 2 SO 4 Weak acids dissociate less than 100% in water. HF H 3 O + + F - Ka = [H 3O + ][F - ] [HF] = 7.1 x 10-4 Leveling Effect HA + B A - + BH + the extent to which this reaction proceeds to the right depends both on how strong an acid HA is and on how strong a base B is. A weaker B can make HA appear stronger; stronger B can make HA appear weaker. When B is water, all acids stronger than H 3 O + appear to be equally strong. By using a weak acid (weaker base than water) we can differentiate between the strong acids: HA + HC 2 H 3 O 2 A - + H 2 C 2 H 3 O 2 + This gives the following order: HNO 3 < H 2 SO 4 < HCl < HBr < HI < HClO 4 Monoprotic having one acidic hydrogen Diprotic having two acidic hydrogens Polyprotic having more than one acidic hydrogens 14-2

Relative Base Strength The strong bases are the soluble metal oxides and hydroxides NaOH (s) Na + (aq) + OH - (aq) Na 2 O (s) + H 2 O (l) 2Na + (aq) + 2OH - (aq) OH - is the strongest base that can exist in water O -2 + H 2 O OH - + OH - PH 2 - + H2 O PH 3 + OH - strong acid <> weak conjugate base weak acid <> strong conjugate base Weak bases react with water to form OH + by accepting a proton from water. Most weak bases are amines (p. 661-662) B + H 2 O B + + OH - K b = [B+ ][OH - ] [B] Weak bases react with water less than 100% to form OH -. NH 3 + H 2 O NH 4 + + OH - K b = [NH 4 + ][OH - ] [NH 3 ] = 1.8 x 10-5 Autoionization of Water amphoteric: can act as either an acid or a base Autoionization of Water H 2 O + H 2 O H 3 O + + OH - proton transfer from one water molecule to another K c = [H 3O + ][OH - ] [H 2 O] 2 but [H 2 O] is a constant, so define K w K w = [H 3 O + ][OH - ] = 1 x 10-14 (at 25 ) 14-3

In neutral solutions, [H 3 O + ] = [OH - ] = 1 x 10-7 In acidic solutions, [H 3 O + ] > [OH - ] > 1 x 10-7 In basic solutions, [H 3 O + ] < [OH - ] < 1 x 10-7 [H 3 O + ] may range from values as high as 20 to as low as 1 x 10-15 14.3 The ph Scale ph = -log[h 3 O + ] In neutral solutions, ph = -log(1 x 10-7 ) = 7 In acidic solutions, ph < 7 In basic solutions, ph > 7 (number of decimal places in log = number of significant figures in number) we also define poh = -log[oh - ] and pk = -log K then K w = [H 3 O + ][OH - ] = 1 x 10-14 gives pk w = 14 = ph + poh 14.4 Calculating the ph of Strong Acid Solutions Calculate the ph of a 0.050 M HCl solution HCl is a strong acid, so [H 3 O + ] = 0.050 M ph = -log[h 3 O + ] = -log(0.050) = 1.30 What is the concentration of a solution of HCl if the ph is 3.75? [H 3 O + ] = 10 -ph = 10-3.75 = 1.78 x 10-4 M 14.5 Calculating the ph of Weak Acid Solutions Weak acids react with water less than 100% to form H 3 O +. HF + H 2 O F - + H 3 O + 14-4

K a = [H 3O + ][F - ] [HF] = 7.1 x 10-4 (see table 13.6, pg 522) Strong acids ionize 100% in water: HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4 Most other acids are weak Determination of K a for weak acids The ph of a 0.100 M solution of a weak acid is 4.26. Calculate K a for the acid. [H 3 O + ] = 10 -ph = 10-4.26 = 5.48 x 10-5 HA + H 2 O H 3 O + + A - K a = [H 3O + ][A - ] [HA] init 0.100 0 0 change -x x x eq 0.100-x x x 0.100 1.73 x 10-4 1.73 x 10-4 K a = (5.48 x 10-5 )(5.48 x 10-5 ) (0.100) = 3.0 x 10-8 Determination of ph for solutions of weak acids Calculate the ph of a 0.050 M HC 2 H 3 O 2 solution K a = 1.8 x 10-5 HC 2 H 3 O 2 + H 2 O H 3 O + - + C 2 H 3 O 2 init 0.050 0 0 change -x x x eq 0.050-x x x K a = 1.8 x 10-5 K a = 1.8 x 10-5 = (x)(x) (0.050-x) x 2 (0.050) x 2 = 9.0 x 10-7 x = 9.49 x 10-4 ph = 3.02 calculate equilibrium concentrations and ph for 0.050 M HF HF + H 2 O H 3 O + + F - K a = 7.1 x 10-4 init 0.050 0 0 change -x x x eq 0.050-x x x 14-5

K a = 7.1 x 10-4 = (x)(x) (0.050-x) x 2 + 7.1 x 10-4 x - 3.55 x 10-5 = 0 x = -7.1 x 10-4 ± (7.1 x 10-4 ) 2-4(-3.55 x 10-5 ) 2 ph = -log(5.61 x 10-3 ) = 2.25 = 5.61 x 10-3 As long as [H 3 O + ] > 4.5 x 10-7 (ph <6.3) we can ignore autoionization of water. The ph of a Mixture of Weak Acids: if one weak acid is much stronger than the others, the dissociation of the weaker acids can be ignored. Percent dissociation % ionization = eq. [H 3O + ] init [HA] x 100% Calculate % ionization for 0.5000 and 0.0500 M HCOOH at 0.5000 M, K a = 1.7 x 10-4 = [H 3O + ][HCOO - ] x 2 = [HCOOH] 0.5000-x assume x<<0.5000, then 1.7 x 10-4 = % ionization = 0.0092 0.5000 x 100% = 1.8% x 2 0.5000 and x = 0.0092 at 0.0500 M, K a = 1.7 x 10-4 = [H 3O + ][HCOO - ] x 2 = [HCOOH] 0.0500-x assume x<<0.0500, then 1.7 x 10-4 x 2 = and x = 0.0029 0.0500 % ionization = 0.0029 x 100% = 5.8% too big. Either solve quadratic or reiterate. 0.0500 1.7 x 10-4 x 2 0.0028 = x = 0.0028, % ionization = x 100% = 5.7% 0.0500-0.0029 0.0500 by quadratic; 8.5 x 10-6 - 1.7 x 10-4 x = x 2 x 2 +1.7 x 10-4 x -8.5 x 10-6 = 0 x = -1.7 x 10-4 ± (1.7 x 10-4 ) 2-4(1)(-8.5 x 10-6 ) 2 Calculating Ka from Percent Dissociation = 2.83 x 10-3 14-6

A 0.0050 M solution of HCN is shown to be 0.038% dissociated. Calculate Ka for HCN. HCN + H 2 O H 3 O + + CN - K a = 1.9 x 10-6 init 0.0050 0 0 change -1.9 x 10-6 1.9 x 10-6 1.9 x 10-6 eq 0.0050 1.9 x 10-6 1.9 x 10-6 K a = (1.9 x 10-6 )( 1.9 x 10-6 ) (0.0050) = 7.2 x 10-10 14.6 Bases The ph of Strong Bases Calculate the ph of a 0.035 M NaOH solution [OH - ] = 0.050 M poh = -log[oh - ] = -log(0.035) = 1.46 ph = 14 - poh = 12.54 The ph of Weak Bases Calculate the ph of a solution of.550 M methylamine (CH 3 NH 2 ) K b = 4.4 x 10-4 = [CH 3NH 3 + ][OH - ] [CH 3 NH 2 ] x 2 + 4.4 x 10-4 x - 2.42 x 10-4 = 0 = x 2 0.550-x x = -4.4 x 10-4 ± (4.4 x 10-4 ) 2-4(1)(-2.42 x 10-4 ) 2 10-14 = 1.53 x 10-2 = [OH - ] [H + ] = 1.53 x 10-6 = 6.52 x 10-13, ph = -log(6.52 x 10-13 ) = 12.2 14.7 Polyprotic Acids 14.8 Acid-Base Properties of Salts The conjugate base of a weak acid is a weak base: F - + H 2 O HF + OH - K = [HF][OH- ] [F - ] HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 - = 1.4 x 10-11 K a = 1.8 x 10-5 14-7

C 2 H 3 O 2 - + H2 O HC 2 H 3 O 2 + OH - K b = 5.6 x 10-10 HCN + H 2 O H 3 O + + CN - K a = 6.2 x 10-10 CN - + H 2 O HCN + OH - K b = 1.6 x 10-5 Relationship between K a and K b HA + H 2 O H 3 O + + A - K a = [H 3O + ][A - ] [HA] A - + H 2 O HA + OH - K b = [HA][OH- ] [A - ] K a K b = [H 3O + ][A - ] [HA] [HA][OH - ] [A - ] = [H 3 O + ][OH - ] = K w Determine the ph of a 0.589 M solution of NaC 2 H 3 O 2 K b = [HC 2H 3 O 2 ][OH - ] - = K w = 1 x 10-14 [C 2 H 3 O 2 ] K a 1.80x 10-5 = 5.6 x x 2 10-10 = 0.589 - x assume x<<0.589, then x = [OH - ] = 1.81 x 10-5 poh = -log(1.81 x 10-5 ) = 4.74, ph = 14- poh = 9.26 the stronger the acid, the weaker the conjugate base. strong acid neutral base weak acid weak base neutral acid strong base If the cation and/or anion are conjugate acids or bases of weak bases or acids, the salt will not be neutral. If both are weak, then ph will depend on which K is greater. If K b > K a, solution will be basic, if K a > K b, solution will be acidic, if K a K b, solution will be neutral. Salts That Produce Neutral Solutions cations from strong bases (Na + ) and anions from strong acids (Cl - ) 14-8

Salts That Produce Basic Solutions cations from strong bases (Na + ) and anions from weak acids (F - ) Salts That Produce Acidic Solutions cations from weak bases (NH 4 + ) and anions from strong acids (Cl - ) Mixtures of Strong and Weak Acids Usually the presence of the strong acid or base swamps out any effect of a weaker acid or base which may be present. 14.9 Influence of Molecular Structure on Acid Strength Binary acids HX HF << HCl < HBr < HI HF is weak, the rest are strong acids. HF is most polar, but bond dissociation energy is strongest (568.2, 431.9, 366.1, 298.3 kj/mol) CH 4 < NH 3 < H 2 O < HF Here, bond dissociation energies (414, 393, 460, 568.2) don t explain why HF is the strongest acid in the series. Here, polarity (HF is most polar) dominates. Oxyacids Z O H When Z is an electropositive metal, the Z-O bond is ionic and ZOH is a base ZOH Z + + OH - When Z is electronegative, the Z-O bond becomes more covalent and the O-H bond becomes more polar. Z O H Z O - + H + Two factors determine acidity of the oxyacid: Greater electronegativity of Z HClO 4 > HBrO 4 Greater oxidation number of Z (more O atoms) HClO 4 > HClO 3 > HClO 2 > HClO 14.10 Acid-Base Properties of Oxides Acidic, Basic and Amphoteric Oxides: All group I and II metal oxides (except BeO) are basic 14-9

Na 2 O + H 2 O 2NaOH BaO + H 2 O Ba(OH) 2 Most nonmetal oxides are acidic CO 2 + H 2 O H 2 CO 3 SO 3 + H 2 O H 2 SO 4 N 2 O 5 + H 2 O 2HNO 3 Amphoteric oxides (Be, Al, Ga, Sn, Pb) can act as either acid or base: Al 2 O 3 + 6HCl 2AlCl 3 + 3H 2 O Al 2 O 3 + 2NaOH + 3H 2 O 2NaAl(OH) 4 Basic and Amphoteric Hydroxides: Be(OH) 2, Al(OH) 3, Sn(OH) 2, Pb(OH) 2, Cr(OH) 3, Ni(OH) 2, Cu(OH) 2, Zn(OH) 2, and Cd(OH) 2 M(OH) x + xh + M +x + xh 2 O x = 2,3 M(OH) x + yoh - M(OH) x+y x = 2,3 and y = 1,2 Strong acid - strong base: All strong acids convert to H 3 O + in water, and all strong bases convert to OH - in water. H 3 O + + OH - H 2 O + H 2 O the resulting solution is neutral Weak acid - strong base: weak acids are mostly unionized in water HF + OH - H2 O + F - The conjugate base of a weak acid is also a weak base F - + H 2 O HF + OH - the resulting solution is basic Strong acid - weak base: H 3 O + + NH 3 H 2 O + NH 4 + The conjugate acid of a weak base is also a weak acid NH 4 + + H2 O H 3 O + + NH 3 the resulting solution is acidic Weak acid - weak base: weak acids and weak bases are mostly unionized in water HF + NH 3 F - + NH 4 + the acidity or basicity of the resulting solution depends on the relative strengths of the acid and base. 14.11 The Lewis Acid-Base Model 14-10

Lewis acid: electron pair acceptor Lewis base: electron pair donor Lewis acid-base reaction: formation of a coordinate-covalent bond AlCl 3 + Cl - AlCl 4 - H + + NH3 NH4 + BF3 + NH3 F3B-NH3 SO3 + OH2 H2SO4 14-11